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NON-METALLIC INCLUSIONS IN
ELECTROSLAG REFINED INGOTS
by
FIDEL REYES-CARMONA
Ing. Quim. Met., U n i v e r s i d a d N a c i o n a l Autdnoma de Mdxico, 1976
M.Sc, The U n i v e r s i t y of I l l i n o i s a t Urbana-Champaign, 1978
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
i n
THE FACULTY OF GRADUATE STUDIES
Department of M e t a l l u r g i c a l E n g i n e e r i n g
We accept t h i s t h e s i s as conforming
to the r e q u i r e d standard
THE UNIVERSITY OF BRITISH COLUMBIA
January 19 83
© F i d e l Reyes-Carmona, 1983
In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.
F i d e l Reyes-Carmona
Department of M e t a l l u r g i c a l Engineering
The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3
Date March 21, 1983
E - 6 (3/81)
ABSTRACT
The o b j e c t i v e of t h i s r e s e a r c h was t o i n v e s t i g a t e how
n o n - m e t a l l i c i n c l u s i o n s ( i n c l u s i o n s ) are p h y s i c a l l y and
c h e m i c a l l y transformed, removed and c o n t r o l l e d from e l e c t r o d e s
to the f i n a l ESR-product.
S e v e r a l 1020, 4340 and r o t o r (Ni-Mo-V) s t e e l e l e c t r o d e s
were r e f i n e d by two ESR-units (7.5 mm and 200 mm i n mould
diameter) under d i f f e r e n t s l a g systems. R e f i n i n g of these
e l e c t r o d e s was done under d i f f e r e n t d e o x i d a t i o n p r a c t i c e s ,
namely pure A l , C a S i , CaSiAlBa and A I S i a l l o y s .
Through t h i s r e s e a r c h i t was found t h a t i n c l u s i o n s i n
the e l e c t r o d e are p h y s i c a l l y and c h e m i c a l l y transformed i n the
e l e c t r o d e t i p by the thermal g r a d i e n t s . I n c l u s i o n s are
c h e m i c a l l y a l t e r e d by the presence of l i q u i d s l a g a t the
l i q u i d f i l m and they are e n t i r e l y d i s s o l v e d i n the m a t r i x
when the d r o p l e t i s completely formed. No i n g o t i n c l u s i o n s
were i d e n t i f i a b l e as of e l e c t r o d e o r i g i n and i t i s concluded
t h a t a l l e l e c t r o d e i n c l u s i o n s are e i t h e r d i s s o l v e d or removed
by the s l a g .
The e f f e c t s o f the s l a g w i t h and without d e o x i d i z e r s
on the chemical composition of the l i q u i d p o o l and i n g o t
were t r a c e d d u r i n g r e f i n i n g and hence the chemistry of
i n c l u s i o n s was determined by e x t r a c t i n g s l a g and l i q u i d
metal samples d u r i n g r e f i n i n g . The t o t a l oxygen content was
measured by the vacuum f u s i o n technique, chemical analyses
i i
o f s l a g by s p e c t r o p h o t o m e t r i c techniques, e l e c t r o n micro
a n a l y s i s by SEM and EPMA and x-ray ( c r y s t a l l o g r a p h i c )
a n a l y s i s . The assays were used to formulate and c o r r o b o r a t e
the d e o x i d a t i o n and p r e c i p i t a t i o n mechanisms.
The chemical composition o f i n c l u s i o n s i n r e f i n e d i n g o t s
are more s t r o n g l y i n f l u e n c e d by the d e o x i d a t i o n p r a c t i c e
than by the e l e c t r o d e or the s l a g composition i n low S i 0 2
content s l a g s . The p r e c i p i t a t i o n of complex A l - C a - S i
i n c l u s i o n s i s p r e d i c t a b l e i n h i g h s i l i c a s l a g s (>10.0 wt%)
and the most a p p r o p r i a t e s l a g system t o perform an e f f i c i e n t
d e o x i d a t i o n i s the 50 wt% CaF 2, 30 wt% A l 2 0 3 and 20 wt% CaO.
The d e o x i d a t i o n i n ESR i n g o t s takes p l a c e by the process
of c o o p e r a t i v e r e a c t i o n s between s l a g and d e o x i d i z e r s i n the
f o l l o w i n g sequences:
2 [Al] + 3 (FeO) t ( A l ^ ) + Fe
[Ca] + (FeO) t (CaO) + Fe
(A1 20 3) + [Ca] t 3 (CaO + 2 [Al]
The p r e c i p i t a t i o n r e a c t i o n s are c o n t r o l l e d by the oxygen
p o t e n t i a l i n the melt, thus the t r a n s i t i o n s t o be expected
are :
A l 2 0 3 « F e O + MnS I I -»• c t - A l 2 0 3 + MnS I I or I I I -»• x Ca0«y A l ^
+ MnS I I I o r (Ca, Mn) S -*• x Ca0«y A l ^ + CaS
An e x c e s s i v e d e o x i d a t i o n w i t h Ca r a i s e s the A l content i n the
i n g o t a c c o r d i n g t o :
X Ca + Y ( A 1 2 0 3 ) * * X CaO* (y - ^) A l 2 0 3 + | X [Al]
i i i
Radial i n c l u s i o n size d i s t r i b u t i o n as well as dendrite
arm spacings i n samples extracted from l i q u i d pool and
ingots were determined. It was found that the inc l u s i o n
size obeys the normal d i s t r i b u t i o n and there i s a normal
v a r i a t i o n of the inc l u s i o n size along r a d i a l distances.
Hence the i n c l u s i o n composition and size i s a function of
l o c a l s o l i d i f i c a t i o n conditions and also of the l o c a l
thermochemical conditions.
i v
TABLE OF CONTENTS
Page
Abstract i
Table of Contents i v
L i s t of Figures i x
L i s t of Tables x v i i
L i s t of Symbols xix
Chapter
I INTRODUCTION 1
II LITERATURE REVIEW 4
2.1 Literature Survey on Electrode Inclusions 4
2.2 Literature Survey on Slag-Liquid Metal Reactions and t h e i r Influence on the ESR Ingot Chemistry 12
2.2.1 P r i n c i p l e s of the Reaction Scheme i n the ESR Process 12
2.2.2 On the Nature of the ESR Reaction Scheme 18
2.2.3 Thermodynamic Approach of the ESR Slag Systems 32
2.2.4 Overall View on the Modelling of ESR Reactions 34
2.3 P r e c i p i t a t i o n of Inclusions 36
2.3.1 General * . . . . 36 2.3.2 Nucleation and Growth of In
clusions 39 2.3.2.1 Homogeneous Nucle
ation 39 2.3.2*2 Heterogeneous Nucle
ation 42
2.3.3 Growth of Inclusions 4 3
V
Chapter Page
2.3.4 Sulfides . . . „ 47 2.3.5 S p e c i f i c Sulfides 51 2.3.6 Oxisulfides 53
2.3.6.1 The Fe-O-S System 5 3 2.3.6.2 The Fe-O-S-Mn E q u i l i
brium 55 2.3.6.3 The Fe-O-S-Si-Mn
Equilibrium 62
2.3.7 Oxides 66
2.3.7.1 Aluminates 66 2.3.7.2 Calcium Aluminates 76 2.3.7.3 Complex Oxides 90
2.4 Inclusions i n ESR-Ingots 94
III NATURE OF THE PROBLEM 106
3.1 Inclusions i n the Electrode 106 3.2 The Chemical Influence of the ESR
components on the Composition of Inclusions 108
3.3 The P r e c i p i t a t i o n of Inclusions from Liquid Pool to Ingot I l l
3.4 D i s t r i b u t i o n of Inclusions During S o l i d i f i c a t i o n 113
3.5 Establishment of the Proposal and Objectives Sought Through t h i s Research 115
IV EXPERIMENTAL WORK AND TECHNIQUES 116
4.1 Experimental Procedure 116 4.2 Analysis of Inclusions 118 4.3 Total Oxygen Analysis 121 4.4 Inclusion Extraction Method 122
4.4.1 Apparatus and Experimental Procedure 123
4.5 Crystallographic X-ray Analysis of Extracted Inclusions 126
4.6 Atomic Absorption Analysis (Spectrophotometry) ? 127
v i
Chapter Page 4.7 Metallographic Analysis -^8
V RESULTS AND DISCUSSION • 130 5.1 Mechanism by which Electrode Inclusions
are Eliminated 130 5.1.1 Behavior of Oxisulfide Inclus
ions i n 1020 M.S. Electrode Tips *. , . . 130
5.1.2 Removal of Oxide and Sulfide Inclusions in 4 340 and Rotor Steels 141
5.1.2.1 Removal of Oxides and Sulfides i n 4340 e l e c trodes 144
5.1.2.2 Calcium Aluminum S i l i cates i n a Rotor (Ni-Cr-Mo) Steel 146
5.1.3 F i n a l Remarks About the Removal Mechanism 148
5.2 The Chemical Influence of the Electrode, Slag and Deoxidizer on the Chemical Composition of Inclusions .. 151
5.2.1 Description of Experimental Findings 151
5.2.1.1 Preliminary Studies on the E f f e c t of the Slag and the Deoxidation 151
5.2.1.2 Intermittent CaSi Additions and the Reaction Scheme 157
5.2.1.3 Refining of 1020 M.S., 200 mm Diameter Ingots Deoxidized Continuously with A l 159
5.2.1.4 1020 M.S. Ingots Deoxidized Continuously with a CaSi Alloy ... ..... 164
5.2.1.5 Corroboration and Extension of Previous Findings to a 4340 Steel CaSi (continuously) Deoxidized
Discussion of Results i n Terms of E l e c t rode and Slag Composition, Related to the Second Question
5.3.1 The E f f e c t of the Electrode on the Inclusion Composition of ESR Ingots
5.3.2 Elucidation of the E f f e c t of Slag and Deoxidizers (preliminary studies) ,
5.3.3 Preliminary Discussion on the Deoxidation Mechanism
5.3.4 Comprehensive Discussion on the Deoxidation Mechanism
5.3.5 F i n a l Remarks
Findings and Discussion Related to the Third Question
5.4.1 Description of Experimental Results
5.4.1.1 The Inclusion Mean Diameter
5.4.1.2 Findings from Individual Experiments
5.4.1.3 Complimentary Studies 5.4.1.4 Summary of Experi
mental Findings
5.4.2 P r e c i p i t a t i o n of Inclusions i n the Fe-Al-Ca-O-S (Mn) system
5.4.3 Discussion of Results »
5.4.3.1 Nucleation Growth and Fl o t a t i o n of Inclusions . . .
5.4.3.2 Comparison between Theoretical and Experimental Results...*
v i i i
Chapter Page
VI THE RADIAL DISTRIBUTION OF INCLUSIONS IN CaSi AND A l DEOXIDIZED INGOTS 2 2 3
6.1 Experimental Details and Techniques...... 223 6.2 Experimental Findings 2 2 4
6.3 Discussion of Results 2 2 5
VII CONCLUSIONS 228
VIII SUGGESTIONS FOR FUTURE WORK 2 32
LIST OF REFERENCES 235
FIGURES 251
TABLES 3 5 1
APPENDIX . . . ........ 3 7 4
ix
LIST OF FIGURES
Figure Page
1. Schematic i l l u s t r a t i o n of an ESR system .... 251
2. Predicted and measured temperature prof i l e s for a 1018 MS electrode 25 mm i n diameter 252
3. Manganese content of the metal for uni-variant equilibrium y - i r o n + "MnO" + "MnS" + l i q u i d (1) for Fe-Mn-S-0 system and univariant equilibrium Y - i r o n
+ "MnS" + l i q u i d s u l f i d e for Fe-Mn-S system 253
4. Univariant e q u i l i b r i a i n Fe-Mn-S-0 system i n the presence of y-iron and Mn(Fe)0 phases 254
5. Univariant e q u i l i b r i a involving s o l i d metal and Mn(Fe)0 i n the Fe-Mn-S-0 system bonded with ternary Fe-Mn-0 and Fe-S-0 terminal-phase f i e l d s (e) , ,(p) , (f) , (n), (g) and (h) 255
6. Location of the planes i n the quaternary (FeO-MnO-MnS-Si02) system 256
7. Equilibrium phases i n three planes of the FeO-MnO-MnS-SiC^ system. a) MnS-FeO-2MnO«Si02, b) MnS-2FeO«Si0 2-2MnO«Si0 2
and c) MnS-FeO-MnO 256
8. Behavior of inclusions enriched i n Mn and Si as a function of temperature 257
9. Schematic i l l u s t r a t i o n of changes i n i n clus i o n composition i n a 1020 MS e l e c t rode produced v i a acid e l e c t r i c furance.... 258
10. Wt. % A l and wt. % Ca and wt. % 0 i n l i q u i d i r o n at unit A l 2 0 3 and CaO a c t i v i t y 259
11. Isothermal Fe-Al-Ca-0 p r e c i p i t a t i o n (Henrian a c t i v i t i e s ) diagram 260
X
Figure Page
12. Ternary Al 20 3~(Ca,X)0-Si0 2 i n c l u s i o n diagram 261
13. Slag chemical composition used i n the ,.(34,53,83) , a. • 4.-past and present i n v e s t i g ation 262
14. Schematic i l l u s t r a t i o n of the ESR arrangement used in this investigation 26 3
15. Schematic i l l u s t r a t i o n of the "inclusion extractor" 264
16. Typical inclusions from 1020 MS electrodes (optical microscopy) * 265
17. Deformed inclusions i n 1020 MS electrodes and t h e i r X-ray spectrum analysis (SEM) ... 266
18. Macrostructure of a 1020 MS electrode t i p where l i q u i d f i l m , p a r t i a l l y molten and f u l l y r e c r y s t a l l i z e d areas are shown 267
19. Macrostructures of a 4340 electrode t i p . Droplet, l i q u i d f i l m , p a r t i a l l y molten, f u l l y and p a r t i a l l y r e c r y s t a l l i z e d zones are shown 268
20. Macrostructure of a rotor (Ni-Cr-Mo) s t e e l . Liquid f i l m , and p a r t i a l l y molten areas are shown 269
21. Schematic i l l u s t r a t i o n of 1020 MS electrode t i p (acid e l e c t r i c furnace produced) subjected to ESR-thermal gradients 270
22. Multiphase ( r e l a t i v e l y grown) inclusions i n a 1020 MS electrode t i p 271
23. Single phase inclusions i n p a r t i a l l y and f u l l y molten regions i n a 1020 electrode t i p 272
24. Complex Ca-Al-Si-Mn inclusions located i n the l i q u i d f i l m and droplets 273
25. Spectrum X-ray analysis of Ca-Al-Si-Mn i n clusions i n a 1020 MS electrode located i n the l i q u i d f i l m and droplets 274
x i Figure Page
26. Changes i n i n c l u s i o n chemical composition i n a 4 340 electrode t i p subjected to ESR thermal gradients 275
27. Changes i n in c l u s i o n chemical composition i n a 4340 electrode t i p with a strong recryst a l l i z e d region 276
28. Behavior of oxide inclusions in a e l e c t - . rode t i p of a rotor s t e e l subjected to ESR thermal gradients 277
29. Aluminum s i l i c a t e s i n a 4340 ESR ingot 75 mm i n diamter. Precipitated inclusions i n a l o c a l i z e d region 278
30. Influence of CaSi and FeO intermittent additions on the oxygen content of a 1020 M.S. (RIII-W) 279
31. Changes i n oxygen content i n a 1020 MS ingot as a r e s u l t of CaSi and FeO i n t e r mittent additions (RII-W) 280
32. Changes i n slag chemical composition i n a 1020 MS (RIII-W) as a r e s u l t of CaSi and FeO intermittent additions 281
33 Changes i n slag chemical composition as a r e s u l t of intermittent additions of CaSi and FeO i n slag during r e f i n i n g (RII-W) 282,283
34. Changes i n ingot chemical composition as a r e s u l t of CaSi and FeO additions i n slag during r e f i n i n g (RIII-W) 284
35. E f f e c t of CaSi and FeO additions i n the slag on the chemical composition of a 1020 MS ingot (RII-W) 285
36. Changes i n in c l u s i o n composition and size i n a 1020 MS ingot (RIII-W) as a r e s u l t of intermittent additions of CaSi and FeO i n the slag 286
37. Chemical analysis of slag samples i n R I - I l . 287
x i i
Figure Page
38. Ingot chemical analysis i n R I - l l 288
39. Slag chemical analysis (wt. %) in a continuously A l deoxidized 1020 MS ingot (RII-Il) 289
40. Slag chemical analysis (wt. %) i n R I I - I l . . . 290
41. Inclusion mean diameter and t o t a l oxygen content i n a continuously (Al) deoxidized ingot, (RII-Il) 291
42. Total oxygen content and inc l u s i o n mean diameter i n ingot (RII-I2) 292
43. Inclusion chemical composition (at. %) as a function of continuously increasing deoxidation rates i n RII- I l 293
44. Inclusion chemical composition (at. %) as a function of the ingot height (or continuously increasing deoxidation rates) i n RII-I2 294
45. Ingot chemical analysis i n RII-Il 295
46. Ingot chemical analysis i n RII-I2 296
47. "Alumina galaxies", (EPMA), associated to MnS II i n i n c i p i e n t aluminum deoxidized ingots, (RII-Il and RII-I2) 297
48. "Alumina galaxies" (EPMA) and MnS II i n A l deoxidized ingots 29 8
49. Faceted alumina (a-A^C^) i n samples from l i q u i d pool and ingot deoxidized with A l .. 299
50. Calcium aluminates low i n Ca from highly A l deoxidized ingots. A l , Ca and S composition RII-I2 300
51. Composition dependence of s u l f i d e phases on the Ca:Al r a t i o i n the oxide phase (RII-Il) 301
x i i i
Figure Page
52. Composition dependence of s u l f i d e phases on Ca-aluminate inclusions phases i n RII-I2 .. 302
5 3 • Segregated material i n an A l deoxidized ingot ( l i q u i d pool) . . 3 0 3
54. Dependence of "FeO" contents i n slag on the Ai2 0-.:CaO r a t i o i n slag of a continuously A l deoxidized ingot (RII-Il) 304
55. Dependence of "FeO" contents i n slag on the A^O^tCaO r a t i o i n slag of a continuously A l deoxidized ingot (RII-I2) 305
56. Changes i n t o t a l oxygen content and i n c l u s ion mean diameter i n a continuously CaSi deoxidized ingot (RIII-Il) 306
57. Inclusion mean diameter and t o t a l oxygen content i n a continuously CaSi deoxidized ingot (RIII-I2) 307
58. Inclusion chemical composition as a function of deoxidation rates i n RI I I - I l 308
59. Inclusion chemical composition as a funct i o n of deoxidation rates i n RIII-I2 309
60. Changes i n slag chemical composition i n a continuously CaSi deoxidized ingot (RIII-I l ) 310
61. Changes i n slag chemical composition i n a continuously CaSi deoxidized ingot (RIII-12) 311
62. Changes i n A l and Si i n RII I - I l as a consequence of continuously increasing CaSi deoxidation rates 312
63. Changes in ingot composition as a consequence of continuously increasing CaSi deoxidation rates i n RIII-I2 313
X I V
Figure Page
64. Dependence of "FeO" contents i n slag samples on the deoxidation rates i n RI I I - I l •
65. Dependence of "FeO" contents i n slag samples on the deoxidation rates i n RIII-I2
314
315
66. Sulfur (as sulfides) content i n inclusions as a function of the Ca:Al r a t i o i n the Ca-aluminate inclusion phases i n RI I I - I l .. 316
67. Sulfur (as sulfides) content i n inclusions as a function of the Ca:Al r a t i o i n the Ca-aluminate in c l u s i o n phases i n RIII-I2 .. 317
68. Chemical composition of inclusions (as Ca:Al ratios) as a r e s u l t of continuously increasing deoxidation rates i n RIII-I2, (a) and sulfu r (as sulfide) i n inclusions i n Ca-aluminates, (b) . s 318
69. Segregate enriched i n A l , Ca and S i i n a sample extracted from the l i q u i d pool of ingot RI I I - I l 319
70. Slag chemical analysis of a 4340 ingot continuously deoxidized with a CaSi a l l o y , [R-4340 (1)] 320
71. Ingot chemical composition i n R-4340 (1)... 321
72. Inclusion size d i s t r i b u t i o n and t o t a l oxygen content i n R-4340 (1) 322
73. Changes i n "FeO" contents i n the slag as a consequence of the continuously i n creasing CaSi deoxidation rates i n R-4340(1) 323
74. Inclusion chemical composition (as Ca:Al r a t i o s i n at. %) i n samples of l i q u i d pool and ingot i n R-4340 (1) 324
75. Inclusion composition as Ca:Al r a t i o s and S content i n R-4340 (1) 325
X V
Figure
76.
77.
78 .
79.
80.
81.
82.
83.
84.
85.
86.
87.
Page
Inclusion composition (oxide and s u l f i d e phases i n a rotor s t e e l deoxidized with Si based a l l o y s , namely: Ca-65 wt.% S i , Al-65% Si and "Hypercal". R-RS(I), R-RS(II) and R-RS(III) 326
Segregate enriched i n A l , Ca and S i from the A l S i deoxidized ingot, R-RS(II) 327
Inclusion p r e c i p i t a t i o n sequence i n a st e e l containing two le v e l s of sulfu r 328
S t a t i s t i c a l determination of the mean i n clusi o n diameter 329
Ce-distribution i n a i n c l u s i o n of a sample extracted from the l i q u i d pool 330
Ce and La d i s t r i b u t i o n i n an inclus i o n of a sample extracted from the l i q u i d pool. La and Ce come from a RE wire located i n the quartz tubing 331
A l , Ca and Zr d i s t r i b u t i o n s i n an inclus i o n of a sample extracted from the l i q u i d pool. Zr was i n the quartz tubing 332
Di s t r i b u t i o n of oxide formers i n an i n clusion of a sample extracted from the l i q u i d pool ,
D i s t r i b u t i o n of oxide-sulfide former i n an i n c l u s i o n of a sample extracted from the l i q u i d pool
Inclusion d i s t r i b u t i o n i n a dendrit i c structure of 1020 MS samples taken from l i q u i d pool during r e f i n i n g
Inclusion d i s t r i b u t i o n i n a dendrit i c structure of a 4 34 0 sample from the l i q u i d pool
Isothermal (1823 K) p r e c i p i t a t i o n (Fe, A l , Ca, 0, S) diagram at 0.1 a c t i v i t y of aluminum
333
334
335
336
337
xvi
Figure Page
88. E f f e c t of the a c t i v i t y of A l ( h A 1 = 0.001, 0.01 and 0.1) on the " p r e c i p i t a t i o n sequence" of Ca-aluminates 338
89. E f f e c t of the a c t i v i t y of S (h = 0.01, 0.01 and 0.001) on the " p r e c i p i t a t i o n sequence" of Ca-aluminates 339
90. Arrangement of aluminates i n r e l a t i v e l y low CaSi deoxidized ESR-ingots 340
91. Typical arrangement of inclusions i n a r e l a t i v e l y low CaSi or high A l deoxidized ingots 341
92. X A1 20_• Y CaO/CaS interface i n an ESR ingot, R-RS (I I I ) , deoxidized with "hypercal". X-ray spectrum analyses are also included 342,343
93. Secondary DAS i n a round (200 mm i n diameter) 1020 MS ESR ingot 344
94. Secondary DAS i n a 1020 MS ESR ingot 345
95. Secondary DAS i n a 4340 ESR ingot 346
96. Radial size d i s t r i b u t i o n of inclusions i n an A l deoxidized ingot, (RII-I2) 347
97. Radial i n c l u s i o n size d i s t r i b u t i o n i n a low CaSi deoxidized ingot (RIII-Il) 348
98. Radial i n c l u s i o n size d i s t r i b u t i o n i n a 200 mm ESR ingot CaSi deoxidized, (RIII-Il) 349
99. Radial i n c l u s i o n size d i s t r i b u t i o n i n a 4340 (ESR) ingot CaSi deoxidized 350
LIST OF TABLES
Table
I. C o r r e c t i o n f a c t o r to the Stokes' Law
I I . Thermochemical data f o r a) I n v a r i a n t e q u i l i b r i a i n Fe-S-0 system and b) Estimated data f o r i n v a r i a n t e q u i l i b r i a i n Fe-Mn-0, Fe-Mn-S and Mn-S-Q t e r n a r y systems
I I I . Estimated data f o r i n v a r i a n t e q u i l i b r i a i n Fe-Mn-S-0 quaternary system
IV. C a l c u l a t e d and p u b l i s h e d f r e e energy data f o r Fe-O-Ca-Al system a t 1823 K (1550°C)...
V. E q u i l i b r i u m c o n s t a n t s f o r d e o x i d a t i o n r e a c t i o n s
VI. Equations f o r i n v a r i a n t e q u i l i b r i a i n the i s o t h e r m a l (Fe-O-Ca-Al) system
V I I . Chemical a n a l y s i s of e l e c t r o d e s used i n t h i s r e s e a r c h
V I I I . Experiments and t h e i r f e a t u r e s used i n t h i s i n v e s t i g a t i o n
IX. Chemical composition of d e o x i d i z e r s used i n the p r e s e n t i n v e s t i g a t i o n
X. a) I n c l u s i o n composition as a r e s u l t of r e m e l t i n g 4340 e l e c t r o d e s i n a s m a l l ESR-furnace (75 mm i n diameter)
b) S l a g - d e o x i d i z e r e f f e c t on i n c l u s i o n composition
c) I n c l u s i o n chemical composition of 4340 e l e c t r o d e s used i n the s m a l l ESR-furnace
XI. Chemical e f f e c t of s l a g and e l e c t r o d e sur face p r e p a r a t i o n on i n c l u s i o n composition. (A 1020 MS and a r o t o r (Ni-Cr-Mo) s t e e l were r e f i n e d i n the 200 mm i n diameter ESR-furnace through s e v e r a l s l a g systems ). .
x v i i
Page
351
353
355
356
357
358
359
360
361
362
362
362
x v i i i
Table
XII.
XIII.
XIV.
XV.
XVI.
XVII.
XVIII.
Page
Slag chemical analysis of ESR (Ni-Cr-Mo) ingots deoxidized with Si based deoxi-dizers 364
Chemical analysis of ingots deoxidized with:
a) the Al-65% Si a l l o y 365 b) the Ca-65% S i a l l o y and 366 c) the CaSiAlBa (hypercal) a l l o y 367
Typical data recorded from EPMA analysis of i nclusions. a) l i q u i d pool and 368 b) ingot
E f f e c t of i n i t i a l number of inclusions on growth during cooling of l i q u i d metal
369
370 Derived equations for invariant (isothermal) e q u i l i b r i a i n the Fe-O-Ca-Al-S system
Computed compositions based on data given i n Table XV and e^ 1 = - 25, e 0^ = -62 and e C * = -40 3 7 1
Computed compositions i n the Fe-Ca-Al-0-S system by using information i n Table XV, variable e~r (-535, -400, -300,
Al -250 and -200) i n addition to the e Q = -62 and e C
sa = -110 3 7 2
373
xix
LIST OF SYMBOLS
l
(A)
[A], A
A: B
a c t i v i t y of the i component
'A' species i n slag
'A' element i n solution i n l i q u i d i ron r a t i o of A to B species
A ( g ) ' A ( l ) ' A ( s ) 'A' species i n gaseous, l i q u i d or s o l i d state
A l S i deoxidant, composition of which i s Al-65 wt.% S i . (Table IX)
A l 2 ° 3 * alumina as a primary deoxidation product
at. % atomic percent
ion i c species with either a pos i t i v e or negative valence
a-Fe a l l o t r o p i c state of iron
a - A l 2 0 3 corundum; i t i s also given as 'A' when i t i s referred to as a part of the Ca-aluminates p r e c i p i t a t i o n sequence
degrees i n the Celsius (centigrade) scale
C:C» supersaturation r a t i o i n terms of concentrations
(1)'(s) l i q u i d or s o l i d CaO
and C 3A 2
stoichiometric Ca-aluminates as given by the pseudo binary (CaO-Al 20 3) phase diagram, i . e . ,
CaO«Al 20 3, CaO-2Al 20 3,
CaO«6Al 20 3, 12CaO«7Al 20 3 and
3CaO«2Al 20 3
X X
(CaO) *
(Ca,Mn)S
CaS
(CaS)*
CaSi CaSiAlBa
DAS
DAS 1 1
D c
6„ or 6-iron Fe
e i
EPMA
f
peripheral phase on a calcium aluminate oxide inclusion
double s u l f i d e with Ca and Mn
phase heterogeneously p r e c i p i t a t e d on a Ca-aluminate phase
peripheral calcium s u l f i d e phase i n equilibrium with the CaO from the Ca-aluminate and oxygen and sulfur i n solution in iron
deoxidant, composition of which i s given i n Table (IX)
deoxidant ("hypercal"), composition of which i s given i n Table IX
dendrite arm spacing
secondary dendrite arm spacing
c r i t i c a l drag force on a spherical p a r t i c l e
a l l o t r o p i c state of iron
electron
i n t e r a c t i o n c o e f f i c i e n t ; the e l e ment for which the a c t i v i t y coe f f i c i e n t i s being calculated i s designated j and the element causing the e f f e c t i s designated i .
electron-probe-micro analyses
Henrian a c t i v i t y c o e f f i c i e n t
FeO" iron oxide, (FexO)
xxi
Fe(x,y,z)-0-S pseudo ternary i n c l u s i o n phase diagrams with an oxide and a su l f i d e phase;(Hilty and co-
,(129) workers)
f^ l i q u i d f r a c t i o n
GR product of growth rate by thermal gradients
Y, k, or 8-Al2C>2 a l l o t r o p i c states of the alumina
Y^ a c t i v i t y c o e f f i c i e n t of species A
h^ Henrian a c t i v i t y of species A
HIC hydrogen induced cracking
HSLA high strength low a l l o y s t e e l
e x c i t a t i o n voltage i n k i l o v o l t s kV
K
£1
absolute degrees- in the Kelvin (absolute) scale
Kg t o n - 1 deoxidation rate i n kilograms of deoxidant per (metric) tonne of remelted ingot
l i q u i d o x i s u l f i d e
£^ l i q u i d metal
L and L„ l i n e s d i v i d i n g the ternary i n -elusion and slag compositions suggesting the formation of low melting phases, Figures (12) and (13)
X wave length i n X-rays
m(CaO) • nfA^O.j) m and n are c o e f f i c i e n t s of the Ca-aluminate phases which are equivalent to those i n the CaO-A l 2 0 3 pseudo-binary phase diagram
x x i i
MnS(I,II,III)
y
yA
ym
o, "o", or oxi
ppm
RII-W, RIII-W
RII-I l and RII-I2
RI I I - I l and RIII-I2
R-43040 (1) R-RS(I), R-RS(II) and
R-RS(III)
P
a
u
V
+
<
manganese s u l f i d e type I, I I , or III
v i s c o s i t y
specimen current density in (EPMA) microamperes
unit length, microns
oxide of the type Mn(Fe)0
concentration in parts per m i l l i o n
ingots i n which CaSi and FeO were added (200 mm i n diameter)
(1020) ESR ingots Al-deoxidized (200 mm i n diameter)
(1020) ESR ingots CaSi deoxidized (200 mm i n diameter)
(4340) ESR ingot CaSi deoxidized (200 mm i n diameter)
Rotor (Ni-Cr-Mo) s t e e l deoxidized with CaSi, A l S i and CaSiAlBa alloys
density <
i n t e r f a c i a l tension
r e s u l t i n g vector v e l o c i t y
v e l o c i t y vector
degree of accuracy (plus or minus) i n chemical analysis
less than
-«- or =
s or ^
reaction in equilibrium
approximately
gaseous phase
ACKNOWLEDGEMENTS
I would l i k e to express my sincere gratitude to my
supervisor Dr. Alec M i t c h e l l for his concise advise. I am
also thankful to Professors R. Butters and B. Hawbolt for
t h e i r contributions and discussions during t h i s work. I
also appreciate the technical assistance of A. Lacis, G. S i d l a ,
R. Cardeno, H. Tump, R. McLeod and M. Mager.
I would l i k e to thank the Banco de Mexico, Consejo
Nacional de Ciencia y Technologia (CONACyT), Universidad
Nacional Autonoma de Mexico (Departamento de Metalurgia de
l a UNAM) and the Department of Met a l l u r g i c a l Engineering of
the University of B r i t i s h Columbia for the f i n a n c i a l support
given during my professional studies. The author i s also
g r a t e f u l for the f i n a n c i a l assistance of the American Iron
and Steel I n s t i t u t e (Project No. 32-445).
This work i s s p e c i a l l y dedicated to my parents,
brothers and s i s t e r s and everyone who has contributed to
reach my goal.
1
CHAPTER I
INTRODUCTION
In today's technology where there i s the demand for
extra-high-quality materials, there are only three second
ary steelmaking processes capable of f u l f i l l i n g the re
quired stringent standards 1) Electron Beam Melting (EBM),
2) Vacuum Arc Refining (VAR) and 3) El e c t r o s l a g Refining
(ESR). The properties r e s u l t i n g from any of the above pro
cesses can be categorized as: c r y s t a l structure, chemical
homogeneity, s u l f u r and phosphorus content and in c l u s i o n
chemistry and size d i s t r i b u t i o n . The EBM and the VAR pro
cesses o f f e r the lowest gas content. The v e r s a t i l i t y i n
control and operation, c r y s t a l structure, r e l a t i v e e l e c t r i
c a l e f f i c i e n c y and r e p r o d u c i b i l i t y are the main features of
the ESR process. *
Research c a r r i e d out since 1967 has widely demonstrated
that the ESR-process o f f e r s d e f i n i t e advantages over con
ventional practice and i n some respects also offers advan
tages over other secondary steelmaking practices. The con
tinuous demand for high q u a l i t y materials has increased
since 1967. From 1960 to 1973 the western world increased
i t s ESR production from 2 600 to about 120 000 tonne/year
and i t i s forecast to increase to 600 000 tonneVyear i n 1985.
The Soviet Union's production i s about three times that * F i r s t Int. Symp. on ESR
2
of the Western world. This marked increase i n production
indicates the a c c e p t a b i l i t y of the products manufactured by
the ESR-technology.
Materials produced for p a r t i c u l a r purposes such as
rotors used i n thermal and nuclear e l e c t r i c a l plants, land
ing gear used i n a i r c r a f t , crank shafts used i n large
vessels, gun barrels, superalloys used i n turbine blades,
high q u a l i t y t o o l and b a l l bearing steels , etc. are ex
amples of the wide variety of materials produced by the
ESR-process. These components are subject to dr a s t i c
temperature and environmental conditions and/or to dy
namic stresses. These two adverse conditions by them
selves generate microcracks which are usually associated
with chemical inhomogeneities and/or with inclusions in
the metal matrix.
Inclusions play a major role i n f a i l u r e s under the
described s i t u a t i o n s . Thus t h e i r index of s p h e r i c i t y ,
degree of cohesion with the metal matrix, i n t e r p a r t i c l e
distance, volume f r a c t i o n , size d i s t r i b u t i o n , p l a s t i c i t y ,
thermal contraction and expansion c o e f f i c i e n t s with respect
to the matrix and chemistry are the parameters which
determine the service l i f e , i n terms of inclusions, for
a given material.
The pote n t i a l of the ESR-process, because of i t s ver
s a t i l i t y , can be extended to other types of uses such as
3
Electroslag Casting (ESC) and Electroslag Welding (ESW).
It i s important to note that while these processes are
very r a r e l y used i n North America, i n the Soviet Union's
technology they are widely practiced.
The ESR process because of i t s multiple degrees of
freedom also o f f e r s a wide range of parameters to be modi
f i e d and improved without modifying the standard furnace.
To optimize the mechanical properties which are s t r i c t l y
related to inclusions i n a ESR-product, a series of i n t e r
actions between a l l the components of the process should be
evaluated, i . e . electrode, slag, deoxidizer and s o l i d i
fying ingot should a l l be considered. If the mechanisms
by which inclusions are formed are known, then the ESR-
process c a p a b i l i t i e s and r e s t r i c t i o n s i n t h i s respect can
be defined.
4
CHAPTER II
LITERATURE REVIEW
2.1 Literature Survey on Electrode Inclusions
From the thermal point of view the electrode t i p
should be considered as one of the sources by which one
part of the t o t a l heat produced by the slag i s consumed,
Figure (1). This amount of thermal energy which i s trans
ferred from the slag to the electrode plays several r o l e s :
1. It i s primarily converted to sensible heat, thus
producing a f i n e l i q u i d f i l m which afterwards w i l l form
droplets, and
2. I t also determines the temperature gradients above
and below the slag/gas interface. Thus, i t establishes the
amount of possible surface oxidation and the d i s s o l u t i o n
of c e r t a i n second phase p a r t i c l e s or inclusions i n the elec
trode . (1-5)
Theoretical and experimental studies have been per
formed to determine whether inclusions from the electrode are
eliminated before or a f t e r the metal droplet i s formed.
Heat and mass transfer models have also been developed to
predict the maximum in c l u s i o n diameter which can be d i s
solved under given ESR-conditions. (6)
M i t c h e l l , Joshi and Cameron have studied the temp
erature d i s t r i b u t i o n above the slag/gas interface i n a
laboratory ESR furnace.
They have i n d i c a t e d t h a t the r a d i a l temperature
g r a d i e n t s i n a l a r g e r e l e c t r o d e - i n g o t c o n f i g u r a t i o n b<
came s i g n i f i c a n t . T h e i r r e s u l t s a l s o suggest t h a t d i ;
s o l u t i o n of second phase p a r t i c l e s to a v a r y i n g ex
t e n t i s f e a s i b l e .
(7)
M a u l v a u l t and E l l i o t t who have developed a one
dimensional model have taken i n t o account the v e r t i
c a l movement of the e l e c t r o d e . T h e i r computations,
which have been based i n an assumed p a r a b o l i c p r o
f i l e , have shown a reasonable agreement with the expe
ment a l l y (37 mm diameter e l e c t r o d e ) determined v a l u e s (8)
Mendrykowski e t a l . ' s work by u s i n g a s i m p l i
f i e d one-dimensional-heat flow model and c o n s i d e r i n g
the e l e c t r o d e p a r t immersed i n the s l a g have a l s o
found t h a t w h i l e n e i t h e r r a d i a t i o n nor gas phase con
v e c t i o n p l a y a major r o l e , f o r a g i v e n s e t of c o n d i
t i o n s , the c o n v e c t i v e heat t r a n s f e r from the s l a g to
the e l e c t r o d e i s indeed more s i g n i f i c a n t . T h e i r r e
s u l t s suggest t h a t c o n d u c t i o n along the e l e c t r o d e pre
dominates as the h e a t - t r a n s f e r - c o n t r o l l i n g mechanism. ( 9 )
Tacke e t a l . along the l i n e s with work per
formed a t U.B.C. (6,10) have coupled two models (one
6
to determine the slag temperature and the other to
determine the heat fluxes) to calc u l a t e the electrode
temperature, i t s melting p r o f i l e and i t s depth of im
mersion i n the slag. It has been claimed that computed
values obtained by th i s two-dimensional flow are in
agreement with the experimental findings. The r a d i a l
e f f e c t was"also t h e o r e t i c a l l y and experimentally anal
yzed. These re s u l t s i n agreement with M i t c h e l l et
(6)
a l . show that the temperature gradients become
steeper i n the electrode nearer the l i q u i d f i l m . A representative example of t y p i c a l gradients i n
an electrode are shown i n Figure (2). It i s important
to mention that i n a l l the above models the thermal
energy spent for r e c r y s t a l l i z a t i o n or grain growth,
as shown by several r e s e a r c h e r s ^ ' 11-14) n Q t j 3 e e n
considered.
The i n c l u s i o n d i s s o l u t i o n phenomenon has also
been approached by several researchers from the mass (12)
transfer view point, Kay and Pomfret were the
f i r s t researchers to have suggested and modelled the
d i s s o l u t i o n of oxide inclusions ( s i l i c a and alumina) i n the
electrode f i l m under normal ESR conditions. They claim that
7
although inclusions can be dissolved as a r e s u l t of the
l i q u i d f i l m - l i q u i d slag i n t e r a c t i o n during the droplet
formation stage, their computed values for d i s s o l u t i o n
rate would only require the time that an electrode material
spends before i t becomes l i q u i d . Their c a l c u l a t i o n s for
d i s s o l u t i o n of alumina and s i l i c a inclusions whose di a
meters were 4 and 20 ym, were performed under the as
sumption that thermodynamic equilibrium at the electrode
t i p / s l a g interface was reached at 1800 and 2000°C.
M i t c h e l l based on heat transfer c a l c u l a t i o n ^ '
has recalculated the d i s s o l u t i o n of inclusions (12)
using the conditions of Pomfret and Kay . M i t c h e l l ' s r e s u l t s show that even by using a two-fold superheating
(8) (70° C) above that found by Mendrikowski et a l . no
solution i s predicted below 1600° C. Hence the i n c l u s i o n -
d i s s o l u t i o n mechanism i n the s o l i d electrode t i p was not
considered to strongly influence the o v e r a l l i n c l u s i o n
removal. (16)
Hajra and Ratnam have also performed mass trans
fer c a l c u l a t i o n s and experimental research i n a laboratory
ESR-furnace. Their approach was on the same basis as the
previously described works. Their r e s u l t s i n agreement
with Mitchell's show that slag/metal reactions play an
important r o l e in the electrode-inclusion removal. They
also found that oxide p a r t i c l e s c h a r a c t e r i s t i c of the
electrode were not traced i n the ingot.
8
Experimental and t h e o r e t i c a l work, so far des
cribed, has only been concerned with laboratory ESR (17)
furnaces. Medovar et a l . also reported that, i n
800 - 1200 mm consumable electrodes refined by ESR,
the i n c l u s i o n removal occurs i n the molten metal f i l m
or i n the process of droplet formation. They have also
indicated that inclusions i n droplets d i f f e r e d from
those i n the electrode. They claim that the i n c l u s i o n
shape, sizes and d i s t r i b u t i o n i n the s o l i d droplets
were sim i l a r i n nature to those i n the ESR ingot. (19 20)
Research ca r r i e d out i n the Soviet Union '
on a quantitative basis has suggested that inclusions
i n the electrode during r e f i n i n g are s p e c i f i c a l l y l o
cated i n the l i q u i d f i l m and they have a well defined
size d i s t r i b u t i o n . Based on these findings i t has been
claimed that inclusions are mechanically and chemi
c a l l y removed by the slag. (19)
On the other hand Roshchin et a l i n agreement
with other i n v e s t i g a t o r s ^ 1 ' ' h a v e established that
due to the "high temperature heating" manganese su l f i d e s
9 are f i r s t l y spherodized and afterwards dissolved. It has
(19) been observed that s i l i c a t e inclusions were sphero
dized, transformed and s l i g h t l y enlarged, i n contradiction
to several investigators' r e s e a r c h 1 3 ^ before they
reach the l i q u i d f i l m . The same contradiction i s found
with respect to second phase p r e c i p i t a t e s . While some
researchers believe that d i s s o l u t i o n occurs i n the s o l i d
s t a g e ^ others have reported that t h i s takes place in
the l i q u i d stage and s t i l l others ̂ have suggested
that they do not dissolve and serve as nucleating agents
i n the r e f i n i n g ingot.
Roshchin et a l . ' s work was performed exclusively
using o p t i c a l and opti c a l - q u a n t i t a t i v e (inclusion size
d i s t r i b u t i o n ) techniques. These researchers claim that
simulated heat treated samples (under an i n e r t atmo
sphere) subjected to several periods of time and temp
erature ranges, produced equivalent re s u l t s to those ob
served i n actual ESR-electrodes. (13)
Other studies i n l i n e with the previous work,
using d i f f e r e n t schedules have agreed with the above f i n d
ings. The major disadvantage of these simulated experi
ments i s that the calculated thermal g r a d i e n t s ^ and
the time-temperature schedules are so d i f f e r e n t to that
experienced by the electrode t i p s that a self-consistent
conclusion cannot be derived, (15)
Studies c a r r i e d out on a quantitative basis (total
oxygen content and i n c l u s i o n chemical analysis) have deter
mined that inclusions are dissolved i n the l i q u i d f i l m .
These a n a l y t i c a l studies however were performed exclusively
on. material belonging to the molten family.
The idea which supports the existence of a continuous
reoxidation due to continuous i n c l u s i o n d i s s o l u t i o n , from
the heat affected region to the electrode l i q u i d f i l m ,
has also been p r o p o s e d 1 3 ^ . The chemical nature of
these inclusions, however, was not investigated.
Theoretical s t u d i e s ^ ' ^ ' ^ ^ indicate that for a given
electrode-mold diameter configuration some superheating
i s expected at the electrode t i p , although for a short period
of time. On t h i s basis i t has been anticipated that i f (18)
inclusions contact the slag or e a r l i e r i f they are s i l i c a type, t h e i r d i s s o l u t i o n rate should be extremely high
(21) for a l l common slag - i n c l u s i o n combinations
(22) Paton et a l . have also suggested that the i n t r i n s i c
l iquidus-solidus length of each a l l o y system and the e l e c -
trode-steelmaking practice also play an important r o l e .
Their studies were performed on 1200 mm diameter electrodes
and a gradual d i s s o l u t i o n of s u l f i d e s was (optically) ob
served. The c r i t i c a l length at which changes i n sulfur con
centrations were observed was about one centimeter above
the "fusion l i n e . " (23) Zhengbang et a l . ' s studies based on the con-
9 5 centrations of a r t i f i c i a l Zr 0 2 inclusions have shown that
the chemistry of inclusions during the ESR process change
gradually from the electrode to the ingot. Other research
ers claim that the major reaction s i t e where inclusions
from the electrode are eliminated i s at the l i q u i d f i l m -(23 24) (15) slag interface ' . M i t c h e l l who has refined
electrodes containing calcium aluminum s i l i c a t e s has re
ported small inclusions i n the l i q u i d f i l m , the composition
of which did not correspond to the stoichiometric 2FeO«Si0 2
phase.
2.2 Literature Review on Slag-Liquid Metal Reactions 12
and th e i r Influence on the ESR Ingot Chemistry
2.2.1 P r i n c i p l e s of the Reaction Scheme i n the ESR Process
Among the conventional and secondary steelmaking prac
t i c e s the ESR-process represents one of the most complex
metallurgical reactors. The degree of d i f f i c u l t y i n i t s
study arises because reactions take place at s i t e s (elect
rode-slag, droplet-slag and l i q u i d pool interfaces) which
have separate and d i s t i n c t chemical and electrochemical re-(26 27)
gimes ' . The droplet, due to i t s size sees no
pot e n t i a l difference between i t and the surrounding slag
therefore i t reacts under the thermochemical conditions
dictated by the slag. The electrode ( l i q u i d film) - slag
and the molten l i q u i d pool - slag interfaces react almost
e n t i r e l y by imposed electrochemical potentials. Reactions
at these s i t e s are controlled by the surface environments
and they are not d i r e c t l y influenced by the slag chemistry.
It i s worthwhile to mention that the above patterns are
mainly applied to DC-ESR and to a given extent to the AC-ESR (26) (2728) operation . Several researchers ' have suggested
that even i n t h i s l a t t e r operating mode slag-metal ex
change and surfaces are ruled by the p o l a r i z a t i o n behavior. (28 29 ) "
Several studies ' on the current density-poten
t i a l behavior have shown that since there i s not a lin e a r
r e l a t i o n s h i p between these parameters for DC and AC ranges used i n ESR, then p o l a r i z a t i o n e x i s t s . Schwerdtfeger 1s
(28)
studies have also shown that af t e r the l i m i t i n g cur
rent i s approached for a given slag system a plateau i s
reached. This l i m i t i n g current can be increased again only
i f the p o t e n t i a l i s markedly increased. Thus, the magni
tude of the l i m i t i n g current from t h i s current density-
p o t e n t i a l r e l a t i o n s h i p has given a clear i n d i c a t i o n that 2+
a surface saturation i n Fe r e s u l t s i n the separation of
an i r o n - r i c h phase which remains fixed on the anode sur
face by i n t e r f a c i a l tension f o r c e s ^ 3 ^ . Therefore, t h i s
incomplete i r r e v e r s i b i l i t y leads to a net "FeO" production
i n the slag and also to a net solution of aluminum or c a l
cium i n the i r o n . Besides the r e c t i f i c a t i o n of AC current caused by i n t e r -
(30 32) f a c i a l e f f e c t s there i s evidence ' i n the l i t e r a t u r e which establishes that r e c t i f i c a t i o n due to current passing
2+ through the slag-skin/mould increases the Fe i n the slag bulk, thus r a i s i n g a l l oxidation rates i n the ESR-reaction
(33) scheme. Hawkins et a l . have shown that i f 5% - 30% of
the t o t a l current i s passed through the mould with an e f f i
ciency of 2% for the anodic reaction a r i s e of about 40 ppm
of oxygen would occur, under normal ESR-conditions. (29)
It has been speculated that the mechanism which con
t r o l s t h i s type of r e c t i f i c a t i o n i s the presence of small arc contacts which pass through the slag-skin into the slag.
(27)
M i t c h e l l has also indicated that due to the high
temperature and high current-density slag-metal interface
the reaction scheme i s probably not a well-defined
14
Faradaic interface. Thus e l e c t r o l y t i c reactions are pos
s i b l e only to the extent permitted by such p o l a r i z a t i o n
phenomena. (28 30)
The suggested ' reaction scheme i s as follows:
FeU) * F e 2 + + 2e , (1)
C a 2 + + 2e * Ca X (2)
Ca X t (Ca) slag or [Ca] (3) and at high current d e n s i t i e s :
A l 3 + + 3e t [Al] (4)
F e 2 + + 2e t . F e ( £ ) (5)
It has also been c l a r i f i e d that other reactions with
higher decomposition potentials than those allowed by the
above scheme w i l l not take place. (26)
From the above reaction scheme i t has been envisaged
that during the anodic h a l f cycle iron oxides w i l l form
adjacent to the metal surface either by discharge of oxy
gen ions or by d i s s o l u t i o n of Fe which w i l l replace Ca-ions
l o c a l l y . This leads to a high iron oxide a c t i v i t y ( aF e 0^ W 1 t h
consequent d i s s o l u t i o n of oxygen into the iron bulk. Simult-2+ 3+
aneously FeO or Fe (or Fe ) w i l l be transported by the
hydrodynamic regime into the bulk of the slag causing a grad
ual increase in the a„ n . The cathodic half cycle w i l l d i s -FeO 3+ 2+ 2+ charge A l , Ca or Fe . Any of these ions will contribute to
reduce the a _ in the slag. It can also be established that i f in this 2+
last electrochemical reaction there i s not s u f f i c i e n t Ca or
15
(33) i f Ca i s extensively evaporated a slag deoxidation i s
necessary to avoid the electrode s a c r i f i c i a l deoxidation.
Up to t h i s point in t h i s review only the reactions as
a r e s u l t of the inherent electrochemical nature of the ESR-
process have been considered. There are, however, other (38)
types of reactions involving the slag atmosphere i n t e r
face which also a f f e c t i t s o v e r a l l reaction patterns. Holz-
gruber ( 3 4^ has claimed that i f remelting i s ca r r i e d out
under a pure oxygen atmosphere the oxygen content i n ingots
ranges from 1.8 to 3.8 times that obtained under an argon
atmosphere. In thi s work i t i s also shown that oxygen
content i n ESR-ingots i s very dependent on the slag chemis-(1 37)
try. Several researchers have established v ' ' that i f
remelting i s not c a r r i e d out under an i n e r t atmosphere and
i f the slag i s not deoxidized then the oxygen content (and
the loss of reactive elements) i n the ingot can only be
controlled by the slag (oxygen p o t e n t i a l ) . Miska et a l . ' s
r e s u l t s (38) also i n favour of the above theory show that
the lowest oxygen content i s controlled s t r i c t l y by the
slag chemistry and intermediate oxygen contents are strongly
influenced by the slag-electrode chemistry.
While some researchers (38) have found that the i n t r o
duction of oxygen into the slag i s a mass transfer controlled
process others d ' 33) believe that in addition to t h i s
mechanism there i s an "oxygen sink" (at the slag-metal i n t e r
face) which acts as a dr i v i n g force. It i s thought that the
d e s u l f u r i z a t i o n reaction i s controlled by t h i s dual mech-
It i s generally accepted that there i s a continuous
introduction of iron oxide into the melt, due to the
continuous oxidation of the electrode surface. M i t c h e l l v
has pointed out that the ESR-reaction pattern i s more
strongly influenced by t h i s phenomenon than by the oxygen
introduced as a r e s u l t of reactions taking place at the
slag-atmosphere interface. Other studies have also
shown that by r e f i n i n g electrodes with d i f f e r e n t chemistry
(36,39) o r different surface preparation under other
wise equivalent ESR-conditions, d i f f e r e n t composition or
d i f f e r e n t mechanical properties are observed. This be
havior although i t i s i n d i r e c t , has been attributed to
the introduction of various quantities of ir o n oxide into
the slag as (electrode) scaling. (40 41)
There are reports i n the l i t e r a t u r e ' x ' ' which
indicate that the evaporation of gaseous f l u o r i d e com
pounds in certa i n slag systems also affects the reaction
pattern. The reaction:
largely contributes to s h i f t the Al 20 3:CaO r a t i o . This
reaction becomes very important where the calcium oxide
anism (1, 12)
(A1 20 ) + 3(CaF 2) t 3(CaO) + 2A1F3+ (8)
a c t i v i t i e s are less than 10 -2 (42, 43) Mi t c h e l l (36)
17
has pointed out that i n order to trace the actual s h i f t i n g
i n the chemical composition p a r t i c u l a r l y i n slags where
the CaF« and AlF have about the same vapour pressure, an ^ 3 2- - 2+ 3+ analysis i n the 0 :F r a t i o as well as the Ca : A l should be considered. Complementary reactions are:
(CaF 2) + H 2 0 ( g ) t (CaO) + 2HF+ (9)
and
2(CaF 2) + (Si0 2) t (CaO) + S i F ^ (10)
Chouldhury et a l . K i i i i ) ±n a recent communication have
pointed out that by remelting ingots with low frequency
current the S i losses from an a c i d i c slag are n e g l i g i b l e .
They notice, however, that f o r a slag where the CaO:Si0 2
r a t i o i s greater than 4, S i losses r i s e to 65% during re
melting. This finding i s also supported i n previous i n v e s t i -(40, 45) m . . , , „. ^, . (39) gations . Tobias' and Bhat's work suggests
that reaction (9) can be considered as an additional source
of oxygen i n the system. In t h i s work although a mechanism
i s not c l e a r l y s p e c i f i e d , i t i s considered that moisture
as a condensed phase in moulds, water vapour from the atmo
sphere or chemically bonded moisture i n the flux markedly
a l t e r s the recovery of titanium and s i l i c o n i n ESR-ingots.
2.2.2 On the Nature of the ESR-reaction Scheme
A great deal of attention has been dedicated to i n
vestigate the o r i g i n , sequence and consequence of the re
actions i n the ESR-process. The major objective of these
studies have been to control the ingot composition with
out s a c r i f i c i n g i t s chemical i n t e g r i t y .
I t i s conventionally accepted that the broadest
c l a s s i f i c a t i o n of reactions taking place during r e f i n i n g
can be subdivided as follows:
Reactions which are controlled by the oxygen pote n t i a l
[Me] + [0] * (MeO) (11)
and reactions as a r e s u l t of the exchange between two
l i q u i d s :
[Me] + (MO) t (MeO) + [M] (12)
Among the reactions comprised i n the f i r s t category, type
(11), are: [Ca] + [0] t (CaO) ( 4 8 ' 6 2 ) (11-i) 2[A1] + 3[0] t (A1 20 3) ( 3 6 ' 4 8 " 5 2 ' 6 2 ) (11-ii) [Si] + 2 [ 0 ] t (Si0 2) (26, 33,35,36,49-51,63,64) ( n _ i i i )
[Ti] + 2[0] t (Ti0 2) ( 5 2 ' 5 8 ) (11-iv) m i r I -*• /i-i ^ \ (26,33,35.36,63,64) . x[Fe] + y[o] (Fe 0 ) ' ' ' • ' (11-v) x y
[Ti] + 3(Ti0 2) t 2 ( T i 2 0 3 ) ( 5 2 ) (11-vi)
[Ti] + 2(Ti0 2) t ( T i 2 0 3 ) + (TiO) ( 5 2 ) (11-vii)
The group of reactions of the kind (12) ± s subdivided i n
the so c a l l e d deoxidation reactions, (12a), namely:
[Mn]+ (FeO) t- (MnO) + Fe (57,59-61) (12-i)
[Si]+ 2(FeO) t (Si0 2) + 2Fe d,57,60,61) (12-ii)
[Til + 2 (FeO) t (Ti0 2) + 2Fe (52,58,60) ( 1 2 - i i i )
2[A1]+ 3(FeO) t (A1 20 3) + 3Fe (35,48,57,60) (12-iv)
[Ca] + (FeO) t (CaO) + Fe ( 6 2 ) (12-v)
and the exchange reactions which also involve the reactive
species, (12-b):
3[Ti] + 2(A1 20 3) t 3(Ti0 2) + 4[A1] (18,39,46-51) (12-vi)
2[Ti] + (Si0 2) t 2(Ti0 2) + 2[Ti] (52,58) (12-vii) 3[Si] + 2(A1,0-) t 3(SiCO + 4 [ Ai]d/44,46,47,49,55)
^ J Z ( 1 2 - v i i i )
2[Mn] + (Si0 2) t 2(MnO) + [Si] (44/56,57,76) (12-ix)
2[A1] + 3(MnO) J ( A l ^ ) + 3 [ M n ] ( 5 7 ) (12-x)
Other reactions included i n t h i s category observed -(̂ 1)
i n laboratory (ESW) experiments are (12-C):
(CuCl 2) + [Al] t Cu + (A1C13)
3(Cu 20) + 2[A1] t Cu + ( A l 2 0 3 )
(Ni0 2) + Fe t Ni + (FeO)
(Si0 0) + 2 Fe t Si + 2(Fe O)
(MnO) + Fe t 2 Fe + (Si0 9)
C + (FexO) t CO ( g ) + Fe
2 0 Although the depicted categorization of the reaction
scheme has been presented i n an oversimplified manner
i t s t h e o r e t i c a l basis should be enunciated to e s t a b l i s h •
the reacting conditions under which i t takes place and
the governing reacting mechanism.
A wide range of opinions and sometimes apparently
controversial r e s u l t s are found in the l i t e r a t u r e i n re
gard to the approach predicting the ESR-reaction sequence. ( 6 3 )
While some investigator's work support the theory that ( 5 8 )
there e x i s t s a state of equilibrium, other studies ( 3 3 )
based i n thermodynamic data and experimental r e s u l t s have
found that either a "dynamic equilibrium" or k i n e t i c fac
tors govern the reaction pattern. It i s well recognized^®^
that i f an ESR-furnace i s considered s t r i c t l y as a reactor
even i n the absence of electrochemical factors, i t s nature i s ( 6 5 6 6 )
such that true thermal equilibrium ' i s never reached
and hence i t should be instead considered as a reactor which
operates under three d i f f e r e n t regimes, namely: 1 ) an
unsteady state which holds for about three times the
ingot diameter from bottom. 2 ) a quasi-steady state
for most of the r e f i n i n g time and 3 ) the hot top
stage which i s at the end of the r e f i n i n g time.
Other p e c u l i a r i t i e s of the ESR process which influence
the reaction sequence are i t s hydrodynamic regime ( 3 0
caused by i t s r a d i a l and v e r t i c a l temperature gradients '
65,67)^ Thus, s t r i c t l y speaking an actual state of e q u i l
brium i s not reached because of the changing thermal cond
tions, hence influencing i t s chemical nature. From t h i s
d escription and from the electrochemical p r i n c i p l e s a l
ready described i t can be seen that the reaction scheme
must be considered according to both thermal regime and
i t s r e s u l t i n g k i n e t i c factors. These ideas have been sup
ported by Kay's s t u d i e s d ' 4 8 ) o n the behavior of the ESR
reaction pattern. In t h i s work i t has been suggested
that slag-metal composition relationships are governed by (33)
k i n e t i c factors. Hawkins et a l . have c l e a r l y shown
that t h i s p a r t i c u l a r state of equilibrium i s not unique
but i t can be represented as a thermal-parameter denomin
ated " c h a r a c t e r i s t i c temperature." This parameter which
was calculated on a thermochemical basis indicates that
the system may see simultaneous reactions at th e i r cor
responding thermal regions. Although t h i s "characteris
t i c temperature" as a parameter does not have any physi
c a l meaning i t does represent the unsteady thermal-chemi
c a l behavior of the ESR process. (33 61)
It has been proposed ' that o v e r a l l (ESR) re
actions are the r e s u l t of a well defined series
of steps which involve mass transfer (di f f u s i o n , convec
t i o n and hydrodynamic flow) and chemical factors (reorgan
zation of the r e l a t i v e p o s i t i o n of ions, atoms or mole-
cules). The k i n e t i c aspect of electroactive interfaces
i s ruled by: the a c t i v i t i e s of reacting species on both
sides of t h i s s i t e , d i f f u s i o n c o e f f i c i e n t s , temperature
and concentration gradients extending from the i n t e r -(33)
face to the l i q u i d iron or slag bulk. Hawkins et a l .
have also suggested that a threefold-stage reaction se
quence can be envisaged, namely
i) Transport of reactants to the slag-metal i n t e r
face. This step i s attained by d i f f u s i o n and convection
of the two contributing phases (Slag and l i q u i d metal).
i i ) • Electrochemical reactions which involve the ion-
electron exchange process, and
i i i ) Transport of reacting products away from the
interface. This step i s again ruled as stage ( i ) .
Experimental (ESW) work ( 6 ^ has shown that the
rate of reaction i s strongly dependent upon the reaction
products at the e l e c t r o a c t i v e interfaces. It has been
found that l i q u i d miscible reaction products r e a d i l y
d i f f u s e and are e f f i c i e n t l y transported away from the
reacting interface. Gaseous reaction products which
are formed at the electro-active interface slow the
reaction by i n t e r f e r i n g with the d i f f u s i o n products
through the boundary layer. And s o l i d reaction prod
ucts also slow the reaction rate by blocking off the
area available for reaction. Patchet (^D has also
investigated " i n d u s t r i a l cases" in ESW involving multi-
component electrode and slag systems. It i s noted i n
these experiments that reactions can take place simul
taneously and they a l t e r the composition of the refined (28
product. Work carried out by several investigators '
33, 57, 5 8 ) i n agreement with Patchet's findings
has suggested that although the process operates under
a (chemical and kinetic) dual regime, the s t a r t i n g thermo
dynamic conditions at least can be used to i n i t i a t e the
calculations involved i n predicting the "dynamic
equilibrium" conditions. On the other hand, Cooper
et a l . have reported that during AC-melting;
metal drops leaving the electrode i n terms of sulfur,
reach chemical equilibirum with the slag and that the
extent of the reactions between l i q u i d pool and slag
were almost n e g l i g i b l e . They also claim that the only
reactions which occurred at t h i s (latter) s i t e were
due to minor temperature and compositional differences
with those of the electrode. This apparent contra
d i c t i o n i s c l a r i f i e d by M i t c h e l l ( 2 7 ' 3 0 ' 4 1 ? and
Hawkins et al.(58) who have established that although
an actual thermodynamic equilibrium may not be attained
i n i n d u s t r i a l ESR-operation; the k i n e t i c s of the pro
cess, however, are so favorable that a state close to e q u i l i
brium i s reached. It has also been found that low. v i s c o s i t y
slags and highly e f f e c t i v e reacting area enable the
(49 53 reactions to reach such a state of n e a r - e q u i l i b r i a ' ' 6 3 7 8 ) (168)
' . Several studies on C a F 2 ~ A l 2 0 3 slags have
shown that although this slag system has high v i s c o s i t y
concentrations of A l , Si and Mn i n ESR-ingots have the
tendency to achieve the t h e o r e t i c a l equilibrium.
Other researchers ( 6 3 , 6 9 ' 7 0 ^ have also found
that regardless of the number of electrochemical or
k i n e t i c parameters of the ESR-process, a simple thermo-
chemical (equilibrium) approach i s s u f f i c i e n t to pre-(71)
d i e t the f i n a l ingot composition. M i t c h e l l
who has studied the S i - S i 0 2 reaction, has also sug
gested that i f there i s a constant i r o n oxide source i n
the slag i n which y° i s high then i t i s f e a s i b l e
that the e f f e c t i v e oxygen p o t e n t i a l w i l l be that of the
Fe-FeO equilibrium. Perhaps the most t y p i c a l work
which supports the (ESR) equilibrium theory i s Holzgruber
and Petersen's (72, 73)_ I n ^h^g W O r k i t has been es
tablished that sulfur removal i s e n t i r e l y dependent
upon the slag chemistry. Other studies along the l i n e s of the previous work was also carried out by Miska et
(38)
a l . • . These researchers have also agreed with
Holzgruber et a l . ' s proposal. Miska et a l . ' s findings
obtained by remelting low a l l o y s t e e l using A l as a
deoxidizer were not i d e n t i c a l to Holzgruber et a l . ' s
who used d i f f e r e n t electrode chemistry and Si-deoxidation
practices. They have attributed these differences to
the deoxidation practice and also to the presence of
Mn and A l i n the electrode. The Si-SiC^ reaction has
been studied under several (ESR) slag systems. Holz-
gruber^ 7 5^, Holzgruber and Plockinger ^7*^ and Miska (38)
and Wahlster have found that i n i n d u s t r i a l ESR-furnaces t h i s reaction reaches a state of equilibrium.
(75) The influence of several a l l o y s and the influence
(38) of the A12C>2 in the slag i s also shown. Kusamichi
(77) et a l . ' s findings in agreement with previous work,
have also shown that the Si-SiC^ behavior i s l i n e a r l y
related to the b a s i c i t y index of the slag. Although
there are indications of the v a l i d a t i o n of the thermo-
chemical equilibrium achieved during r e f i n i n g i t
should be pointed out that this equilibrium i s very
temperature-dependent^ 4 8^. Retelsdorf and Winterhager ^ 9 ^ f Boucher and
(79) Jager and Kuhnelt have studied the Al-A^O^ re
action and conclude that the Al and the oxygen content
from ESR-ingots indeed follow the the o r e t i c a l (thermo
dynamic) equilibrium. (46 47 49-51) Abundant information v ' ' ' exists i n
the l i t e r a t u r e which establishes the v a l i d i t y of the (83)
equilibirum-reaction theory. Rehak et al . ' s studies
on the A l and Si d i s t r i b u t i o n i n ESR-ingots also favour
the thermodynamic approach of the ESR-reaction system.
They claim that reactions involving these species are ruled
by the electrode composition and by the i n d i v i d u a l a c t i v i t y
of the components of the slag. (76)
Holzgruber and Plockinger's work and Choudhury (44)
et a l . ' s are also i n agreement with Holzgruber and (72 73)
Petersen's ' . Their thermodynamic approach on
slag chemistry as a function of the a c i d i t y - b a s i c i t y
concept and A l deoxidation techniques i n i n d u s t r i a l
p r a c t i c e , c l e a r l y reveal that indeed equilibrium i s reached.
Kamardin et a l . who have refined 0.38 wt. % carbon
low a l l o y Cr-Mo steels deoxidized with aluminum have
concluded that despite t h e i r approximate method to c a l
culate the a c t i v i t y of the slag components (by means of
ternary diagrams of the CaD-A^O^-SiG^-system) , the pre
d i c t i o n of the A l and Si-concentrations i n refined ingots
can be estimated using a thermodynamic treatment.
The previous description has e s s e n t i a l l y been em
phasized on reaction of the type ( 1 1 ) . The other cate
gory which involves reacting species between two l i q u i d s
has also been extensively studied. The importance
of t h e i r study i s that they largely contribute to
a l t e r the chemistry of i n d u s t r i a l ESR - ingots. The
modification of the ingot chemistry ("longitudinal seg-
27
regation") becomes more c r i t i c a l where high a c t i v i t y of
reactive elements i s present. Because of the important
ro l e played by the T i in either superalloys or T i -
s t a b i l i z e d s t a i n l e s s s t e e l s , a considerable e f f o r t has
been devoted to understand the mechanism by which a
homogeneous longitudinal T i - d i s t r i b u t i o n i s reached
in refined ingots. Although there are evidences in the 1-4. a. (37, 48, 71, 74) . . . . ,. ^ , l i t e r a t u r e • • which indicate the mechanism,
there are few studies which c l e a r l y reveal i t . Pateisky's (46 47)
studies ' have shown that the reaction (12-vi)
indeed attains equilibrium. It i s also indicated that
t h i s equilibrium i s strongly affected by the thermal (52)
reaction conditions. Krucinski's studies i n agree
ment with Pateisky's have also indicated that the
reaction (12-vi) actually reaches a state of e q u i l i
brium. In addition to the above facts Krucinski has
also pointed out that Ti-oxides of lower valency i n the
slag should be considered, i . e . reactions (11-vi and 11-vii)
Another inte r e s t i n g reaction from the i n d u s t r i a l (47 49 55)
viewpoint which has been extensively studied ' '
i s the reaction ( 1 2 - v i i i ) . A l l i b e r t et a l . '
who have remelted ingots under SiC^-slags of lower vola
t i l i t y have shown that t h i s reaction does follow i t s
stoichiometric r a t i o . The [Al] vs [Si] p l o t whose
slope i s 0.781 indeed corroborates t h i s f a c t and also
indicates the r e v e r s i b i l i t y which i s reached as a mani
fe s t a t i o n of a state of equilibrium. Kay's s t u d i e s ^
suggest that t h i s reaction i s not influenced by the
oxygen p o t e n t i a l . This conclusion i s also supported (81) (78) by Opravil . Choudhury et a l . who have refined
ingots (2300 mm i n diameter) using 2.5 Hz AC. ESR have
studied the reaction (12-ix). Their r e s u l t s i n agree
ment with Holzgruber 1 s and Holzgruber and Plockinger 1 s ^
c l e a r l y show that the slag b a s i c i t y plays a s i g n i f i c a n t
role i n the Si-Mn d i s t r i b u t i o n i n ESR-ingots. Choudhury 1s
re s u l t s also show that the type of current does not i n
fluence the ingot chemistry. I t i s also pointed out
that Mn losses take place only when the CaOtSiC^ r a t i o
i n the slag i s less than 3. They also claim that despite
the continuous Al-addition during r e f i n i n g the loss of S i
and Mn are unavoidable. The stoichiometry of t h i s reaction
was not investigated. (37)
Buzek and Hlineny concerned with the low Mn-recovery rates (50-58%) have used isotopic oxygen
18
{O } above the slag to determine the reaction mechanism.
Their findings* indicate that by Al-deoxidation im
proved Mn-recovery i s attained. Although they have not speci
f i c a l l y established i t s thermodynamic behavior they did
indicate i t s high effectiveness. They claim that the
reduction-oxidation mechanism i s ruled by the reaction
(12-x).
Regarding the so-called deoxidation reactions, Kay ^
has suggested the use of extreme care in any approach
since several reactions may operate simultaneously. It
has been pointed out that reaction (12-ii) may i n some
instances be controlled by reaction (8). Since the re
action (12-i) has a well-defined equilibrium constant
of a value close to unity i n the range of ESR-tempera-
tures, Fraser has d e l i b e r a t e l y selected i t to e l u c i
date i t s mechanism. He has noted that t h i s reaction
may proceed to the l e f t at the electrode-slag interface
and subsequently reverse at the higher temperature of
the slag-ingot interface. This proposal has led to the
b e l i e f that the steady state conditions which are f r e
quently observed i n reactions of t h i s type are a con
sequence of a dynamic balance created by the thermal
difference between the two major electroactive s i t e s .
M i t c h e l l ( 3 0 ) has extended t h i s proposal to reactions of
the type (12-vi).
Krichevec et a l . (^O) h a v e studied the S i , T i ,
Mn and the A l d i s t r i b u t i o n s by r e f i n i n g ingots under
several slag systems have found a " s a t i s f a c t o r y " stab
i l i t y of these elements. They claim that these findings
are an i n d i c a t i o n of the " p r a c t i c a l " equilibrium conditions
attained during r e f i n i n g . Their studies were performed
on "deoxidation reactions" (12-iv).
30
As described i n previous sections the formation of
iron oxide during r e f i n i n g i s almost unavoidable, e i
ther as a r e s u l t of electrochemical reactions or as a
r e s u l t of the reactions between the slag and the atmo
sphere during r e f i n i n g . • These sources of iron oxide in
conjunction with the oxide on the electrode surface formed
before and during r e f i n i n g , in the presence of the oxide
components of the slag generate a l i q u i d with extensive (1 38 82)
im m i s c i b i l i t y ' ' . Hence, the aF e 0 r i s e s rapidly
to unity at very low "FeO" concentrations. Studies on
binary (CaF2~FeO) ̂ , ternary (CaF2-CaO-FeO) ̂ 1 *, (CaF 2~
Al 20 3~FeO) ̂ and i n quaternary (CaF 2-Al 20 3-CaO-Al 20 3-FeO) ^
systems have shown t h i s behavior.
A CaF 2 slag can permit very l i t t l e oxygen before i t
becomes oxidizing with respect to ir o n . Consequently, any
element which forms an oxide more stable would then be
oxidized from the metal into the slag. This f a c t becomes
more c r i t i c a l where reactive metals such as A l , S i , Zr, and T i are present.
Several techniques have been proposed to overcome
such a problem: 1) A complete removal of scale on the electrode surface and the use of an i n e r t atmosphere
(74) (He or N 2) , 2) painting the electrode surface after scale i s removed with an A l or a magnesia-alumina spinel
(38) paint to prevent oxidation of the electrode , 3) en-
(83) r i c h i n g electrodes i n oxidable elements such as
Zr, A l , S i , etc., 4) continuous additions of a) strong
oxide formers as elements (Al, S i , T i , Zr, e t c . ) , b) ferro
a l l o y s (FeAl or FeSi) ^ 8 4^ and c) slag-deoxidizer composites
(CaF 2-Ca> ( 8 5' 8 6>. (34)
Holzgruber suggests that i n addition to the
use of a protective atmosphere a more e f f i c i e n t deoxi
dation i s achieved i f a deoxidizer (Al, S i or Ti) i s added
i n a slag system which does not contain i t s oxide. Other . (39 , 46, 55, 60) . _ ^ researchers ' ' ' have proposed that i f an
element i s prone to oxidation l i k e T i , S i , Zr, during
ESR-process then additions of i t s respective oxide pre
vents i t s losses. Kay's ^ findings indicate that by re
f i n i n g a l l o y s which contain A l , T i and Zr i n free-titanium
oxide slags the three elements i n the electrode act as
deoxidizers. As the amount of Ti02 i n the slag increases
the deoxidation i s only c a r r i e d out by Al and Zr. The
Al-increment and the Si-decrement i n the ingot are con
t r o l l e d by the exchange reaction ( 1 2 - v i i i ) . Kay has
also found that by adding 0.5% Zr.0 2 to the slag i n i t i a l l y
containing 15% T i 0 2 i s s u f f i c i e n t to protect the Zr con-(79 )
tent of the refined a l l o y . Jager et a l . have shown
that the e f f e c t of the Al-content from the electrode or as
a deoxidizer i s very dependent upon the slag chemistry.
The net content of A l transported into the ingot i s almost
constant for a given A^O^-content (up to 24 wt. %) in
the slag and also for a given CaO: S i 0 2 r a t i o (up to 2).
If these two parameters are increased the net Al-content
of the ESR-ingot increases d r a s t i c a l l y hence leading to (78)
deleterious mechanical properties. Chouldhury et a l . (79)
and Jager et a l . have claimed that by using an "im
proved technique" a maximum A l content of 0.01 wt %
can be attained i n i n d u s t r i a l 125 tonnes ESR-ingots.
In these two communications the deoxidation rates, how-(55)
ever, are not given. Kajioka et a l . have studied the
e f f e c t of Al-deoxidation under several slag systems, namely
CaF2~CaO, C a F 2 - A l 2 0 3 and CaF 2-Al 20 3-CaO-Si0 2. They have found
that deoxidation rates between 0.05 to 0.1 wt. % A l produce
the best " r e s u l t s " , i . e . an almost steady A l - d i s t r i b u t i o n (34)
i n the ingot. On the other hand, Holzgruber proposes
a deoxidation rate of 6.2 wt. % A l .
2.2.3 Thermodynamic Approach of the ESR-Slag Systems
Another important area of the ESR-slag system i s i t s
thermo chemical approach. In t h i s f i e l d although very important, very l i t t l e information has been reported.
(45)
M i t c h e l l has found that in the common system CaO +
A1 20 3 + S i 0 2 + CaF 2 there are only three fluoride-containing
compounds other than the in d i v i d u a l f l u o r i d e s , i . e . C 1 1 A 7 F ' C 3 A 3 F a n d C 9 S 3 F ' w n e r e c ' A ' F a n d s stand for
33
CaO, A1 20 3, CaF 2 and S i 0 2 respectively. This observation
has been taken as an i n d i c a t i o n that the CaF 2 may be con
sidered as an i n e r t diluent in highly basic areas of
these systems, since the acid-base interactions involving 2- - (53
0 would be stronger than those for F . A l l i b e r t et a l . ' 54)
who have remelted a l l o y s through 70 wt. % C a F 2 '
30 wt. % A1 20 3 with 0, 2, 5, 10, and 15 wt. % S i 0 2 res
pectively, have applied Mitchell's c r i t e r i a . Their
findings show that the acid-base interactions i n the quat
ernary CaO + A l ^ O ^ + S i 0 2 + CaF 2 are approximately the
same as those i n the ternary CaO + A^O^ + SiC^ s Y s t e m f
d i l u t e d i n the CaF 2. There are other indications i n the 1 -4. t.. *_ , ^ • , (38, 58, 88 , 89) l i t e r a t u r e which also support t h i s theory
(14 53 54)
A l l i b e r t et a l . ' ' also suggest an alternate method
for reactions related primarily to b a s i c i t y , i . e . the
CaO/Si0 2 r a t i o concept which i s strongly supported i n the (34, 49-51, 72-76) M . , . , . (38) German l i t e r a t u r e ' ' . Miska and Whalster
who have studied the CaF 2-CaO-Al 20 3-Si0 2-FeO system have
found that i f the Al 20 3:CaO r a t i o i s greater than 3.0,
the CaF 2 at ESR-temperatures remains i n e r t and the slag
composition behaves as i t were i n the CaO-Al 20 3 binary.
Chai's and Eagar's s t u d i e s o n CaF 2~metal oxide welding
fluxes have concluded that the oxidizing p o t e n t i a l of
these types of slags i s reduced only by the d i l u t i o n of
these species i n the CaF 2 < The addition of CaF 2 i n these
34
slag systems has almost no e f f e c t on the more stable ox
ides and hence the CaF 2 was v i r t u a l l y considered as a
diluent.
2.2.4 Overall View on the Modelling of ESR-Reactions (57)
A recent work has c l e a r l y revealed the current
understanding of the ESR-reaction system. I t has been
indicated above that there are e s s e n t i a l l y two ways to ap
proach the reaction pattern. These are the equilibrium
reactor and the "single-stage reactor" concepts. The
equilibrium-reactor method i s by far a simpler approach.
It i s supported by the idea that an actual state of e q u i l i
brium i s reached during r e f i n i n g , as previously i n d i
cated. Its p r i n c i p l e i s to (thermodynamically) e q u i l i b r a t e
a (ESR) slag with an electrode of a given composition This
technique i n t r i n s i c a l l y assumes a unique equilibrium temp
erature and a mass transfer flow through a fixed slag v o l
ume. These two major assumptions as already indicated,
are not necessarily true.
The second approach considers a t r a n s i t i o n a l phase con
tact and a "lumped mass-transfer c o e f f i c i e n t . " I t has been
pointed out that although t h i s model resembles the non-
equilibrium nature of the process i t r e l i e s on a mass-
transfer c o e f f i c i e n t which does not have a t h e o r e t i c a l
background i n i t s computation. Another disadvantage of
t h i s technique i s that i t i s not able to account for
reactions of the type (12). M i t c h e l l has also pointed out 2+
that both techniques re l y on the Fe concentrations as an
input datum. This parameter, however, i s unknown and has
to be determined experimentally.
As described here, these two concepts have both ad
vantages and disadvantages. Nevertheless, i t can be un-
mistakeably seen that the main objective of these two a l
ternatives i s to predict and control the ingot chemical homo
geneity .
36
2.3 P r e c i p i t a t i o n of Inclusions
2.3.1 General
Since endogeneous inclusions are no longer considered
as foreign p a r t i c l e s but "natural" components of s t e e l ,
today's technology demands from metallurgists s k i l l f u l
control of them to y i e l d products which could f u l l y sat
i s f y the required stringent standards. A vast amount of (90-98)
research has been devoted to study the e f f e c t s of
deoxidizers and deoxidation techniques i n the past. It
i s known that since elemental oxygen i s highly soluble in
l i q u i d iron (0.168 and 0.20 wt. % at the monotectic and
at ordinary steelmaking temperatures r e s p e c t i v e l y ) , then
an appropriate deoxidizer can be selected to maintain the
oxygen content to a given l e v e l . The f i r s t requirement
expected from any deoxidizer i s obviously a high c a p a b i l i t y
to react with oxygen and i n p a r t i c u l a r cases simultaneously
with sulfur i n the melt. The second important requirement
i s i t s a b i l i t y to be removed from the melt once oxidation
has taken place. „ . .. .. . ,. (99-106) . S o l i d i f i c a t i o n - p r e c i p i t a t i o n studies have
shown that the i n c l u s i o n p r e c i p i t a t i o n sequence, as a
measure of the degree of deoxidation i s a series of con
tinuous processes. These comprise the nucleation, growth
and elimination of the deoxidation products. The nucle
ation phenomena can be homogeneous or heterogeneous and
i t can take place i n the l i q u i d stage or during s o l i d i
f i c a t i o n . To homogeneously nucleate a phase i t i s neces
sary to originate a supersaturation and i t can be reached
by undercooling of the species i n solution, by additions (9
of either a deoxidizer or oxygen and during s o l i d i f i c a t i o n
If there are some s o l i d p a r t i c l e s i n a melt where deoxid
ation products can di f f u s e to then a heterogeneous nucle
ation (precipitation) occurs.
At steelmaking temperature some oxides, o x i s u l f i d e s and
su l f i d e s are generally found i n solution with l i q u i d s t e e l .
Once s o l i d i f i c a t i o n s t a r t s , segregation (due to incomplete
d i f f u s i o n i n the solid) i n the in t e r d e n d r i t i c l i q u i d occurs.
As soon as t h i s i n t e r d e n d r i t i c l i q u i d i s saturated pre
c i p i t a t i o n of inclusions takes place. P r e c i p i t a t i o n of i n
clusions continues u n t i l the solidus temperature of the melt
i s reached. Under p a r t i c u l a r circumstances as w i l l be
further described, p r e c i p i t a t i o n takes place at even lower
temperature ranges. Inclusion s i z e , shape, quantity and
d i s t r i b u t i o n i s p r i n c i p a l l y given by the s o l i d i f i c a t i o n
rate and by the s o l u b i l i t y of ce r t a i n chemical species
i n the l i q u i d s t e e l . Maximum s o l u b i l i t y of these
species influences the p r e c i p i t a t i o n sequence during s o l i
d i f i c a t i o n . As an a p r i o r i rule, for species which have low
s o l u b i l i t y i n l i q u i d s t e e l p r e c i p i t a t i o n starts when i n
c i p i e n t s o l i d i f i c a t i o n i s observed. These types of i n -
38
elusions w i l l have longer time for growth.
Oxi-sulfides and s u l f i d e s which are p r e c i p i t a t e d as
a r e s u l t of monotectic reactions are included i n t h i s cate
gory. On the other hand, i f s o l u b i l i t y i s r e l a t i v e l y high
p r e c i p i t a t i o n takes place at the l a s t stage of s o l i d i f i
cation and hence these phases are confined to l o c a l i z e d
areas. While in the f i r s t case inclusions are globular,
large and have large i n t e r p a r t i c l e distances, i n the sec
ond group inclusions are very small and usually located
around grains or dendrites. Single phase or composite
films around dendrites are also included i n t h i s second
kind. The s o l u b i l i t y of some species i n l i q u i d s t e e l can
a l t e r the s u l f i d e chemistry and hence i t s p r e c i p i t a t i o n , . (90,106,107) sequence and size '
2.3.2 Nucleation and Growth of Inclusions
2.3.2.1 Homogeneous Nucleation
According to the homogeneous nucleation theory pro
posed by Volmer and Weber-Becker and D o r i n g ^ ® ^ ' ^
the formation of a new phase from a l i q u i d phase occurs
only i f supersaturation e x i s t s i n the parent phase. Sev-
, , (99,101,110,111) . i T x. e r a l researchers ' • have revealed that
the i n t e r f a c i a l tension (a) and the degree of super-
saturation (C:C») of a melt influence the nucleation rate.
P o p e l ^ 1 1 ^ has calculated the rates of nucleation for
i n t e r f a c i a l tensions ranging from 180 to 100 ergs/cm 2
and supersaturation r a t i o s from 1 to 10. His studies
show that for systems (FeO-MnO) where the a = 180 ergs/cm 2,
2 8
he obtained a nucleation rate of about 10 at super satur
ation r a t i o s as low as .1.5. With C:C°° r a t i o s of about 10, 35
the nucleation rates were increased to 10 . On the other
hand i n systems (FeO-MnO-Si02) where the i n t e r f a c i a l
tension i s about 700 ergs/cm 2 the nucleation rate of -79
inclusions i s extremely small (10 ) for supersaturation
40
r a t i o s of less than 3. It becomes appreciable, however,
at C:C°° r a t i o s of about 10. For i n t e r f a c i a l tensions of
about 1000 ergs/cm 2 as i n the CaO-A^O^-SiC^ system,
homogeneous nucleation of inclusions becomes very d i f f i
c u l t . These conclusions are s i g n i f i c a n t for deoxidation
during cooling and s o l i d i f i c a t i o n . During cooling of a
well-deoxidized melt which does not contain any inclusions
there i s a p o s s i b i l i t y of formation of an oxide with lower
i n t e r f a c i a l tension, e.g. FeO'A^O^ and FeO-rich s i l i c a t e s .
Turpin and E l l i o t using Volmer and Weber's ^^8)
c l a s s i c a l theory obtained an expression for c r i t i c a l super-
saturation i n terms of the free energy for homogeneous nucle
ation. They have found that hercynite was formed i n iron
melts where thermodynamically alumina should have pre
c i p i t a t e d . Thus, they suggested that supersaturation i s
required to produce homogeneously nucleated alumina phases.
Chipman ̂ X 1 1 ^ has suggested that both A l ^ O ^ and FeO'A^O-j may
p r e c i p i t a t e simultaneously i f there i s not complete mixing
(112-113) i n the melt. McLean and Ward have indicated that
hercynite may also p r e c i p i t a t e af t e r an i n i t i a l alumina pre-
41
c i p i t a t i o n when the metal i s depleted in A l . The e q u i l i
brium i n t h i s case is between A^O^, FeO-A^O^ dissolved (99)
oxygen and the l i q u i d iron. Turpin*s and E l l i o t ' s work
shows that i f the c r i t i c a l free energy to homogeneously
nucleate A^O^ were much greater than that for hercynite,
the p r e c i p i t a t i o n s of the l a t t e r phase becomes more fea
s i b l e . These researchers have also concluded that at high
concentrations of deoxidizer or a l t e r n a t i v e l y of oxygen
r e l a t i v e to normal bath compositions in equilibrium with
pure oxide i s required to overcome the surface tension
e f f e c t and thus to p r e c i p i t a t e phases homogeneously.
They have also pointed out that the common method of adding
the deoxidizer to a molten pool provides s u f f i c i e n t super-
saturation to form very small inclusions. This con
clusion was also reached by Turkdogan ̂ ^1) showed
that i f the deoxidant i s assumed to be evenly d i s t r i b u t e d
i n the melt before nucleation. then the supersaturation at
tained would be an order of magnitude less than that neces
sary for homogeneous nucleation. He therefore postulated
that the d i s s o l u t i o n of deoxidizer in l i q u i d s t e e l takes
a f i n i t e time during which c e r t a i n regions of the melt
are expected to be very r i c h i n solute concentration. In
these regions l i q u i d metal attains s u f f i c i e n t l o c a l super-
saturation for homogeneous nucleation.
Investigations c a r r i e d out by Von Bogdandy^ 1 1 4^' on
nucleation of aluminum containing phases showed that when
the deoxidant i s c a r e f u l l y introduced into the melt the
inclusions form i n a layer at a d e f i n i t e time and location.
They have postulated that very high super-saturation r a t i o s 14
(10 ) are required for homogeneous nucleation of these oxides. These values are several orders of magnitude
(99) higher than those obtained by Turpin
2.3.2.2 Heterogeneous Nucleation
In actual practice large degrees of supersaturation
are not required i f the p r e c i p i t a t i o n of inclusions take -i , . , . (115) place on s o l i d surfaces
From the above discussion i t i s evident that nucle
ation of inclusions i n s t e e l i s not the rate c o n t r o l l i n g
step because high supersaturation i s reached near the
regions where the deoxidizers are transported into the
l i q u i d pool. These nuclei are uniformly d i s t r i b u t e d due
to s t i r r i n g of the melt (during refining) and p r e c i p i t a t e
further oxides as cooling and s o l i d i f i c a t i o n takes place.
However, the presence of pre-existing inclusions from
electrodes ( i f any), make heterogeneous nucleation more
favorable where r e f i n i n g conditions are not optimum.
2.3.3 Growth of Inclusions
By using Shewman's f o r m u l a t i o n ^ one can see that
among the phenomena which contribute to growth of i n
clusions from nuclei (0.1 ym) to sizes found i n ESR i n
gots (2-4 ym), the growth by d i f f u s i o n of solutes (oxy
gen and deoxidizers) and p r e c i p i t a t i o n on the nuclei
i s the mechanism which i n the least amount of time (̂ 1-2
sees.) could generate i n c l u s i o n sizes in the range ordin
a r i l y found i n the ESR process. The above calc u l a t i o n s 7
are very dependent on the number of nuclei present (10 cm - 3). Turkdogan u s e ( j a d i f f e r e n t number of nuclei
5 (10 ) at the s t a r t i n g conditions and hence his c a l c u l a t i o n s
showed that a d i f f e r e n t maximum radius of inclusions
(23 ym) was reached i n a s l i g h t l y higher period of time
(6 sees ). The above information plus some other theor-(117)
e t i c a l c a l c u l a t i o n s performed by Lindberg and T o r s e l l
i n a similar a p p l i c a t i o n of Wert's and Zener 1s model^ 1 1 8^
c l e a r l y indicate that t h i s growth mechanism i s a very
f a s t process. Experimental work performed by several re-(107 117 119 120) searchers ' ' ' agree with previous c a l c u l a t i o n s .
During cooling and s o l i d i f i c a t i o n the growth of inclusions
becomes more s i g n i f i c a n t as p r e c i p i t a t i o n continues on pre
ex i s t i n g p a r t i c l e s . I y e n g a r ^ 1 2 ^ has formulated a model
44
to predict the growth under the above conditions. His re
su l t s have shown that the time for completion of growth i s
dependent on the i n i t i a l size of inclusions. This time de-6 7
creases with increase i n radius. With 10 -10 nuclei/cm 3
the growth was almost n e g l i g i b l e . Thus, these r e s u l t s
are c l e a r l y i d e n t i f i e d with r e s u l t s found i n E S R ' " ' ' 2 1 '
27 30)
' l i q u i d pools and ingots, i . e . growth i s almost en
t i r e l y achieved by d i f f u s i o n of solutes. (117
The second phenomenon which has been observed ' 120-122)
to largely contribute to the growth of inclusions
i n conventional melting practices i s the c o l l i s i o n co
alescence mechanism. This mechanism i s s t r i c t l y related
to the motion of oxide inclusions i n l i q u i d s t e e l .
For a given size d i s t r i b u t i o n of inclusions i n a melt
there i s a d i s t r i b u t i o n of r i s i n g v e l o c i t i e s ' 2 1 ^ . The
larger inclusions w i l l l e v i t a t e more rapidly than the smaller , ,,. . (117,122) ones and as a r e s u l t c o l l i s i o n may occur '
The coalescence of inclusions depends on impact,
speed and angle, surface properties such as surface ten
sion, chemical composition and t h e i r physical state ( i . e .
l i q u i d or s o l i d ) .
It has been observed that the motion of inclusions
i n s t e e l f a l l s within the viscous flow regime where the
Reynolds number of the p a r t i c l e i s less than one. This
45
includes i n c l u s i o n sizes i n the 0-50 ym range. Under t h i s
condition, Stokes 1 Law
Ust = (13)
i s applicable provided the system i s considered to be d i
lute and p a r t i c l e i n t e r a c t i o n can be neglected. The deriv
ation of Stokes' Law^ 1 2 6^ depends upon the following condi
tions: 1) incompressibility of the medium, 2) i n f i n i t e
extent of the medium, 3) very small and constant terminal
v e l o c i t y , 4) r i g i d i t y of inclusions, 5) absence of s l i p p i n g
at the f l u i d - p a r t i c l e interface and 6) sp h e r i c i t y of p a r t i
c l e s . The parameter involved i n t h i s expression are:
Ust = terminal v e l o c i t y according to Stokes' Law,
(cm/sec.)
p , p = density of the p a r t i c l e and the medium res-s pectively, (g/cm3)
r - radius of the sphere, (cm)
y = v i s c o s i t y of the medium, (g/cm. sec.)
The above conditions are very d i f f i c u l t to s a t i s f y
i n r e a l i t y . Corrections, however, can be applied to ac
count for the deviation from i d e a l i t y . Table (I) l i s t s
the a v a i l a b l e corrections. Stokes' Law i s a p p l i c a b l e ^ 1 2 6 ^
provided that gravity i s the only external force acting on
inclusions. If an inclu s i o n i s i n a melt as i n the ESR
46
process, which i s i t s e l f i n motion, then the drag on a
spherical p a r t i c l e i s given by:
Dc = -6iryr (v-u) (14)
where v = v e l o c i t y (vector) of sphere due to gravity and
u = r e s u l t i n g v e l o c i t y (vector) due to movement of the sur
rounding f l u i d .
In order to obtain an expression for the v e l o c i t y of
the inclusions r e l a t i v e to a fixed coordinate system i t
i s necessary to know the r e s u l t i n g v e l o c i t y (u) at every
location i n the melt.
Iyengar's work' 2 1^ has c l e a r l y revealed through sev
e r a l mathematical approaches that growth of inclusions by
any other mechanism i s almost n e g l i g i b l e .
47
2.3.4 Sulfides
The idea of "hot shortness" was conceived i n the
l a s t century. This topic, however, received more attention (127 128)
i n the early 30's ' . The so c a l l e d " s t e e l burning"
or "overheating phenomenon" has been studied since then.
Research through the years has led to the conclusion that
s u l f u r was less detrimental when manganese was present.
Since then Mn has been used to modify the s u l f i d e phase.
If Mn i s absent or present i n i n s u f f i c i e n t amounts the
formation of FeS i s almost i n e v i t a b l e . This phase has a
very low melting point (1190° C) and when i t i s combined
with i r o n or FeO i t forms a lower melting point eutectic phase (940°C). This eutectic p r e c i p i t a t e s between grains
(129-132) or dendrites . I f t h i s s t e e l i s to be heated at
a u s t e n i t i c temperatures for further mechanical working
then grain detachment o c c u r s ^ ^ " ^ ^ . This problem (133) • (overheating") has been observed even i n ESR-mater-
i a l s . Thus Mn has the c a p a b i l i t y to modify the s u l f i d e
phase from a l i q u i d "FeS" to a s o l i d "MnS" at hot working
temperatures (1900- 1200°C) . This t r a n s i t i o n occurs where (92)
the Mn:S r a t i o i s higher than 4 . The i d e a l form for
s u l f i d e inclusions i n low carbon steels i s to have a pure
MnS-phase which melts at about 1610°C. Three d i f f e r e n t (90 134)
kinds of MnS have been usually reported ' , MnS I-III.
Their s t a b i l i t y depends on the cooling rate and on the
chemistry of the melt (136-139) ̂ T ^ e t r a n s - L t i o n f r o m
type I to II occurs i f the oxygen content i n the s t e e l i s
diminished to l e s s than 100 ppm. The t r a n s i t i o n of type
II to III i s obtained by the influence of two parameters,
a low oxygen content and a high a c t i v i t y of s u l f u r . It
has been reported that the presence of C, S i , P, A l and
C a(140 142) p r o m 0 4 - e formation of MnS I I I . Fredriksson
and H i l l e r t ' 3 ^ who have studied the Fe-O-S ternary sys
tem have claimed that as a r e s u l t of a cooperative-
eutectic reaction where MnS forms as a c r y s t a l l i n e phase
together with the Fe-rich phase, a MnS-type IV i s formed.
The morphology of these s u l f i d e s has generally been des
cribed as globular,branched rod and idiomorphic for type (93 142)
I, II and I I I , respectively ' . The mechanism by
which they are formed i s as follows. Sulfide inclusions
type I are usually associated with o x i - s u l f i d e s (eutectic
type) and are p r e c i p i t a t e d as a r e s u l t of a monotectic
reaction. Since p r e c i p i t a t i o n of s u l f i d e s takes place
almost simultaneously with the deoxidaton process then
an oxide^sulfide and a s u l f i d e (type I) enriched phase
w i l l be formed. The phase which p r e c i p i t a t e s f i r s t i s (91 142)
richer i n oxygen than the second phase ' . Rimmed
or semikilled i n d u s t r i a l ingots (with r e l a t i v e l y high
oxygen content and low sulfur s o l u b i l i t y ) contain t h i s
49
type of s u l f i d e . Ingots which have been deoxidized with
the t h e o r e t i c a l required amount of aluminum w i l l have
low oxygen content and high sul f u r s o l u b i l i t y . This
deoxidation practice produces MnS II. This type was
i n i t i a l l y thought to be formed as a r e s u l t of a eutectic
reaction ^ 1 3 ^ . More recent ideas indicate that i t i s
originated by a cooperative monotectic reaction. It i s
also believed that t h i s s u l f i d e p r e c i p i t a t e s at i n -(91)
c i p i e n t s o l i d i f i c a t i o n stages. K i e s s l i n g has pointed
out that since alumina and MnS II are frequently observed
together but as d i s t i n c t phases then the alumina phase
acts as a substrate for the s u l f i d e (type I I ) .
Lower oxygen content and lower sulf u r s o l u b i l i t y are
required to p r e c i p i t a t e MnS III i n s t e e l s , than i n MnS II.
These conditions are attained because of the excess of Al
in solution. This s u l f i d e p r e c i p i t a t e s i n the early stages
of s o l i d i f i c a t i o n as a single phase. It has been observed^ 1 4 2^
that the three types are p r e c i p i t a t e d i n between grains or
dendrites.
Recent communications^ 1 3 8' 1 4 3^ have suggested that
MnS IV which presents a "ribbon" pattern i s a modifi
cation of either I - I I I ( 1 3 9 ) or I I ( 1 4 3 ) . Ito et a l . ( 1 3 9 )
who have studied the k i n e t i c s and chemical influence of
melts on the MnS-shape claim that type II r e s u l t s from
a eutectic reaction and type I and III (type N) are pre-
50
c i p i t a t e d from s o l i d s t e e l . A mechanism, however was not
given. Steinmetz et a l . d 4 3 ) who have studied the Fe-Mn-O
and the Fe-Si-0 systems at 1500° and 1600°C have shown
that there are areas of s t a b i l i t y for the three MnS types
when the [S] vs. [0] and the [S] vs. [Mn] are plotted.
In t h i s work, i t i s also proposed that MnS-morphology i s
very dependent upon the l o c a l a c t i v i t y conditions of oxy
gen and the degree of deoxidation (Si or Mn). Thus, i f
a succession of d i f f e r e n t conditions i n terms of l o c a l
a c t i v i t y or degree of deoxidation of melts i s made then
a continuous series of morphological changes i n MnS's would
take place, i . e . from spherical (oxisulfide) at high oxy
gen a c t i v i t i e s , r o d l i k e , d e n d r i t i c and "skeleton" shape
at medium oxygen a c t i v i t i e s and pseudo and highly crys
t a l l i n e (MnS) at very low oxygen (local) a c t i v i t y . This
l a s t " t r a n s i t i o n " (MnS II to MnS III) has been observed
when a melt i s deoxidized either by A l , Ca or Ca-bearing (140,142,144,145)
a l l ovc: ' ' '
51
2.3.5 S p e c i f i c Sulfides (91) (92) Kies s l i n g and Lange and Salter and Pickering
have established that double s u l f i d e s of the type (Mn, Me)s
are frequently found. Me represents any of the following
elements T i , V, Ni, Cr, Fe, Zr, Mg and Ca. Salter and
Pickering ̂ h a v e studied the replacement of Mn by Fe
in the double s u l f i d e . They found that i t ranges from
0.5 to 32 wt. % Fe. The maximum s o l u b i l i t y l i m i t , however, (91)
disagrees with previous work performed by Kiessling , (41.0). The Cr substitution for Mn i n these s u l f i d e s
has also been studied by these researchers. While Salter (92)
and Pickering have reported a replacement of Mn by (91)
Cr ranging from 0-5 to 25 wt. % (Cr), Ki e s s l i n g who
has studied these compounds i n synthetic s u l f i d e s has
found a maximum replacement of 26 wt. % Cr. Perhaps the
most important double s u l f i d e which has become a focus of
attention with the advent of the Ca-injection processes
i s the (Ca, Mn)S. There i s abundant information i n the , . ̂ . (140, 144, 158) . . . ^ , , . , . . l i t e r a t u r e ' ' which establishes that t h i s compound i s pr e c i p i t a t e d on complex Ca-aluminates forming a peripheral envelope. Salter and Pickering d 4^) n a v e
found that since CaS i s isomorphous with the MnS (NaCl type of l a t t i c e ) then extensive CaS s o l u b i l i t y i n MnS should be expected. They have also found a lim i t e d solub-
i l i t y of FeS (4% Fe) i n the above (CaS-MnS) system. Kiess- ,
l i n g and Westman' 4^ have determined the existence of a
(triangular shaped) m i s c i b i l i t y gap which shows a maxi
mum i m m i s c i b i l i t y at approximately 120 0°C and at 50 Ca/
Ca + Mn, (in a t . % ) . At approximately 1000°C i t i s increased
from 2 5 to 8 5 Ca/Ca + Mn. These researchers suggest that
t h i s m i s c i b i l i t y gap may divide the (Ca, Mn)S into two types
i . e . Ca-rich and Mn r i c h phases. There i s c e r t a i n disagree
ment with respect to the Ca and Mn s o l u b i l i t i e s i n the CaS
or MnS phases. Church et al.'^®^ have reported from 3.0 to
6.0 wt.% Mn i n CaS and from 3.0 to 7.0 wt. % Ca i n MnS.
Salter and P i c k e r i n g ' 4 * ^ report from 1.0 to 12.0 wt. % and
from 1.0 to 19.0 wt. % for Mn i n CaS and Ca i n MnS respect
i v e l y . Eklund s t u d i e s ' ^ ^ ^ supported by K i e s s l i n g and West-
man's f i n d i n g s o n the i n i t i a t i o n of corrosion i n i n c l u s
ions, has also i d e n t i f i e d the existence of these phases. In
t h i s study i t i s shown that while Ca-sulfide enriched i n Mn
remains almost i n e r t the s u l f i d e phase enriched i n Ca was
severely attacked. This finding i s i n agreement with Kiess
l i n g and Westman's who have also observed the CaS decompos
i t i o n to hydrogen s u l f i d e and calcium hydroxide i n the pre
sence of water.
53
2.3.6 Oxisulfides
2.3.6.1 The Fe-Q-S System
Rosenqvist and Dunicz (161) and Turkdogan et a l . (162)
have determined the s o l u b i l i t y of sulfur i n high purity
ir o n . I t i s established that the maximum s o l u b i l i t y of
sulfur i n d e l t a - i r o n i s 0.18 wt.% at 1365°C. At the same
temperature the gamma-iron holds only 0.06 wt.%. The solu
b i l i t y generally decreases with temperature but alpha-
i r o n at 913°C holds 0.02 wt.%. The Fe-O-S ternary system
has been studied extensively. H i l t y and C r a f t s ^ ^ 3 ^ have
determined the liquidus surface based on chemical and metallo-
graphic analysis. Their r e s u l t s show that the Fe-FeS-FeO
as the most important part of the Fe-O-S system i n the
range of p r a c t i c a l i n t e r e s t , i s constituted by a ternary
eutectic at 67 wt.% Fe, 24 wt.% S, 9 wt.% 0 and about
920°C. A m i s c i b i l i t y gap which extends into the system
from the Fe-0 side to a sulfu r content at 21.5 wt.% with
a minimum point ( p l a i t point) at approximately 81.5 wt.%
Fe, 16.5 wt.% S, 2 wt.% O and at 1345°C. The s o l u b i l i t y of
oxygen i n ir o n at several temperatures has also been deter
mined by H i l t y and C r a f t s ^ ^ 3 ) ^ j t i s indicated that for
increasing sulfur concentrations the oxygen content of
iro n f i r s t decreases s l i g h t l y up to about 0.1% S and then
increases r a p i d l y . The s o l u b i l i t y of sulfur i n iron as a
54
r e s u l t of the oxygen influence has been determined by Turk-
dogan et a l . 166)^ Their findings indicate that i n
the equilibrium Fe-S-O. saturated with wustite i n the
temperature range between 913°C to 1375°C, there i s a pro
nounced expansion on the su l f u r s o l u b i l i t y curve which
reaches a maximum of 143 ppm of S at about 1200°C. This
value corresponds to almost half of that of the solidus on
the Fe-S diagram.
Yarwood et a l . ' ^ " ^ have proposed an inc l u s i o n pre
c i p i t a t i o n model for the Fe-FeS-FeO system. Their f i n d -4. - 4 . U 4 - v , u (90,134,135, 163) mgs i n agreement with other researchers ' ' ,
show that the p r e c i p i t a t i o n sequence as a r e s u l t of the
monotectic reaction, i s indeed affected by the %0:%S r a t i o ,
(0:S), i n the a l l o y . If thi s r a t i o i s greater than 0.05.
inclusions w i l l have a wide range of compositions. They also
found that while the maximum oxide content i n inclusions
was dependent on the i n i t i a l 0:S r a t i o , the minimum content J m i J • (164-166) . -it, , •
was not. Kor and Turkdogan's i n q u a l i t a t i v e agreement with Yarwood et a l . ' s ' 0 5 ^ indicate that for 0:S ra t i o s greater than 0.12 two l i q u i d s form at a given temperature and composition. Their experiments on an al l o y containing 0.02% C and an 0:S r a t i o of 0.29, c l e a r l y reveal the presence of the l i q u i d oxysulfide.
55
2.3.6.2 The Fe-O-S-Mn "Equilibrium"
H i l t y and C r a f t s ^ 1 2 9 ' i n t h e i r aim to gain a bet
ter understanding of the deoxidation practice and the i n
clusion chemistry have studied the Fe-S-O, Fe-S-Mn, Mn-O-S
and the Fe-O-Mn systems to develop the Fe-O-S-Mn system.
They have suggested that the Fe-S-0 i s strongly modified
i f the Mn content i n the a l l o y i s high enough to enhance
the p r e c i p i t a t i o n of the s u l f u r - f r e e oxide and the oxygen-
free s u l f i d e s , simultaneously with the s o l i d i f i c a t i o n of
the metallic phase. Semi-quantitative work performed on
the Fe-O-S system by these researchers indicates that
by adding 0.3% Mn the im m i s c i b i l i t y regions from the Fe-O-S
and the Fe-S-Mn are encountered and hence a continuous im
miscible region i s formed. They propose that the metal
oxide and metal s u l f i d e eutectics are i n i t i a l l y formed close
to the iron corner, diagonally intersect the imm i s c i b i l i t y
region and meet the "ternary" eutectic. It was also found
that the eutectic i n the modified ternary remained almost
i n the same location as in the o r i g i n a l Fe-O-S system.
Van Vlack et a l . ^ 1 3 ^ also concerned with the "hot
shortness" problem have observed that i f the Mn content
i s about 0.8% i n the Fe-O-S system, t h i s a l l o y originates
during quenching duplex oxide-sulfide inclusions. The
s u l f i d e phase enriched i n Mn was a r e s u l t of a primary
c r y s t a l l i z a t i o n . It was also noted that i f the Mn content
56
i n the ternary a l l o y (Fe-O-S) was low then FeS and Mn-
r i c h oxide phases were pr e c i p i t a t e d . Turkdogan and Kor (164-166) . f , .. . _
m a series of papers have compiled thermodynamic
information concerning the Fe-Mn-S-0 system. Oxygen and
sulf u r p o t e n t i a l diagrams which are involved i n th i s quater-(93)
nary have been developed '. Turkdogan and Kor based
on H i l t y and Crafts observations'** 3^ and on t h e o r e t i c a l
thermodynamic data developed by Darken and Gurry have pro
posed to represent the s t a b i l i t y phase f i e l d s for the
Fe-Mn-S-0 system under several conditions. They have e s t i
mated the coexistence of gamma iron, Fe(Mn)0, FeS, Mn(Fe)S
and a l i q u i d oxysulfide, £^ , as the equilibrium (condensed)
phases at about 900°C, Figure (3). The a p e S and a M n g were e s t i
mated to be unity and 0.4 respectively and hence the e q u i l i -_3
brium manganese i n gamma ir o n was computed to be lOppm (10 % ) .
The four f o l d phase equilibrium involving the gamma iron,
Mn(Fe)0, Mn(Fe)S and l i q u i d oxysulfide (£^), considered as
perhaps the most important univariant equilibrium i n the
quaternary (Fe-O-S-Mn) system has been estimated on the
premise of an id e a l l i q u i d oxysulfide solution, i . e . a + a + a _ + a = i . They claim that since i d e a l FeO FeS MnO MnS J
mixing behavior i s observed i n the FeO-FeS l i q u i d i n e q u i l i
brium with gamma iron and MnO-MnS and since the FeO-MnO forms
an i d e a l solution then the id e a l behavior considered i n
the FeO-FeS-MnO-MnS i s a reasonable assumption. The behavior
of Mn i n iron under the above conditions i s given i n F i g
ure (3) and Table (II).
The reaction scheme used to estab l i s h the phases i n
volved i n such e q u i l i b r i a are:
M n 0 ( s ) + F e ( s ) * F e O ( 1 ) + Mn ( s ) (15)
M n S ( s ) + F e ( s ) t F e S ( 1 ) + Mn ( s ) (16)
MnO ( s ) t MnO ( 1 ) < 1 7)
MnS ( s ) t M n S ( i ) ( 1 8 )
It has been elucidated that i f a., _ = a.. _ s 1 (be-MnS MnO
cause of th e i r low Fe i n solution and the Mn a c t i v i t i e s
or atom fr a c t i o n s i n iron) then an expression i s obtained
to represent the Mn content of s o l i d i r o n for t h i s e q u i l i
brium: 1 - N
(K, + K_) ( — - — — ) + K-, + K. =1; where N = atom f r a c t i o n . Mn
By taking a., „ = 0.4 and a., = 0.5 at the invariant 3 MnS MnO equilibrium located at 900°C and using the above expression,
the dotted curve i n Figure (3) i s obtained. The Mn contents
of gamma iron i n equilibrium with s o l i d Mn(Fe)S and l i q u i d
s u l f i d e , i n curve (k) have been established using the "FeS"-
"MnS" phase diagram. Raoult's Law was assumed for the solub
i l i t y of "FeS" i n "MnS".
The curves j and k show the strong e f f e c t of oxygen on the
melting point of the oxysulfide phase i n the Fe-Mn-O-S (j) and
58
the Fe-Mn-S (k) systems respectively. It i s seen that while
a l i q u i d phase (point A) i s formed when the Mn content i s
less than 10 % i n the former system ( j ) , the minimum Mn
content to suppress the l i q u i d phase on the Fe-Mn-O-S system
at 1200°C i s 1%. The invariant equilibrium (j) which i n
volves the gamma iron, Mn(Fe)0, Mn(Fe)S and the l i q u i d oxy
su l f i d e phases i s the most important equilibrium i n the
Fe-Mn-O-S system by which the "hot shortness" can be avoided.
Turkdogan and Kor have shown that as long as the s t e e l con
tains Mn(Fe)0 and Mn(Fe)S i n equilibrium with the metal, a
l i q u i d oxysulfide may form between 900° and 1225°C depending
on the concentration of Mn i n solution i n s t e e l . The higher
the Mn-content i n solution the higher i s the temperature
above which a l i q u i d phase i s present. (165
Experimental work performed by Kor and Turkdogan ' 166)
have c l e a r l y shown the above by oxidizing iron
(0.34% Mn and 150 ppm S) at about 900°C. They found a de
p l e t i o n of Mn and accumulation of S i n the metal close to
the scale-metal interface. These changes bring about the
formation of l i q u i d oxysulfide near the surface above 900°C
and the p r e c i p i t a t i o n of pyrrhotite below 900°C. Because
of low i n t e r f a c i a l tension the l i q u i d oxysulfide has been
seen to penetrate into the grain boundaries i n the metal
and the scale. They have also observed that most of the
l i q u i d phase i s found at the scale-metal interface. These (130,
observations have also been reported by other researchers 1 6 9 ) who have studied more complex systems.
Turkdogan and Kor who have taken as a basis the i n
formation and technique used to construct figure (4),
have extended t h e i r experimental and th e o r e t i c a l data,
table (III), to describe the Mn behavior in the presence
of other (terminal) phase f i e l d s . Namely the Fe-S-O,
Mn-S-0 and Fe-Mn-O. They have suggested that due to the
low s o l u b i l i t y of oxygen in iron stable deoxidation prod
ucts i n commercial steels must be present. In the quater
nary Fe-Mn-O-S system the Mn(Fe)0 has been taken as the
most stable oxide which i s present at a l l temperatures of
int e r e s t . As shown i n Figure (4), the Mn-potential
i s given by two invariants at two temperatures, 900 and
1225°C: 1) the invariant given by the in t e r s e c t i o n of the
m-n-j univariants which i s constituted by the gamma iron,
Mn(Fe)0 as (oxi), FeS, i^, "MnS" and the gaseous phase at
about 900°C and 2) the invariant given by the inte r s e c t i o n
of the univariants j , p and q. The involved phases i n
thi s equilibrium are an Fe/Mn s o l i d phase, "MnS", "MnO",
l i q u i d oxide (SL^) and l i q u i d metal {l^) at about 1225°C.
The most complete representation of the phase changes i n
cluding the l i q u i d , delta and gamma iron has also been
developed by Kor and Turkdogan ^ and i t i s given i n
Figure (5). The invariant VII represents the immiscible
region i n the Fe-Mn-0 system at 1527°C. It i s important
to point out that this invariant was assumed to be equi
valent to the Fe-0 m i s c i b i l i t y gap. The phases i n e q u i l i
brium at VII are delta-iron, Mn(Fe)0, (Oxi), l i q u i d oxide
(£ ̂ ) and l i q u i d metal " i ^ - • The univariants V and VI were
previously described i n Figure (4). The equilibrium phases
involved i n the univariant e which are for the Fe-Mn-0
system, are delt a - i r o n "MnO" and l^- Since the "MnO" has
a low s o l u b i l i t y product at high Mn-activities (>1% Mn)
the equilibrium oxygen i n solution i s so low that the s o l i -
dus i n the Fe-Mn-0 i s almost equivalent to the Fe-Mn system.
The univariant g was also assumed equivalent to the Fe-Mn
binary system. In general i t can be said that the univariants
g and f represent the gamma to delta and the delta to
l i q u i d i ron transformations respectively. The three new
phase f i e l d s in t h i s figure are the delt a - i r o n + Ox +
which i s lim i t e d by the e-f univariants, the gamma-iron +
Ox + £ 2 which l i e s between f';^ e univariants and the
6 iron + Ox + which i s located between the univariants
g and f. The f - f ' univariants correspond to the Fe-Mn-0
system already described i n Figure (4).
Crafts and H i l t y ' s w o r k ' 2 9 ' * 6 7) on pseudo-equilibrium
i n c l u s i o n p r e c i p i t a t i o n diagrams has also included the
representation of the Fe-O-Si-Mn-S system as a pseudo-
ternary, Fe(Mn, Si)-0-S. It i s proposed that t h i s system
61
i s integrated by metal-oxide, metal-sulfide and s u l f i d e -
oxide m i s c i b i l i t y gaps which int e r s e c t themselves prod
ucing an almost isometric i n t e r n a l t r i p l e l i q u i d region.
Equivalent to their proposed Fe(Mn)-0-S pseudo-ternary
there are metal-oxide and metal s u l f i d e eutectics which o r i
ginate i n the metal corner and pass through the three
f o l d immiscible region. These binary eutectics continu
ously decrease i n temperature u n t i l they reach the pseudo-
t r i p l e e u t e c t i c . It i s also postulated that while the
above "ternary" i s useful to represent s i l i c a saturated
melts a pseudo ternary which could describe low Si to Mn
r a t i o s should be d i f f e r e n t . They claim that since the Mn
fluxes s i l i c a a s h i f t of the metal oxide-eutectic towards
the oxide corner of the diagram would be expected. I t has
also been proposed that since there i s s o l u b i l i t y between
"MnS" and MnO-SiC^ then the sulfide-oxide m i s c i b i l i t y gap
may disappear and the pseudo-ternary eutectic would move
towards the oxide corner. It i s anticipated that more i n t e r -
granular s u l f i d e s are expected i n t h i s case than i n the
higher s i l i c o n s t e e l s .
62
2.3.6.3 The Fe-Si-O-S-Mn System
Other more complex systems have also been generalized
i n the modified pseudo-ternaries, namely the Fe(Si)-0-S,
Fe(Mn, S i , high Al)-0-S, Fe(Mn, S i , low Al)-0-S and the
Fe(Al,Ca)-0-S s y s t e m s ( 1 2 9 , 1 6 7 ) .
The extensive e f f o r t dedicated to control the "hot-shortness" problem i s c l e a r l y revealed by Crafts and H i l t y
(129)
studies . I t i s seen that the search for adequate de-
oxidizers which could promote the formation of high melting
point phases has been the main aim. The most common
feature of these diagrams i s that the l a s t l i q u i d to
s o l i d i f y i s a ternary eutectic. These ternary diagrams
as t h e i r authors have stated are not true ternaries and
hence "the r a t i o n a l i z a t i o n of the problem i s i n t u i t i v e in
character and l i a b l e to considerable error, but should be
hel p f u l i n the e f f o r t to bring inclusions under control".
S i l v e r m a n ^ also concerned with the "hot shortness"
problem has studied the Fe-Mn-Si-S-0 system. He has
claimed that i f the metallic phase i s r e l a t i v e l y neglected
the inc l u s i o n chemistry of the f i v e f o l d system can be
reasonably well represented by the MnS-MnO-FeO-Si02 system.
This system was " s l i c e d " , as depicted i n Figure (6) i n three
ternary planes, shown in Figure (7a-c). Two of these planes
are pseudo ternaries the 2FeO«Si0 2-2MnO«Si0 2-MnS and the
FeO-MnO-MnS. The t h i r d plane. FeO-MnO•Si02~MnS i s more d i f -
63
f i c u l t to analyze because of i t s f i v e primary phase f i e l d s .
The "A" area i s present i n the three planes and i t rep
resents the "FeS" s t a b i l i t y f i e l d .
The primary c r y s t a l l i z a t i o n product on the 2FeO«SiC>2-
2MnO«Si0 2-MnS-plane i s a s o l i d solution consisting of
2FeO«SiC>2 and 2MnO«SiC>2. The second and t h i r d products
are the FeS'-phase at "A" and a mixture of two immiscible
l i q u i d s at "B". Silverman's observations can be summarized
as follows: 1) The MnS-FeO-MnO•Si02 plane shows that l i q u i d
s i l i c a t e s enriched i n "FeO" allow more "MnS" i n solution
than l i q u i d s i l i c a t e s enriched i n "MnO". 2) This plane
also shows that "MnS" i s more soluble i n "FeO" than i n
the l i q u i d s i l i c a t e s i n t h i s plane. 3) As the "FeO"
content of the l i q u i d s i l i c a t e decreases and the S i 0 2 con
tent increases, the s o l u b i l i t y of "MnS" i n l i q u i d s i l i c a t e
decreases. 4) Samples from area "A" i n the MnS-FeO-MnO«Si0 2
plane suggest that the "FeS"-phase s o l i d i f i e s as a eutectic
at about 910°C. S i l v e r m a n ^ has concluded that since
MnS i s fluxed by s i l i c a t e s and oxides i n the planes studied
then inclusions i n th i s system are p a r t i a l l y l i q u i d at r o l l i n g
and f i n i s h i n g temperatures.
Van Vlack et a l . ' s r e s e a r c h ' 3 ^ i n l i n e with Silverman's
work'^ 9^ has semi-quantitatively shown how the s i l i c o n
a f f e c t s the in c l u s i o n chemical behavior i n various systems.
64
In t h i s work the metallic phase neglected by Silverman i s
now taken into account. Van Vlack et a l . ' s findings can
be summarized i n the following points: 1) If s i l i c o n i s
added i n "small or moderate" quantities to an Fe-S a l l o y
undetectable changes i n i n c l u s i o n shape and composition w i l l
be observed. "FeS" was the only phase present. 2) In an
Fe-S-0 a l l o y s i l i c o n was also added. In t h i s a l l o y two
non-metallic phases were present. One phase was enriched
i n "FeS" and the other enriched i n " S i 0 2 " . 3) If s i l i c o n
i s added to a ternary Fe-Mn-S a l l o y whose Mn:S r a t i o was
3:1, inclusions remain s o l i d at a u s t e n i t i c temperatures
(about 1200°C). Globular inclusions of the type (Mn,Fe)S
were observed. 4) If s i l i c o n and oxygen were added to
the Fe-Mn-S system (they generate the Fe-Mn-Si-S-0 system)
a l i q u i d phase s i l i c e o u s i n character was observed. This
l i q u i d phase was associated with saturated MnS. They also
observed that i f the Si:0 r a t i o was larger than that
required to form Si02 then a glassy type of i n c l u s i o n was
formed during cooling. On the other hand, i f t h i s r a t i o
was smaller the glassy phase disappeared. Van Vlack et a l .
also observed that i f oxygen was i n excess of the s i l i c o n
content then the l i q u i d composition was s h i f t e d from the
s i l i c e o u s range to a more oxidizing compositions. This
l a t t e r phase was similar in nature to that encountered i n
the Fe-Mn-O-S-system. This finding was also observed by
65
H i l t y and C r a f t s < 1 2 9 ' l 6 7 ) .
Van Vlack et a l . ' s work based on t h e i r own findings
and also on Silverman's work (MnS-MnO-FeO-Si02~system)
have i l l u s t r a t e d the phase changes i n the
l i q u i d phase using Figures (8 a-b) which represent the
MnS-MnO-FeS-Si02-system. It has been pointed out that
although t h i s system i s q u a l i t a t i v e in character and the
metallic phase i s "temporarily" ignored the approach
used i n t h e i r experimental work indeed describes phase
changes to which the inclusions are subjected. Van Vlack
et a l . d 3 ( ^ have proposed that since the l i q u i d phase
varies from a s i l i c a enriched composition (A) to a MnO
enriched composition (C) then the quaternary (at high
temperature ranges) can be represented by a Mn0-Si0 2 b i n
ary equilibrium. Once the s o l u b i l i t y product for S i 0 2 i n
the melt i s reached the excess s i l i c o n goes into solution
i n the metal and the excess oxygen reacts with Mn to produce
MnO (and l i m i t e d l y with iron to produce FeO). These re
action products w i l l generate a composition B i n Figure (8)
which fluxes the MnS, as indicated by Silverman's work i n
Figure (76).
The r e s u l t i n g l i q u i d w i l l be composed of 50 % "MnS" at
about 1320°C and hence, during s o l i d i f i c a t i o n a mixture
of "MnS" and a s i l i c a t e either tephroite (2MnO*Si02) or
Rhodonite (Mn0«Si0 o) depending on the MnO:Si09 r a t i o w i l l
p r e c i p i t a t e at low au s t e n i t i c temperature. Van Vlack et a l . ^ 1 3 0
have also observed and q u a l i t a t i v e l y predicted with t h i s
s i m p l i f i e d model that the l i q u i d phase becomes a s i l i c e o u s
glass i f the l i q u i d contains an excessive amount of s i l i c o n .
Under t h i s condition a l i q u i d which i s rapidl y s o l i d i f i e d
can remain as a "glassy" i n c l u s i o n at room temperature.
Composition C i n Figure (8a) i s attained i f the oxygen
content exceeds the s i l i c o n content. Under t h i s circumstance
the l i q u i d phase dissolves a considerable amount of "MnS"
and the excess oxygen may react with some Mn from the MnS.
It i s also believed that some of the iron i s transferred
into the l i q u i d to balance the su l f u r . These s h i f t s i n
compositions are sketched i n Figure (9). In subsequent
s o l i d i f i c a t i o n stages the formation of FeS i s expected.
2.3.7 Oxides
2.3.7.1 Aluminates
Experimental and th e o r e t i c a l studies on the Al-0 e q u i l i
brium have been traced i n the l i t e r a t u r e since early i n t h i s
century. The d i f f i c u l t i e s presented i n determining the
thermodynamic equilibrium are obviously observed i n Figure (10).
This summarizes the equilibrium values found by several
researchers ( 1 7 ° " 1 8 3 ) at 1600°, 1800° and 1900° C. The
l a t e s t equilibrium values given by Gustafsson and Melberg^ 1 7^ .. - , , u (176, 177, are summarized work from several researchers
179—18 2) Gustafsson and Melberg claim that t h i r d order
67
polynomial (regression) parameters should be included i n the
determination of the involved a c t i v i t i e s where there e x i s t s
strong oxygen-metal (Al or Ca) i n t e r a c t i o n . Their tech
nique unfortunately only works when binary oxygen-aluminum
or oxygen-calcium systems are considered. Sims^ 9^ has
pointed out that the observed discrepancies can be reconciled
on the basis that the oxide phase i n equilibrium with the
Fe-O-Al i s not pure A^O^ but instead FeO'A^O^ and hence
t h i s spinel phase w i l l always be present. Other studies on
t h i s matter have shown that by deoxidizing a melt with
aluminum at several lev e l s three general stages can be ob
served: 1) If i n s u f f i c i e n t aluminum i s added to an iron
melt ( i . e . there i s an excess of oxygen i n solution) ferrous
oxide and hercynite should be p r e c i p i t a t e d as dictated by (112-113)
the FeO-Al^O^ equilibrium diagram . 2) At i n t e r
mediate A l - l e v e l s (about 0.4-0.5% Al) a mixture of hercy-* . • - a - 4- ^d83, 184) _. .
nite and alumina i s p r e c i p i t a t e d ' . The hercynite phase w i l l be the major phase present and 3) At high Al-contents i n an i r o n melt almost pure A^O^ i s f o u n d ' 8 3 ' 1 8 4 ) .
According to the deoxidation diagram proposed by H i l t y
and F a r r e l l ^^5) a f u n v k i n e < 3 carbon or low a l l o y
s t e e l deoxidized with aluminum, the s o l i d i f i c a t i o n of the
metal-oxide-sulfide system starts by s o l i d i f y i n g oxide
c r y s t a l s (A^O^) instead of metal. As the temperature
68
decreases s o l i d i f i c a t i o n of metal and some oxide takes
place simultaneously u n t i l the metal-oxide binary eutectic
i s reached. Soli d metal, s o l i d oxide and a phase r i c h i n
su l f i d e w i l l p a r t i a l l y p r e c i p i t a t e i n further s o l i d i f i c a t i o n
stages. The remaining phase r i c h i n s u l f i d e w i l l f i n a l l y
p r e c i p i t a t e , i n the same manner as the Fe-O-S system, as
the temperature approaches the ternary eutectic. McLean (112-113)
and coworkers who have studied the thermodynamic
behavior of the Fe-O-Al system have found that there are
clear thermochemical conditions under which either pure
A^O^ or hercynite are formed. In thi s work i t has been
pointed out that to prevent the formation of hercynite the
oxygen a c t i v i t y should be reduced to le v e l s below 0.058
at 1600°C. A c t i v i t i e s of oxygen equal to 0.058 represent
the location of the (A^O^-FeO• A^O^) t r a n s i t i o n point.
Several researchers have also agreed that k i n e t i c fac
tors are involved in the FeO-FeO'A^O^-A^O^ p r e c i p i t a t i o n
sequence. T o r s e l l and O l e t t e ^ 1 2 ^ have observed inclusions
i n the submicron sizes one second after aluminum was added.
Hammar's'^^ th e o r e t i c a l predictions are i n agreement with
T o r s e l l * s and O l e t t e 1 s findings. Hammar1s experimental
work, however, did not follow such a behavior. He claims that the f i r s t transformation i s given by FeO'A^O^dT
* FeO'A^O-jCs) and i t i s very dependent of the inclu s i o n s i z e .
This transformation has been traced 17 seconds afte r the
Al-addition. Later deoxidation stages transform the
FeO'A^O^s) to almost pure A^O^. The mechanisms proposed
to control these transformations are the simultaneous d i f
fusion of oxygen from the p a r t i c l e and d i f f u s i o n of aluminum
into the p a r t i c l e . Since, Hammar1s res u l t s were not i n
agreement with his theory i t was proposed that inclusions
may have a peripheral case of FeO-A^O^Csf which enclose
the FeO «A1 20 3 ( i f thus d i f f u s i o n of A l into l i q u i d p r e c i p i t
ates was prevented. This proposal was indeed correct, p n u „ . ,. (184, 186-188) , . . , since EPMA-studies ' showed an enriched Al-case
surrounding the FeO'Al^O^ which was o r i g i n a l l y i n l i q u i d
state. Another i n t e r e s t i n g observation traced by Hammar
was that the FeO-phase was not detected one second aft e r the (18 7)
addition of aluminum. Wadby and Salter have noted
that a sharp decrease i n oxygen as well as pure alumina
inclusions are seen within a period of 30 seconds. Cremer
and Driole s t u d i e s ' 8 9 ^ on the influence of electromagnetic
s t i r r i n g and the removal of inclusions, have established
that a f t e r 20 seconds of the aluminum addition spherical
p a r t i c l e s were grown inside c l u s t e r s . Hammar has also ob
served r e l a t i v e l y large FeO-A^O^ inclusions transforming to
pure A^O^ i n the period of one to three minutes a f t e r the
addition of A l into the molten iro n . * FeO«Al 20 3 i s not intended to indicate stoichiometry and i n fact the FeO«Al 20 3(s) would have a d i f f e r e n t Al 20 3/FeO r a t i o than FeO«Al 20 3(1).
70
Straube and P l o c k i n g e r ^ ' 1 9 1 ^ who have studied the pre
c i p i t a t i o n of alumina i n melts containing some manganese,
have stated that the primary deoxidation products are lean
i n alumina and contain e s s e n t i a l l y Mn-oxides. The A^O^-
content increased very rapidly during the next few seconds
u n t i l the composition reached that of the spinel (Fe,Mn)0«
A^O-j. It was observed that inclusions were i n a f l u i d or (18 7)
p a r t i a l l y f l u i d state. Waudby et a l . have also agreed (190 191)
with Straube and Plockinger's findings ' . Waudby (187)
et a l . noted that once the spinel type was formed i t reacted with A l i n further stages to produce i r r e g u l a r highly aluminous inclusions which enclosed i r o n . Morgan
(188)
et a l . who have studied the deoxidation e f f e c t on the
i n c l u s i o n chemistry have also observed s i l i c a t e deoxidation
products peripherally p r e c i p i t a t e d on A^O^ or FeO'A^O^
phases. Waudby's and Salter's experiments were performed
at several Al-deoxidation l e v e l s , namely 0.05, 0.15, 0.3
and 0.5 wt. %. Their samples were quenched and heat treated
afterwards for 7 days at 1150°C. Under the above experimental
conditions, i t was found that by deoxidizing the iron
melt with 0.5% A l i n as cast condition duplex hercynite-
alumina inclusions were observed. After the heat t r e a t
ment, however, inclusions dissolved more oxygen u n t i l the
hercynite equilibrium composition was reached. Hammar
•has also i d e n t i f i e d similar compounds, (Fe,Ni) 'A^O^,
71
ir r e s p e c t i v e of the quenching time. The outstanding crys-(183)
tallographic work performed by Watanabe and coworkers (192)
along the l i n e s of Sloman's and Evan's work on alum
inum deoxidized melts has c l e a r l y revealed the nature of (183)
the transformation i n the Fe-O-Al system. Their work
was c a r r i e d out by melting l e v i t a t e d samples under a p u r i
f i e d Ar-atmosphere and s o l i d i f i e d under three d i f f e r e n t
cooling conditions. The 0:A1 r a t i o was 1.45 which i s close
to the stoichiometric r a t i o found i n the A1 20 3 phase. Ex
tracted inclusions from the met a l l i c matrix were analyzed
by X-ray (powder) d i f f r a c t i o n using Cr-Ka ra d i a t i o n . Their
r e s u l t s showed that since the FeO«Al 20 3 phase had i n t e r -
planar spacing approximately equivalent to the y'-A^O^a')
then the y'-A^O^ was though to originate from the spinel
phase, as follows: F e O A l 2 0 3 -•• Y ' - A l 2 0 3 ( a ' ) -> y l - A l 2 0 3 ( a ) + Y'-AljOg ( b )
The f i n a l Y , - A 1 2 0 3 i s obtained according to the degree of
simultaneous migration of iron and aluminum out and into
the o r i g i n a l FeO«A1 20 3 phase. I t was also proposed that
since i n the process of K - A 1 2 0 3 formation the Fe0«Al 20 3
phase disappears and Y ' - A 1 2 0 3 (b) shows up, then K - A 1 2 0 3
i s derived from Y ' ~ a 1 2 ° 3 ^ • T n e t h i r d cooling condition
i n t h e i r experiments produced Y ' - A 1 2 0 3 (b), 0-Al 2O 3 and
a - A l 2 0 3 . The a - A l 2 0 3 phase was the more abundant phase.
Hence, the o v e r a l l transformation sequence was proposed
to be:
FeO-Al 20 3 + y ' - A l 2 0 3 (a') y' A 12 ° 3 (a> + ^' ~ A l 2 ° 3 ( b ) "*" *~ A l2°3
-* 6 - A l 2 0 3 a - A l 2 0 3 .
Since y' -A12C>3 was found i n a l l the cooling conditions, i t
was suggested that the rate of transformation from
FeC"Al 20 3 to y '-A12C>3 i s f a s t and that the y '-A^C^ (b) to
a - A l 2 0 3 i s r e l a t i v e l y slow.
The morphology of Al 2C> 3 type of inclusions has been widely
studied. T o r s e l l and O l e t t e ^ 1 2 ^ were the f i r s t researchers
who proposed that i n addition to a l o c a l supersaturation, a
continuous d i f f u s i o n of aluminum and oxygen into these
regions i s required to p r e c i p i t a t e the dendritic type of
inclusions. Under these conditions i t has been proposed
that d e n d r i t i c alumina was a r e s u l t of a homogeneous nucle-(189)
ation. Cremier and Driole have suggested that spherical
alumina inclusions are products of heterogeneous nucleation.
This type grows i n areas low i n deoxidizer. T o r s e l l and
Olette have also proposed that clusters are not formed i n
regions enriched i n aluminum. It i s also indicated that
c l u s t e r s of alumina which are very commonly observed i n
aluminum deoxidized melts are formed due to c o l l i s i o n of
single p a r t i c l e s as a r e s u l t of thermal, mechanical or
electromagnetic a g i t a t i o n of the molten bath. Cremier and
Driole have concluded that the importance of magneto-hydro-
dynamic phenomena on the k i n e t i c s of deoxidation of s t e e l
i s mainly a physico-chemical process, i . e . shape and type
of inclusions i n the Fe-Al-0 system are co n t r o l l e d by the
l o c a l flow and a c t i v i t y conditions. Steinmetz et a l . ' 8 4 ^
who have studied the above system under several conditions
( i n d u c t i o n - s t i r r i n g , convection-free and i n the gas phase)
have also agreed with that proposal. They found that with
an oxygen supply high enough compared to the supply of res
pective elements, l i q u i d phase of high FeO-activity pre
c i p i t a t e . I t i s also suggested that t h e i r growth i s a l
most s t r i c t l y controlled by the flow conditions at which
the given region i s subjected. They claim that the spheri
c a l contours become unstable and change to "rosettes" and
f i n a l l y to dendrite shape as the "phase-specific concen
t r a t i o n s " or "materials flow" are changed. The degree of
l o c a l deoxidation and the type and shape of oxides and s u l
fides have also been shown i n t h i s work. The association
of p l a t e - l i k e alumina and the MnS III are given where the
l o c a l concentration of aluminum i s higher (2 to 3%). At
intermediate deoxidation (0.25-1.0% Al) a mixture of coarse
s u l f i d e type II and a c i c u l a r oxide i n the presence of s u l
f i d e are found. Between these two ranges (1.25-1.75% Al)
a mixture of MnS II and III i s expected and at very low
aluminum contents (0-0.25%) oxysulfides, primary s u l f i d e s , type (184)
II and de n d r i t i c oxides may be formed. Steinmetz et a l . have
74
a l s o proposed the mechanism already described for su l f i d e s
for the alumina, i . e . 1) dendrites and coarse globular
hercynite at high i n i t i a l oxygen contents, 2) i n i t i a l
globular to " c o r a l " shape alumina for low oxygen contents
under very d r a s t i c cooling conditions. Above 0.019% oxy
gen only d e n d r i t i c A^O^ i s expected. Slower growth i s re
quired for the branched-irregular shape alumina type. They
have also proposed that under low i n i t i a l oxygen contents
r a d i a t e d - c r y s t a l l i n e to compact n i t r i d e s are grown on the
alumina.
Braun et a l . ^ l 9 3 ^ , who have studied the influence of
s t i r r i n g time, s t i r r i n g rate and i n i t i a l oxygen i n iron melts
have c l a s s i f i e d the morphology of the alumina inclusions into
f i v e types, namely: de n d r i t i c , faceted, p l a t e - l i k e ,
spherical and c l u s t e r s . It was found that these types are
not exclusively found as a unique type but as a mixture for
a given set of experimental conditions. They have also
observed that inclusions which integrate the clusters
change from de n d r i t i c to p l a t e - l i k e shapes at low oxygen
contents to spherical at high oxygen l e v e l s . T o r s e l l and
O l e t t e ( 1 2 0 ) , Braun et a l . ( 1 9 3 ) , Okohira et a l . ( 1 9 4 ) , Ooi
et a l . ( l 9 5^ and Cremer and D r i o l e ^ 1 8 9 ^ have agreed on the
mechanism by which alumina c l u s t e r s are formed, i . e . c o l l i s i o n
and coalescence of single p a r t i c l e s as a r e s u l t of f l u i d
motion i n the melt. Ooi et a l . 's studies on non-stirred
75
and s t i r r e d melts have shown that under the former condi
t i o n dendritic and alumina clusters are the major types.
Spherical inclusions were almost absent. Their second
type of experiment produced 1) inclusions larger than
20 ym i n diameter with adhered p a r t i c l e s of about 0.5 -
2.0 ym i n diameter and 2) clus t e r s composed of very small
spherical inclusions with the maximum diameter of which was
about 2 ym. Ooi et a l . ^ 1 9 ~ ^ have corroborated the " c o l l i s i o n
coalescence and sin t e r i n g theory" on the formation of alumina
cl u s t e r s by measuring the neck growth and assuming volume
d i f f u s i o n as the c o n t r o l l i n g mechanism.
H i l t y and C r a f t s ^ 1 6 7 ^ have found that 0.5% Mn i n l i q u i d
i ron enhances the Al-deoxidation power as much as f i v e times.
McLean^ 1 1 3^ suggests that Mn lowers the oxygen and raises
the Al concentration for univariant t r a n s i t i o n from (Fe, Mn).
A l 2 0 3 to h l ^ O ^ i as suggested by Plockinger ) and Waudby ^ 1 8 7 \
Sims^ 9^ has studied the dual A l - S i deoxidation of melts
(0.4% A l and 0.5% S i ) . He noted that the inclusions are
c h a r a c t e r i s t i c of those exclusively deoxidized with A l
and stronger deoxidation was reached than when the melt i s
Al-deoxidized. Sims points out that i f s i l i c o n i s added
before or with the aluminum one minute i s s u f f i c i e n t to
obtain the maximum cleanliness • Waudby1s and Wilson's
s t u d i e s ^ l 9 ^ on progressive deoxidation of iron-oxygen
melts by A l - S i a l l o y s (0.3, 0.6 and 0.9% Al-Si) . have
76
found that because of the much more rapid formation of
alumina compared with s i l i c a , the alumina content of
the inclusions proportionally increase with the A l - S i
addition. Hence, they concluded that the dual (Al-Si)
deoxidizer behaves i n a complex manner only when a melt
i s (Al-Si) deoxidized i n r e l a t i v e l y c r i t i c a l additions.
Deoxidation of melts by large quantities of A l - S i behave
as conventional additions of the strongest element i n the
deoxidizer. As proposed by Sims^ 9^ formation of alumina
clust e r s i s expected. G a t e l l i e r et a l . ' 9 ^ i n agree
ment with Waudby's and Wilsons's and Sim's findings
have noted that i f aluminum i s f i r s t l y introduced to the
metal bath the s i l i c o n remains as a passive deoxidizer.
G a t e l l i e r et a l . ' s t h e o r e t i c a l consideration have shown
that three d i f f e r e n t behaviors might be observed, at 1600°C: 3/4
1) If the a s i / a A l ° 6 0 ° / alumina should be pr e c i p i t a t e d , 3 /4
2) i f a / a A l " 1400/ pure SiC>2 precipitates and 3) i n the 3/4
intermediate range of these s t a b i l i t y ranges (600 S a c - / a
A l i 1400), mullite i s the most stable phase. 2.3.7.2 Calcium aluminates
Although the use of calcium as a deoxidizer and de-
s u l f u r i z e r i n iron melts has become an a t t r a c t i v e alternative (198)
since Sponseller's and Flinn's research i t s use i n
the foundry industry, to desulfurize, inoculate and to
77
enhance the s p h e r o i d i z a t i o n o f g r a p h i t e has been used s i n c e
e a r l y i n t h i s c entury. S p o n s e l l e r ' s and F l i n n ' s work
has been c o n s i d e r e d as one of the most fundamental p i e c e s
of r e s e a r c h t h a t has c o n c l u s i v e l y c o n t r i b u t e d to the develop
ment of the Ca-treatment of l i q u i d i r o n . They have a l s o
s t u d i e d i t s i n t e r r e l a t e d e f f e c t s w i t h other elements, i . e .
A l , C/ N i , S and Au. They found t h a t the s o l u b i l i t y of
l i q u i d c a l c i u m i n l i q u i d i r o n under p r e s s u r e i s 0.032% a t
1880°K. At t h i s temperature i t s vapour p r e s s u r e i s
1 . 8 7 ( l 9 8 ) - 1 . 6 4 5 ( 1 9 9 ) atm. and i t b o i l s a t about 1 7 8 0 ° K ( 1 9 9 ) .
In l i g h t of S p o n s e l l e r ' s and F l i n n ' s f i n d i n g s i n t e n
s i v e r e s e a r c h has been d e d i c a t e d to improve i t s s o l u b i l i t y ,
t o overcome the problem of d e n s i t y w i t h r e s p e c t t o l i q u i d
i r o n and to d i m i n i s h i t s vapour p r e s s u r e i n the l a s t 20
y e a r s ( 1 4 4 ' 1 5 5 ' 1 9 7 ' 1 9 9 _ 2 0 2 ) . W o r k has been devoted to
re d u c i n g i t s vapour p r e s s u r e by a l l o y i n g i t wit h S i , C,
A l , Ba or as m u l t i p l e mixtures of v a r i o u s elements, C a A l S i , (9 5)
Ca A l S i F e , CaAlBaFe, e t c . P h i l b r o o k who has reviewed
the s t a t e o f the a r t of oxygen and i t s r e a c t i o n s with d i f
f e r e n t d e o x i d i z e r s , has d e s c r i b e d the c u r r e n t understanding
of Ca-treatment up to 19 77. In June of the same year i n
Sweden, the F i r s t I n t e r n a t i o n a l Conference on I n j e c t i o n
M e t a l l u r g y took p l a c e . In the proceedings of t h i s meeting^ 9*^
a new d i r e c t i o n on the d e o x i d a t i o n p r a c t i c e i s c l e a r l y seen.
78
Thermodynamic and k i n e t i c theory to support the experi
mental work, f i n a l l y give c r e d i t to the deoxidation capa
b i l i t y of Ca-bearing mixtures. The second conference on
(97)
i n j e c t i o n processes also held i n Sweden i n 1 9 8 0 , once
more, reconfirms the advantages (deoxidizer and desulfur-
izer) and disadvantages (low yield) of i t s usage. Holappa^ 9 8^
i n a more recent communication also presents an o v e r a l l
view of the Ca-treatment i n the l a d l e .
In addition to the previously described advantages of
the usage of Ca-bearing mixtures the major att r i b u t e s of
t h i s deoxidation practice i n terms of inclusions are:
1) the elimination of c l u s t e r s and angular alumina i n
clusions which otherwise would be formed from the A l -
deoxidation p r a c t i c e ' 4 ^ 1 6 8 ) ^ 2) ^he t r a n s i t i o n of
MnS type II to MnS III or peripheral calcium s u l f i d e
around C a - a l u m i n a t e s ( 1 4 4 ' 1 4 5 ' 1 4 7 ' 1 6 8 ' 1 9 7 ] . The presence
of the MnS III has been observed at r e l a t i v e l y low Ca-
content (< 20 ppm) and the "CaS" at much higher Ca-t O A , . , ̂ ( 1 5 7 , 158, 168) TJ_ . , content (> 20 ppm) i n the melt ' • . I t has also j. * • *.u T * . ( 1 4 6 , 159, 160) _ . been reported i n the l i t e r a t u r e • • that i n
addition to the above c h a r a c t e r i s t i c s excellent de-
79
(96—98) oxidation, d e s u l f u r i z a t i o n and some dephosphoriz-
( 1 5 7 , 1 5 8 , 2 0 4 ) , , , i . . . . ation ' ' are reached by the calcium i n j e c t i o n
processes. Gaseous and metallic calcium has been
added"to the st e e l stream during tapping ̂ 8 , ' p l u n g i n g '
^ t ( 2 0 4 ) o r s n o o t i n g i t as b u l l e t s i n t o the metal bath.
When metallic calcium i s added to the melt i t turns into
vapour bubbles which rapidly r i s e to the metal surface and
subsequently react v i o l e n t l y with the slag and the oxygen
i n the a i r producing considerable f l a r e , splashing and
fumes. Burn-off and reactions with the slag and l i n i n g
materials reduce the y i e l d of the Ca-treatment to about
1 0 - 5 0 % ( l 9 7 ' 2 0 5 ) m Thus, new means to u t i l i z e calcium as ( 9 7 )
calcium-composite wires or iron tube containing Ca-( 1 5 5 )
alloys have been developed . In addition to these
techniques obviously the simultaneous deoxidizer additions
(as mixtures of deoxidizers with or without slags) either
top or bottom blown into the converter have also been
i n d u s t r i a l l y practiced. It i s a general practice in
conventional steelmaking, to f i r s t l y deoxidize e f f i c i e n t l y
the melt with A l and secondly by the calcium t r e a t m e n t ^ .
Regarding the mechanical properties, i t i s generally
accepted that globular inclusions improve the anisotropy ( 9 2 9 5 - 9 7 )
of the mechanical properties ' . I t has been pro
posed as a p r i o r i rules that ei t h e r a Ca:S r a t i o greater than
1 . 2 5 ( 1 5 3 ) or more than 2 0 - 3 0 ppm of C a ( l 5 1 ) i s required to
80
achieve the t r a n s i t i o n from the alumina (clusters) to (144) calcium aluminates and the MnS II to at least MnS III
or to peripheral calcium s u l f i d e s . G a t e l l i e r et a l . ' 9 7 ^
have found i n experimental melts that a complete elimination
of A l 2 0 3 as clust e r s i s attained when the 0:Ca r a t i o
(wt %) in inclusions i s about three. This rule was found
to be independent of the ingot chemical composition. (209)
Faulring et a l . have reported that the nozzle block
age by the aluminate type of inclusions i s eliminated when
the Ca:Al r a t i o (in wt.%) i s greater than 0.14. This
r a t i o represents compositions which c l o s e l y correspond (133)
to the CaO • 2A1 20 3 phase. Boldy et a l . have sug
gested that "burning" which occurs even i n ESR-ingots
where the sulfur content i s usually thousandths of a
percent might be eliminated only by rare-earth or Ca-
treatments. These researchers have reported that the
problem was eliminated, i n conventional p r a c t i c e : when
manganese su l f i d e s were completely transformed to CaS. (157 158)
Japanese researchers ' who have studied the hy
drogen induced cracking (HIC) on pipeline steels have
concluded that a Ca:S r a t i o greater than or equal to 1.5
i s required to prevent the reoxidation of Ca during the
teeming of the melt. Under these conditions high r e s i s
tant or t o t a l l y insusceptible steels to HIC were developed.
It has also been established that Ca-aluminates are also
81
present in steels produced v i a the basic e l e c t r i c arc
f u r n a c e ^ 2 ^ ^ . Their presence arises due to the aluminum
which i s used as a deoxidizer and the b a s i c i t y of the slag or by the Ca-(Si) treatment. While some work i n the
l i t e r a t u r e reports the presence of Ca-aluminates by the
Ca-treatment ( 1 4 4' 1 4 7 ' 1 4 8 ' 1 5 1 ' 1 5 3 ' 2 0 7 ) , o t h e r s ( 9 6 ' 9 7< 19 7 20 8)
' have shown that complex calcium-aluminum-silicates (159)
are formed. Church et a l . ' s studies on a s t e e l
processed under two d i f f e r e n t conditions, a i r melt and
deoxidized i n the lad l e and carbon deoxidized i n vacuum
have found globular (galaxite) oxides, s u l f i d e s and stringer
type of inclusions. In the s t e e l treated under vacuum,
inclusions were smaller, fewer and were less complex than
i n that treated under a i r . These inclusions consisted
of a nucleous of galaxite surrounded by a Ca-Al-Si matrix.
Whereas the a i r melted steel contained (Mn, Ca)S around the
globular oxides and MnS I with some Ca, Cr and Fe i n
solution, i n the s t e e l melted under vacuum only the l a t t e r
type was observed. The presence of slag i n some conventional
processes and the chemistry of the deoxidizers employed
i n the Ca-treatment plus the chemistry of the melt indeed
complicate the elucidation of the mechanism(s) by which the i n c l u s i o n chemistry i s controlled. Ja*ger and Holz-
(202) gruber have found that a l l o y s of Ca with A l , Mn or S i
used as deoxidants i n 18-8 steels after the Al-treatment
increase the Ca y i e l d when 0.1 wt.% (Ca + Ba) i s also
present. D^type of inclusions consisting of 40-60 wt.%
CaO and 60-40 wt.% ^2°3 w i t h peripheral CaS are found
under t h i s treatment.
Salter and P i c k e r i n g ' 4 0 ^ i n th e i r studies on C-Cr
bearing steels dexodized with a CaSi a l l o y and H i l t y and
c o w o r k e r s ' 4 4 ' 1 6 8 ^ who used CaSi, CaSiB'a and CaSiBaAl al l o y s to deoxidize an iron melt and CaSiTi to deoxidize
some casting melts, have highlighted the p r e c i p i t a t i o n
scheme. These researchers have found that inclusions
generally obey the sequence of phases given by the pseudo-
binary Ca0-Al 20 3 diagram. Although these phases did not
necessarily follow a stoichiometric r e l a t i o n s h i p the
calciim aluminates i d e n t i f i e d were: CaO*f>Al203 (CAg),
CaO-2Al 20 3 (CA 2), C a O - A l ^ (CA) , 12CaO• 7 A l 2 0 3 ( C 1 2 A 7 ) .
Salter and Pickering have reported as an exceptional case
a Ca-aluminate the composition of which corresponded
cl o s e l y to the C^ 2A 7. P i c k e r i n g 1 s ^ studies on de
oxidation i n the ladle, have also found the same sequence
of reactions. In these studies the C^A^ was i d e n t i f i e d .
Faulring et a l . ^ 2 < ^ 9 ^ and Salter et a l . ' 4 ( ^ and others
(201, 207) have agreed that for a given l e v e l of deoxi
dation with calcium a mixture of at least two d i f f e r e n t
stoichiometric calcium aluminate phases are found. (144)
H i l t y and coworkers have approached the i n
clus i o n formation mechanism as another p a r t i c u l a r case of
the general theory to explain the metal-oxide s u l f i d e
83
co-pr e c i p i t a t i o n . These researchers' work coi n c i d e n t a l l y
to Salter's and P i c k e r i n g • s ( 1 4 0 J and o t h e r s ( 1 4 7 ' 1 4 8 '
190, 210) S U g g e s t that since CaO substantially fluxes
A^O^ then the CaO decreases the melting point of the
" A l 2 0 3 " to produce Ca-aluminates which melt within the
range of steelmaking temperatures. H i l t y et a l . ^ ^ 8 ^
propose that the pseudo ternary eutectic i n the i r ternary
(metal-oxide-sulfide) diagram should be moved closer to
the oxide corner and hence a higher melting point "eutectic"
should be expected. The Ca-aluminate p r e c i p i t a t i o n se
quence given by H i l t y and F a r r e l l suggest that t h i s i s
modified by the sulfu r content, i . e . while a st e e l con
taining 0.015% S pre c i p i t a t e s (CA 2), a steel with 0.005% S
and the same Ca- content ('v, 40 ppm) , i t w i l l p r e c i p i t a t e
(CA) .
Laboratory and i n d u s t r i a l work performed by Takenouchi (154)
and Susuki have agreed with the res u l t s previously
described. The shape control of the Ca-aluminates and the
disappearance of the manganese su l f i d e s was also obtained.
The deoxidizer used was a CaAl a l l o y as wires 4.8 and 7 mm
diameter shielded with a s t e e l plate 0.2 mm i n thickness.
Emi et a l . ^ X 1 ^ who have used the same deoxidizer and
technique, have reported e s s e n t i a l l y the same trend of (144)
r e s u l t s as that given by H i l t y and F a r r e l l . The
stoichiometric C^A-y phase was c l e a r l y seen at approxi
mately 75-80 ppm of Ca i n the (HSLA) s t e e l .
84
Researchers at (Wakayama works) Sumitomo Metal i n
dustries i n Japan^^3) a i o n g the l i n e s of Takenouchi
et a l . ' s and Emi et a l . ' s work (pipeline steels and
Ca treatment) have introduced the Ca-alloy by the " A l -
b u l l e t shooter" into the Steel contained i n the 160 ton
L.D. converter. The CaO to A^O^ r a t i o was equivalent to
the above research. The presence of the i n t e r n a l (Ca-
aluminates) and the external (A^O^-MnS and CaS) phases
have shown a clear dependence on the t o t a l calcium con
tent of the previously (Al)-deoxidized s t e e l .
Saxena and coworker's ' research on i n
j e c t i o n of CaO-bearing slags into the (30 kg) melt which
was previously deoxidized with aluminum, have shown that
by t h i s treatment alumina clu s t e r s into the melt are
changed to CaO-A^O^ inclusions and that MnS gradually
'disappears."
I t i s also indicated that the inclusions, present
aft e r the prefused slag i s injected are spherical calcium (147,
aluminates with peripheral s u l f i d e s . Saxena et a l . 1 4 8 J have proposed that as soon as the CaO-bearing
slag i s i n contact with the melt two primary reactions take
place, namely:
mCaO, , . + nAl_0 * t mCaO-'n A1,0,(1) (19) (slag) 2 3 2 3
and
3 C a 0 ( s l a g ) + 2 [ A 1 ] * A l 2 0 3 * ( s ) + 3[Ca] (20-a)
85
where the c o e f f i c i e n t s m and n i n reaction (19) represent
stoichiometric factors according to the equilibrium pseudo *
binary (CaO-A^O^) phase diagram. A l ^ O ^ represents the primary (Al) deoxidation products.
(19 7) (210) G a t e l l i e r et a l . and Holappa also support
the deoxidation mechanism given by the reaction (20).
Holappa gives an equivalent reaction which comprises both
e q u i l i b r i a ; namely Al-deoxidation and Ca-treatment:
x[Ca] + y ( A l 2 0 3 ) . n c l u s i o n t xCaO.[y - K r]Al 20 3 + |x.[Al] (20-b)
Saxena and c o w o r k e r s ^ 1 4 7 ' 1 4 8 ^ have pointed out that these
reactions (19) and (20) or (21) take place insofar as the
bath contains s u f f i c i e n t aluminum and hence low oxygen a c t i
v i t y .
Since a simultaneous deoxidation and d e s u l f u r i z -
ation^ ' ' ' ; i n Ca-injection processes , . , _ (147,148) , , ̂ has been observed then Saxena has proposed to
represent t h i s equilibrium by the reaction:
(CaO)* + [S] = (CaS)* + [0] (21)
It i s anticipated that i f the CaO and CaS have unit a c t i v i t i e s then a^ = 0.0266 a and hence Saxena and co-
O s
workers predict the p r e c i p i t a t i o n of CaS sol e l y i f the
oxygen l e v e l i n the melt i s lower than or equal to 10 ppm.
Thus, i f a strong deoxidation i s obtained to reach such
oxygen leve l s pure CaS would p r e c i p i t a t e . They propose
86
that since the CaO has also a very high a f f i n i t y for A l 2 0 3
then a series of calcium aluminates would be formed, namely:
CaO + 6A1_0_ Z Ca0«6Al_0 o (CA,) (19-a) 2 -i 2 3 6
CaO + 2A1 20 3 t Ca0-2A1 20 3 (CA2) (19-b)
CaO + A1 20 3 t CaO-Al 20 3 (CA) (19-c)
12CaO + 7A1 20 3 t 12CaO-7Al 20 3 ( C1 2
A7 ) (19-d)
3CaO + 2A1 20 3 t 3Ca0*2Al 20 3 ( C ^ ) (19-e)
Although i n these laboratory s t u d i e s ' 4 7 ' * 4 8 ^ oxides en
riched i n calcium were i d e n t i f i e d , reaction (19d) and (19e)
which p r e c i p i t a t e the C^ 2A 7 and the C 3A 2 phases were not , , (147,148) ,̂ ^ c l e a r l y revealed. Saxena et a l . propose that
the CaO-CaF2 slags do not contribute to form CaS on i n
clusions unless the A1 20 3 i s f i r * s t l y transformed into Ca-
aluminates. H i l t y et a l . ( 1 6 8 ) , Salter et a l . ( 1 4 0 ) and
Nashiwa et a l . ' 5 * ^ have also agreed with Saxena' s proposal.
G a t e l l i e r et a l . ( 1 9 7 ) , Saxena et a l . ( 1 4 7 , 1 4 8 ) and
Holappa( 2*°) who have studied the deoxidation i n ladles with
Ca-treatments have suggested that the in c l u s i o n morphology
can be retained i n the f i n a l ingot only i f sources of oxy
gen, which produce reoxidation are r e s t r i c t e d . (159)
Church et a l . have proposed that nucleation of Ca-bearing s u l f i d e s take place exclusively on Ca-aluminates.
• ^ - . - ^ • , ̂ , (147,148) Experimental evidence given by Saxena and Engh
shows that as the i n j e c t i o n time of CaO-slags into the melt
87
increases., the amount of s u l f u r i n the (Ca) s u l f i d e phase
also increases gradually up to a l e v e l which i s thought to
be the maximum sulfur s o l u b i l i t y i n calcium aluminates.
As a further corroboration of these observations
chemical analyses of samples extracted during the i n j e c t i o n
process show a gradual and continuous increment of Ca
which reaches a plateau at approximately 20 ppm at l a t e r
i n j e c t i o n stages. Saxena's and coworkers previous lab
oratory work on CaO-CaF2 i n j e c t i o n has been extended to
i n d u s t r i a l t r i a l s ^ 2 1 4 ) . Although the re s u l t s on a lab
oratory scale have indicated a r e l a t i v e l y high y i e l d , i n
terms of transformation of Al 2°3 t o Ca-aluminates and MnS II
to Ca-sulfides, the i n d u s t r i a l scale t r i a l s did not show
such e f f i c i e n c y . The MnS II was only transformed to duplex-
(Ca, Mn)S- s u l f i d e and pure CaS by i t s e l f was not traced.
In an apparent disagreement with a l l of the previously , , . (140, 147, 148, 159, 168) . .. described investigations with
respect to the required conditions to change the A^O^
and the MnS II morphology, i t has been reported ̂ 2 1 ! ^ that
"pure" CaS i s formed exclusively a f t e r the "A^O^" content
i n the Ca-aluminates i s reduced by Ca to less than about
40.0%, i . e . when the CaO:Al2C>2 r a t i o i s 3.0 or when the
3CaO«Al20.j stoichiometric compound i s formed. I t i s also
indicated that once t h i s r a t i o i s reached the CaS i s
sharply increased. (209)
Faulring and H i l t y have observed CaS i n the pre-
88
sence of CaO-A^O^ and CaO^A^O^ as the major and minor
compounds respectively. According to the schematic trans-( 9 7 )
formation model proposed by Tahtinen et a l . and sup-( 9 4 )
ported by Holappa i t i s seen that faceted inclusions,
probably as the hexagonal Ca-aluminate which corresponds
to CaO'GA^O^, represent the i n c i p i e n t t r a n s i t i o n of the
a-A^O-j to the Ca-aluminates and the simultaneous tran-(94)
s i t i o n of the MnS to (Ca, Mn) ,S. Gustaffson and Melberg (209 217)
Faulring et a l . ' have observed these phases i n (15)
Ca-treated ingots. M i t c h e l l has also reported these
phases i n ESR-ingots.
The most comprehensive work which analyzes, on thermo
dynamic p r i n c i p l e s , the p r e c i p i t a t i o n sequence of A^O^
and Ca-aluminates i s that developed by Faulring and
Ramalingam^ 2 1^. These researchers have developed a
ternary Al-O-Ca equilibrium, isothermal (1550°C, 1823°K)
p r e c i p i t a t i o n diagram based on Henrian a c t i v i t i e s . They
have established that although diagrams of thi s kind (three
components, isothermal and Henrian behavior) are hypo
t h e t i c a l i n nature, these are very h e l p f u l i n the
understanding and predicting of the id e n t i t y of inclusions
from the chemistry of the bath of vice versa.
It i s also emphasized by these researchers, that
Henrian a c t i v i t y behavior was assumed due to the incon
sistency found i n the thermodynamic data available for
89
calcium. This three dimensional diagram (h^, ^ c a ' ̂ A l ^
was developed by projecting the isothermal Al-O, Ca-O
and Ca-Al binary e q u i l i b r i a . Thus, or i g i n a t i n g the sat
urated and unsaturated surfaces which w i l l give the volume
of s t a b i l i t y , Figure (11). Thermodynamic data used to
construct t h i s diagram i s condensed i n tables (IV, V and
VI) .
Faulring's and Ramalingam's experimental and t h e o r e t i
c a l findings are condensed i n the following points:
1) The Ca:Al r a t i o determines the i d e n t i t y of the i n
clusion phases. 2) The amount of calcium for a given
amount of aluminum varies over a narrow range for each Ca-
aluminate phase. 3) If h A ^ > 0.01 i n s t e e l , calcium has
a n e g l i g i b l e e f f e c t as a deoxidizer but does a l t e r the
composition and thus the morphology of the inclusions and
4) Close control of the Ca:Al r a t i o to obtain a desired
Ca-aluminate as the major phase
F i n a l l y , Faulring and Ramalingam have determined em
p i r i c a l l y several correction parameters based on the
h C a : h A l a n <3 t n e %Ca:%Al r a t i o s from
Ca _ Ca • %Ca h A l ' f A l % A 1
2.3 x 10~ 6 for CA C, CA 0 and CA and when i.e. Ca "Al
90
alumina i s present as one of the phases = 10 x 10 u . r A l
2.3.7.3 Complex Oxides (92)
Pickering i n his aim to c l a s s i f y the nature of
non-metallic inclusions i n complex s i l i c a t e systems has
defined f i v e d i f f e r e n t categories -, namely: 1) Pyroxenes,
2) Olivines, 3) Garnets, 4) Feldspars and 5) Cord-
i e r i t e s . At the same time, these categories can be sub-
c l a s s i f i e d as follows:
1) Pyroxenes, these are compounds of the type
MO«Si0 2. Where M can be Fe, Mn and Mg. Their names are
grunerite (FeO«Si0 2), rhodonite (MnO«Si0 2) and enstatite
(MgO«Si0 2), respectively. Since there i s extensive solu^
b i l i t y between CaO and MgO i n the presence of S i 0 2 then
the diopside (CaO«MgO•2Si02) may be considered as a mix
ture of CaO*Si0 2 and MgO«Si0 2-
2) Olivines, t h i s category comprises the same e l e
ments as the previous c l a s s i f i c a t i o n ; t h e i r stoichiometry,
however, i s given as: 2 MO«Si0 2. Thus, f a y a l i t e
(2FeO-Si0 2), tephroite (2MnO«Si0 2) and f o r s t e r i t e
(2MgO«Si0 2) are the main phases of t h i s kind. A compound
with Ca i n this category i s not included due to t h i s large
ion i c s i z e . It i s anticipated, however, that since the
main three phases have complete mutual s o l u b i l i t y they can
dissolve up to 50% CaO.
3) Garnets. This series of compounds follows the
general stoichiometry given by 3M0«A1 20 3• 3SiC>2 . M i n t h i s
case can be Fe, Mn, Mg and Ca. Thus, almandine (3FeO*
A1 20 3«3Si0 2), spessartite (3MnO«A1 20 3•3Si0 2), pyrope
(3MgO«Al 20 3«3Si0 2) and g l o s s u l a r i t e (3CaO•A1 20 3•3Si0 2) are
the s u b c l a s s i f i e d phases in t h i s group. A l l these phases
show v i r t u a l l y complete mutual s o l u b i l i t y .
4) The feldspar group includes phases of the general
form: MO•A1 20 3•2Si0 2 where M represents Mn and Ca. These
phases also show mutual s o l u b i l i t y and take into solution
c e r t a i n quantities of FeO or MgO replacing MnO or CaO.
5) Cordierites. This i s a group which encompasses
compounds of the following general chemistry: 2MO«2Al 20 3«
5Si0 2. M there represents Fe, Mn and Mg. (91) (92) Kie s s l i n g and Lange i n agreement with Pickering
have c l a s s i f i e d the most common compounds i n the CaO-Al 20 3~
S i 0 2 (C-A-S) system, i n the following manner: 1) C«A«S 2 ,
anorthites. 2) C 2A«S, gehlenite. 3) C 2 « A 2 « S 5 , Ca-
Corderite and 4) C 3«A«S 3, g l o s s u l a r i t e . Kiessling and
Lange have established that the C 2«A«S, C 3«A«S 3 and the C2* A2* S5 t v P e s a r e n o t common inc l u s i o n phases.
( 9 1 9 2 ) It i s generally agreed ' that to avoid misleadin
chemical analysis by EPMA, due to the extensive mutual solu-
92
b i l i t i e s and the wide v a r i a t i o n of compositions around the
stoichiometric values, i t i s required to know not only the
Ca, Al and S i but also the amount of Mn, Mg, Fe and T i . (91)
A summary of work performed by Kiessling and Lange
i n the C-A-S system i s graphically shown i n Figure (12).
The L^ and l i n e s represent the maximum and the minimum
MeO:SiC>2 r a t i o s in the phases of the inclusions. Although
the binary oxide MeO p r i n c i p a l l y represents CaO, i t can
frequently contain various amounts of FeO, MnO and MgO.
It has also been pointed out that the central area
between L^ and L 2 largely corresponds to the low-melting
parts of the above systems. The open c i r c l e s represent
chemical composition of indigeneous inclusions determined
by K i e s s l i n g and Lange. The major area, on the Al 202-Si02
side are chemical analysis of extracted samples from a Ca-Si A -A- A • a. (207) deoxidized ingots
(91 92) It i s suggested ' that to trace the o r i g i n of
the deoxidation products a knowledge of the furnace and
ladle slag and refractory composition as well as the deoxid
ation practice i s required. Salter and P i c k e r i n g ' 4 ^
have reported that i n c l u s i o n phases i n the range of the C2*A'S-C2*M*S2 types are commonly found i n Ca-Si de-
(208) oxidized melts. Other studies on deoxidation of s t e e l
with complex deoxidizers (CaSiAl and MgSiAl) have reported
i n c l u s i o n compositions as follows: A^O^/ 5.0 - 82.7 %,
93
CaO, 6.6 - 37%; FeO, 1.4 - 6.0%; and S i 0 2 , 2.4 - 64.4%.
Lindon and B i l l i n g t o n ^ 1 1 5 ^ have also found that the
alumina content i n the C-S-A products increases to approach
pure alumina as the A pet. 0: pet. A l added decreases to
less than the stoichiometric r a t i o . The chemistry of the
deoxidation products indicate that the degree of u t i l i z a t i o n (181
of calcium i s maximum only for a short period of time ' 202 20 5)
•' . Hence, i f there i s not s u f f i c i e n t residual c a l
cium i n the melt although CaO i s present i n the deoxidation
product, i t w i l l mainly contribute to reduce the a c t i v i t y
of s i l i c a and thus to achieve a lower oxygen content. (94)
Holappa's re s u l t s also exhibit similar trends.
Experiments in t h i s research show that the aluminum i n
solution controls the Al 20 3:Ca0 r a t i o and also the S i 0 2
content i n the deoxidation products. If the aluminum i n
solution increases from 0.05 to 0.4% the Al 20 3:CaO r a t i o
also increases whereas the S i 0 2 gradually decreases i n
the endogenous p r e c i p i t a t i o n products.
As depicted i n the Ca0-Si0 2~Al 20 3 ternary, t h i s be-(91)
havior agrees with Kiessling's and Lange's proposal,
i . e . i n c l u s i o n chemistry w i l l follow a tendency to f a l l
within the area of low melting point enclosed by and
94
2.4 Inclusions i n ESR-ingots
In spite of the improvement i n mechanical properties
mainly due to i n c l u s i o n size and quantity obtained by the
ESR-process/ l i t t l e work has been published with respect to
the i n c l u s i o n chemistry. One of the major drawbacks found
in approaching the i n c l u s i o n chemistry either i n s i t u by
metallographic or microprobe (EPMA) techniques or by ex
t r a c t i o n methods i s the small i n c l u s i o n s i z e . While en-
dogeneous (primary deoxidation products) inclusions i n con
ventional steelmaking practices are 15-40 ym i n diameter,
products manufactured by the ESR-technology ; they range
between 2 to 10 ym i n diameter. The second major d i f f i c u l t y
of t h e i r study i s the complexity of the reaction scheme.
The p r e c i p i t a t i o n of inclusions i n the ESR-process has
been studied by B e l l ( 2 1 8 ) and M i t c h e l l ( 1 5 ' 2 1 9 ) under d i f
ferent slag and deoxidation practices. B e l l ' s findings on
the Fe-O-Al system and CaF 2~30% A^O^ slag, indicate that
the f i n a l i n c l u s i o n composition cannot be explained by
the c l a s s i c a l nucleation theory applied to the l i q u i d pool,
i . e . i n s u f f i c i e n t supersaturation for nucleation. It was
observed, however, that t h i s requirement was f u l f i l l e d
e xclusively i n l a t t e r stages of s o l i d i f i c a t i o n .
(15)
M i t c h e l l i n an attempt to influence the p r e c i p i t
ation s i t e i n laboratory ESR-ingots has induced an instant
aneous supersaturation by adding f e r r o s i l i c o n into the
95
metal pool through the slag. Under these conditions, i t
was anticipated that the i n c l u s i o n size d i s t r i b u t i o n i n the
ingot would show the e f f e c t of growth time and possibly
f l o t a t i o n . His r e s u l t s i n agreement with B e l l ' s , however,
produced no detectable change i n either oxygen content or
i n c l u s i o n d i s t r i b u t i o n .
Other studies on F e - C u ( 1 5 ) and F e - N i ( 2 1 8 ) (ESR)-alloys
which exhibit well developed dendrites, have shown either
inclusions aligned along the i n t e r d e n d r i t i c region or
dendrites folded around inclusions. These observations have
enabled them to suggest that inclusions i n the f i r s t case
were formed at early stages and i n the second case - they
might be formed at the beginning of s o l i d i f i c a t i o n . Hence,
i t was concluded that deoxidation i n the ESR process occurs
by chemical e q u i l i b r a t i o n of oxygen and deoxidant with the
slag and since almost a l l inclusions are nucleated and
grown i n i n t e r d e n d r i t i c regions during s o l i d i f i c a t i o n , the
i n c l u s i o n f l o t a t i o n mechanism was discarded. (15 i
M i t c h e l l ' has reported that only very few inclusions
from the electrode penetrate to the l i q u i d pool bulk and
those which do so are s u b s t a n t i a l l y altered i n composition.
It has been widely recognized that due to the surface
area available for thermochemical and electrochemical re
actions and temperature differences i n ESR-furnaces; two
96
d i f f e r e n t behaviors i n terms of inclusion chemistry are
also observed. Work on the Fe-O-Al and on the Ni-O-Al (218 220)
systems performed at U.B.C. ' i n agreement with (38)
Miska's and Wahlster's work i n laboratory ESR-furnaces has shown that the Al-0 behavior does not obey the s t o i c h i o -
(218) metric r a t i o expected from equilibrium conditions. B e l l
who has remelted electrodes i n CaF 2-25% A^O^ under an argon
atmosphere has found that i f an (anodic) electrode with a
t o t a l oxygen content of 30 ppm i s remelted, a d r a s t i c i n
crement of oxygen i s observed at the droplets 1600 ppm)
and t h i s i s reduced down to about 70 ppm af t e r they have
passed through the cathodic ingot surface. Under the above
conditions hercynite or a mixture enriched i n hercynite and alumina as inclusions would be expected. Miska et a l . ' s
(38)
work on aluminum alloyed steels remelted i n a labor
atory ESR-furnace under alumina saturated CaF 2~slags has also shown that the oxygen content far exceeded the e q u i l i -
2 3 brium, [Al] [0] , product.
B e l l has also performed (ESR) laboratory scale experi
ments i n which aluminum deoxidation was carried out. Elec
trodes containing 700 and 30 ppm of oxygen were refined
through a CaF 2~25 wt% A l2 ° 3 s l a 9 ~ s ' deoxidized at various
lev e l s (1.15, 10.3 and 44.0 grams). The introduction of
deoxidizer i n the melt was carried out by attaching aluminum
97
wires to the electrodes. Results of these series of ex
periments showed an 'apparent equilibrium temperature'
of 2000 to 2100°C / rather than 1700°C. Thus, whereas the
metal was expected to lose aluminum and oxygen as alumina
to the slag sometimes the opposite was observed. I t was
suggested that t h i s difference i s due to the i n e f f i c i e n c y
of the aluminum deoxidation and that an excess of aluminum
i s required to lower the oxygen content. Since calculations
have shown that losses of aluminum to atmospheric oxidation
would be as important as the slag deoxidation reaction
then i t was proposed that the reactions:
2 A 1 ( 1 ) + 3(FeO) t F e ( i ) + A l 2 0 3 ( s ) (12-iv)
and
2 A1 ( 1 ) + | ( O ) t A l 2 0 3 ( s ) ( l l ' - i i )
take place. This l a s t reaction was the most probable, since
the condensation s i t e would be the cold mold wall where the
aluminum i s not refluxed but removed from the system.
Miska et a l . ' 8 ^ i n agreement with Burel's findings'"'"' 2 2 0 ^ have found . that the amount of alumina or hercynite
i n (ESR) laboratory scale ingots was strongly increased
as compared to the i n i t i a l quantity traced i n the electrode. (55)
Kajioka et a l . have reported that better cleanliness i s
achieved i n larger rather than i n smaller ESR-furnaces. Sev
e r a l explanations have been proposed to account for the above
98
facts; i t i s , however, accepted that electrochemical rather
than chemical reactions control the ingot and hence (63)
the i n c l u s i o n chemistry. Boucher's work
on equivalent slag compositions to the previous works
have suggested that i n i n d u s t r i a l ESR-ingots where there
i s a larger surface area exposed to the slag, chemical
instead of electrochemical reactions govern the ingot and
thus the in c l u s i o n chemistry. Boucher's findings, although
with some scatter show that there i s a li n e a r r e l a t i o n s h i p
between the "FeO" content i n the slag and the aluminum i n
the ingot. This behavior was an in d i c a t i o n that thermo
dynamic equilibrium was p r a c t i c a l l y achieved. While B e l l ' s
findings suggest an "apparent equilibrium temperature" i n
the 1900° to 2000°C range for alumina i n small ESR ingots,
Boucher's i n d u s t r i a l scale ESR-experiments suggest that
an almost true thermodynamic e q u i l i b r i a i s reached at
1700°C. He also claims that almost pure alumina inclusions
were i d e n t i f i e d when the protective (Ar) atmosphere was
t i g h t l y maintained throughout the r e f i n i n g period.
Retelsdorf and Winterhager^ 9^ i n the i r aim to produce
alumina free high carbon ferrochrome and metal chrome by
ESR. have found that r e l a t i v e l y large diameter ingots,
about 200 mm, contained either a-A^O^ 'C^O^ or a-alumina
(corundum) and the aluminum and oxygen contents always cor-
responded to the Al-0 equilibrium. Their "unsuccessful"
findings were claimed to be due to the high oxidation
p o t e n t i a l of the slag (CaF2-CaO-Al2C>3 with 30% A l 2 0 3 )
and to the i n e f f i c i e n c y of the protective atmosphere. (15)
M i t c h e l l who has remelted electrodes containing
complex (calcium alumina s i l i c a t e ) i n c l u s i o n phases has
reported that ingots remelted through CaF 2~20 wt.% A1 20 3
and high aluminum deoxidation (0.5% Al) are prone to con
t a i n calcium bearing inclusions. It has also been reported
i n t h i s study that although less than one percent of these
inclusions contained s i l i c a , where i t was found i t was as
high as 50%. (74)
Holzgruber's work on the e f f e c t of s i l i c a of the
CaF 2~CaO-Al 20 3 slag system, has found that at low s i l i c a
l e v e l s (4.2%) i n the slag, calcium aluminates containing
84% Al 2C» 3, 12% CaO and about 1% s i l i c a with a peripheral
s u l f i d e phase—probably (CaMn)s—are commonly found.
Another peculiar in c l u s i o n composition (14% S i 0 2 ,
32% A l 2 0 ^ and 56% CaO) reported by Holzgruber which also
contained a peripheral s u l f i d e phase was obtained under
the above experimental conditions, the s i l i c a i n the slag,
however was about 12.0 wt.%. Holzgruber's findings indicate
that about 12-15% s i l i c a i n the slag y i e l d s the highest Ca
100 content i n inclusions.
These re s u l t s were l a t e r confirmed by A l l i b e r t et (53 54)
a l . ' . I t should be mentioned that Holzgruber's experi
ments were performed under variable CaOrSiC^ r a t i o s and a l
though s i l i c o n was used as deoxidizer the amount was not
s p e c i f i e d . (53 54)
A l l i b e r t et a l . ' s studies ' on the acid-basic
reactions i n the CaF2-Al202-CaO-SiC>2 slag system i n agree
ment with Holzgruber's findings have reported that at low
s i l i c a content i n the slag, calcium aluminates associated
with a s u l f i d e phase are c o i n c i d e n t a l l y precipitated.
Holzgruber's and A l l i b e r t ' s et a l . ' s findings i n terms
of slag and i n c l u s i o n chemical composition are shown i n (53 54)
Figures (12) and (13). A l l i b e r t et a l . ' have c l a s s i
f i e d the i n c l u s i o n composition i n oxides plus s u l f i d e s and
s u l f i d e s . The f i r s t types are located i n the ternary where
a r e l a t i v e l y high a c t i v i t y of s i l i c a i s found, Zone (1).
The s u l f i d e zone (2) i s located at moderate s i l i c a a c t i v i t i e s .
Holzgruber*s r e s u l t s , c i r c l e s enclosing a s o l i d square, also
r e f l e c t similar trends. (74)
It has been reported that i f th,j s i l i c a content i n
the ESR-slag i s higher than 10%, the percent of s i l i c a i n
inclusions w i l l be higher than the percent i n the slag. Sim
ultaneously to t h i s s i l i c a increment the size of inclusions
w i l l be larger and hence the t o t a l oxygen w i l l also be i n
creased. (92)
Holzgruber i n agreement with Pickering and Kiessling (91) and Lange has also found that higher s i l i c a content
101
in the slag, the alumina s i l i c a t e phase i n inclusions contains
either CaO or MnO, but not both compounds together.
Holzgruber has also noted that i f r e f i n i n g i s carr i e d
out i n alumina free CaO-CaF2 slags small alumina free
manganese s i l i c a t e s which contain a maximum of 10 wt %
CaO w i l l be the most common type of inclusions found. (83)
Rehak et a l . who have studied the e f f e c t of elec
trode and slag chemistry on inclusions i n ESR-ingots have
remelted a s t e e l (CSN 19.426 used for cold r o l l i n g rods)
produced v i a e l e c t r i c arc furnace under either CaSi or A l
deoxidation practices. The chemical composition of i n
clusions i n electrodes Al-deoxidized i s : 92.25 wt% A^O^,
2.23 wt% CaO and 5.43 wt % MgO. In the CaSi deoxidized
electrodes i t was as follows: 58.48 wt.% A l 2 0 3 , 19.46 wt. %
CaO, 5.36 wt. % S i 0 2 and 16.7% MgO.
Three major slag types were selected to refi n e these
electrodes; namely basic, neutral and a c i d i c . While the
series of aluminum deoxidized ingots showed almost pure A l 2 0 3
i n inclusions and only traces of Ca (less than 1%), s i l i c a
was detected exclusively i n one case where the s i l i c a con
tent i n the slag was as high as 25%.
The CaSi deoxidized series of electrodes showed,
after r e f i n i n g a higher Ca-content (2.41 to 6.91%). The
e f f e c t of the s i l i c a content i n the slag was strongly
r e f l e c t e d i n the i n c l u s i o n chemistry. The slag and inclusion
chemistry are also shown i n Figures (12 and 13). A slag con
taining 50% CaF 2 30% CaO and 20% S i 0 2 and a pure CaF 2 slag,
both considered as neutral and the highly a c i d i c slags pro
duced S i 0 2 enriched inclusions (53.0 to 71.36% S i 0 2 ) . Their
alumina content ranged from 27.0 to 39.4%. The CaO content
varied from 2.4 to 5.0%. The MgO ranged from approximately
1.5 to 3.5%.
The CaSi series of electrodes remelted through basic
slags showed a s l i g h t l y higher CaO-contents (3.2 to 6.9 5%)
than i n the previous ESR ingots. Traces of s i l i c a i n i n c l u
sions of about 0.3 to 0.57% were f o u n d s The MgO contents,
on the other hand varied from 5.1 to 10.0% and the alumina
content which was the major component, was 86-87%.
Rehak et a l . have concluded that oxide inclusions are
sequentially formed as their thermochemical a f f i n i t y for
oxygen i s dictated. They have suggested that the a-A^O^
(corundum) w i l l form f i r s t and as the a c t i v i t y of aluminum
in the melt decreases other elements with lower a f f i n i t y
for oxygen, such as calcium-aluminum-silicate inclusions,
w i l l subsequently be formed.
Thus, the parameters which influence the f i n a l i n
clusi o n chemical composition are the slag chemical comp-
(49 51 53 54 74) o s i t i o n and the deoxidation of the elec-
t r o d e ( 8 3 ) .
Several studies on mechanical properties of ESR-ingots
obtained under d i f f e r e n t practices have been recently pub-
(133)
li s h e d . Work carr i e d out by Boldy et a l . has dealt
with the e f f e c t of s u l f i d e inclusions on the "overheating"
phenomenon. Overheated materials, as a r e s u l t of p r e c i p i
t a t i o n of manganese s u l f i d e s onto high temperature (1100°-
1400°C) aus t e n i t i c grain boundaries show a reduction i n
toughness. These researchers describe that a faceted ap
pearance of the fracture surface i s c h a r a c t e r i s t i c of an
"overheated" material.
It has been found that although r e f i n i n g processes
such as ESR and VAR ingots are capable of reducing the
sulfur content i n ingots down to very low levels and hence
promoting the p r e c i p i t a t i o n of a fine dispersion of s u l -
104
fid e s , i t enhances the intergranular (austenitic) p r e c i p i t
ation which causes overheating. Several counteracting measures
have been proposed to overcome such a problem, i . e . changes
i n ingot chemistry or deoxidation practices or cooling rates
other than a i r cooling (^100°C/min). Another communication
(221)
which has evaluated and compared mechanical properties
between VAR and calcium-treated ESR-ingots under several slags
and deoxidation rates, has shown that impact strength i s
strongly affected by these variables. This work shows that
at high calcium deoxidation rates (0.14% Ca) the aluminum
and oxygen and p a r t i c u l a r l y the s i l i c o n content are sharply
increased during remelting. The ESR ingots exhibited a
gradual increment i n the inc l u s i o n size as the Ca-deoxidation
l e v e l was increased (0.032%, 0.047% and 0.14% Ca). As a
r e s u l t of the above parameters the lowest toughness was found
at the highest calcium deoxidation l e v e l s . In these
studies neither the inclu s i o n chemical composition nor a
self - c o n s i s t e n t explanation as to why the mechanical prop
e r t i e s exhibited such a behavior, have been considered. Although the Ca/CaF 2 solution has been used i n d u s t r i -
(86) a l l y i n ESR for removing phosphorous from st a i n l e s s steels
(85)
and sulfur from rotor steels and as a r e s u l t improved
mechanical properties have been reported, the inclu s i o n
chemical composition has not been investigated.
105
(222) Viswanatan and Beck have reconfirmed findings from R a t l i f f (223)
and Brown i n terms of determining the influence of
A l i n the mechanical properties of a rotor (Cr-Mo-V) s t e e l .
Their r e s u l t s c l e a r l y show that the presence of aluminum
(more than 230 ppm) i n s o l i d solution without forming n i
t r i d e s , markedly reduces the rupture d u c t i l i t y and hence
leads to premature f a i l u r e s .
CHAPTER III 106
NATURE OF THE PROBLEM
A comprehensive work on semi-industrial or f u l l - s c a l e
ESR experiments, which would account for a l l of the steps-
at the electrode, electrode-slag, s l a g - l i q u i d pool and i n
the process of s o l i d i f i c a t i o n during r e f i n i n g and the ef
fects of slags and deoxidizers on the f i n a l ingot and hence
the i n c l u s i o n chemistry, has yet not been performed.
3.1 Inclusions i n the Electrode (2-5)
Several studies have attempted to elucidate the
nature of the transformations of inclusions from electrodes
to ingots during r e f i n i n g . Mathematical models^ 9^ which
have analysed the thermal history of electrodes have demon
strated that c r i t i c a l thermal gradients are developed at
the electrode t i p . Regarding the mechanism by which inclusions i n the
electrode are removed, controversial and inconsistent models -,(1,5) ., . (1,12,13,15,16) have been proposed ' . While some researchers '
have reported that inclusions are gradually dissolved as a
consequence of the thermal gradients at the electrode t i p , (17 19 20)
others •' have reported the opposite. Other research-( ? 3 5 i
ers ' have suggested that the elimination of inclusions
from electrodes i s by mechanical action. On the other hand,
107
(15) studies performed on inclusions at the l i q u i d f i l m have
demonstrated that inclusions do not chemically show any simi
l a r i t y with inclusions i n areas where electrodes experience (22)
low thermal gradients. Other studies i n f u l l scale (ESR)
electrodes, i n agreement with t h i s proposal, have also sug
gested that there i s a c r i t i c a l length above the l i q u i d f i l m
where ce r t a i n volumetric changes of inclusions take place. (19 20)
Russian investigators ' have also pointed out
that the c h a r a c t e r i s t i c liquidus-solidus length of a l l o y s
also plays an important role i n the removal of inclusions
from the electrode. As seen i n t h i s summarized review,
there i s a vast quantity of q u a l i t a t i v e information but
only a l i m i t e d amount of quantitative information a v a i l a b l e .
Since the chemical nature of inclusions i s related to
t h e i r transformation as a r e s u l t of the steep thermal gradi
ents at the electrode t i p i t has only been approached on a
q u a l i t a t i v e basis (in terms of the chemical composition of
the electrode) and quantitative analysis (in terms of i n
clusion size d i s t r i b u t i o n s ) , i t was, however, very clear
that a deeper study which could r a t i o n a l i z e the reported
research, would be very valuable.
108
3.2 The Chemical Influence of the ESR Components on the
Composition of Inclusions T T • (37,55,58,75-77,79) . . - , Various studies have been performed
to optimize the chemical homogeneity of ingots manufactured
by the ESR technology. It i s widely accepted that i f re
melting i s conducted under atmospheric conditions, an en
hanced and continuous accumulation of iron oxide i n the slag
occurs. It i s also known that the oxidative state of the
slag with respect to the l i q u i d pool i s related to the slag ^ (38,48,82) system ' '
On an a p r i o r i basis, i t i s understood that i f deoxi
dation i s not c a r r i e d out during remelting then a s a c r i f i
c i a l oxidation of reactive a l l o y i n g elements takes place.
Several workers'^'^'^' have pointed out that the chemical
composition of the slag strongly enhances or suppresses cer
t a i n reactions during r e f i n i n g . The net production of iron (28 29 58 71)
oxides and the production of calcium or alum
inum at the e l e c t r o - a c t i v e interfaces dictates the oxidative
state of the molten pool and hence the f i n a l ESR-ingot and i n
clusion chemical composition.
It has been proposed that p o l a r i z a t i o n due to current
passing through the slag-^skin/mould wall interface, which
generates small arc contacts, increases the r e c t i f i c a t i o n
i n the A.C. ESR process and hence i t enhances the net as-
symetry of reactions (1-5). This p o l a r i z a t i o n increases the
109
2+ Fe content of the slag bulk and thus, accentuates a l l oxid
ation rates i n the system. This proposal has been used to ex
p l a i n the chemical composition of inclusions in the Fe-Al-0 . (219,220) system • ' .
The b a s i c i t y index of the slag (as a measure of i t s
chemical potential) and i t s e f f e c t on the chemical composition (49-51)
of ingots and to a limited extent on the chemical na
ture of i n c l u s i o n s ' 8 ' 7 4 ^ , has been strongly supported i n
the German l i t e r a t u r e . These studies, however, have not con
c l u s i v e l y determined the r o l e of the deoxidizer, the chemistry
of the slag and/or deoxidizer, and/or the chemistry of the
electrode or the combined e f f e c t of these parameters on the
incl u s i o n chemical composition of ESR ingots. Several deoxidation practices have been suggested i n
/ ̂ g 74 8 3)
the l i t e r a t u r e ̂ ' ' . Among the uneconomic deoxidation
techniques, to overcome losses of reactive elements (Al,
T i , S i , Mn, e t c . ) , i t has been proposed to: eliminate the
hard scale from r o l l e d electrodes, deliberately increase the
r a t i o of these species in the electrode, use protective paint
ings based on Mg or A l , to deposit oxidative elements on the
electrode surface, etc. The most frequent deoxidation tech
nique developed excl u s i v e l y on an empirical basis has
been the external addition of either aluminum or s i l i c o n as
wire, p e l l e t s , f e r r o a l l o y s , etc. into the slag to achieve a
desired ingot chemistry. The conventional widespread deoxi-
110
dation l e v e l i s about 0.2 wt. % A l or S i . I t has been pro-(74)
posed that e f f i c i e n t deoxidation i s achieved when a de
oxidizer i s introduced into the slag which does not contain i t s oxide. On the other hand, other researchers have pro-
(39 48)
posed ' that i f an element i s prone to oxidation ( i . e .
reactive elements as T i , S i , Zr, A l etc.) during r e f i n i n g ,
then additions of i t s respective oxide into the slag pre
vents i t s losses. The pot e n t i a l harm of inappropriate de
oxidation w i l l manifest i t s e l f i n an uneven d i s t r i b u t i o n (55 75 79)
of c r i t i c a l a l l o y i n g elements i n ingots ' ' and re-(221)
s u i t i n deleterious mechanical properties
These studies, however, have not approached the re
action mechanisms and hence ingot and inc l u s i o n chemical
composition has remained unexplored. In summary, although
the need to introduce a deoxidant into the slag, has been
i d e n t i f i e d , the net e f f e c t of i t has not been elucidated.
Thus, to a large extent the explanation for the ingot chem
i s t r y and hence the composition of inclusions has remained
obscure. In addition, since the reaction mechanisms which
control the chemistry of ingots have not been completely
understood, the exploration of other a l t e r n a t i v e means of
deoxidation—with th e i r p o t e n t i a l advantages and disadvant
ages—has never been properly investigated.
I l l
3.3 The P r e c i p i t a t i o n of Inclusions from Liquid Pool to
Ingot , . ,. (1,33,36,48,58,61,64) .. . ,
Several studies ' ' • ' ' ' ' which have approached the reaction scheme i n the ESR process have c l e a r l y demonstrated the existence of oxidation-reduction reactions at the e l e c t r o a c t i v e (electrode t i p - s l a g and s l a g - l i q u i d pool) interfaces which largely contribute to control the
(218—220) ingot and the i n c l u s i o n chemistry. Research car
r i e d out at U.B.C. has c l e a r l y revealed the electrochemical
nature of i n c l u s i o n p r e c i p i t a t i o n i n the Fe-O-Al system.
Chemical analysis (in s i t u by metallographic and electro-
microscopic techniques and by extracting inclusions and an
alyzing them by X-ray techniques) have conclusively shown
that a state of thermochemical equilibrium i s only q u a l i t a t
i v e l y obeyed. From thermochemical conditions i n slags where
alumina was expected as the only type of i n c l u s i o n , a mix
ture of iron oxide and hercynite, hercynite and alumina were
found instead of pure alumina. The majority of studies on the chemical composition of
inclusions have been performed on samples from the l i q u i d (15)
f i l m (at the electrode t i p ) , droplets i n process of form
ation which have been i n contact with the s l a g ' ^ ' ^ " ^ (also
at the electrode tip) and from samples of already s o l i d i f i e d
ingots.
112
I t should be pointed out that these studies have been
performed i n ingots r e f i n e d i n laboratory ESR-furnaces and
as previously c i t e d , the surface area a v a i l a b l e for reactions
i s smaller than i n i n d u s t r i a l s i z e furnaces. In a d d i t i o n
p h y s i c a l l i m i t a t i o n s i n the small furnaces ( i . e . electrode
mould wa l l spacing) pose another drawback for e x t r a c t i n g
samples from e i t h e r l i q u i d pool or slag. These f a c t o r s have
not allowed researchers to elucidate the o r i g i n of i n c l u s i o n s .
Thus, the need to investigate the thermochemical or e l e c t r o
chemical influence of the s l a g - l i q u i d pool i n t e r f a c e on the
chemical composition of i n c l u s i o n s and therefore to unambigu
ously i d e n t i f y t h e i r o r i g i n i s e s s e n t i a l .
113
3.4 D i s t r i b u t i o n of Inclusions during S o l i d i f i c a t i o n
It i s generally accepted that inc l u s i o n size d i s t r i
butions i n ESR-ingots are markedly smaller compared to
ingots produced under conventional and most of the secondary
(refining) steelmaking processes.
While i n conventional processes l o c a l i z e d concen
t r a t i o n of inclusions have been widely reported due to
the i r c h a r a c t e r i s t i c c r y s t a l l i z a t i o n mode, i n semi-industrial
or f u l l scale ESR-ingots t h i s phenomenon has almost never been
reported. Other in t e r e s t i n g features commonly observed i n
ESR-ingots obtained under conventional deoxidation practices, (224)
are t h e i r i n c l u s i o n size d i s t r i b u t i o n s (about 2 to 12ym) (225)
and t h e i r r a d i a l l o c a l s o l i d i f i c a t i o n times , expressed
as a regular decreasing v a r i a t i o n of th e i r primary and second
ary dendrite arm spacings along t h e i r r a d i a l directions from (65)
the centreline towards the mould wall . Several detailed (218)
studies on inclusions i n ESR ingots where r e f i n i n g of a
ferrous Ni enriched a l l o y was performed through a CaF^^CaO
slag, have found round inclusions aligned along primary
dendrite arms. These observations led to the b e l i e f that
p r e c i p i t a t i o n of inclusions takes place homogeneously from
the i n t e r d e n d r i t i c l i q u i d . Complementary studies also per-(15 219)
formed at U.B.C. ' have also corroborated t h i s proposal.
In t h i s study i t i s reported that inclusions were located i n
in t e r d e n d r i t i c spaces and very r a r e l y were dendrite arms seen
114 folded around inclusions.
Since attempts^ 1 5^ to generate a s i l i c o n supersaturation
i n laboratory ESR melts did not produce any detectable change
i n the i n c l u s i o n size or i n the t o t a l oxygen analysis, i t was
proposed that nucleation and growth of inclusions takes place
almost exclusively as a r e s u l t of r e j e c t i o n of solutes during
s o l i d i f i c a t i o n . It was therefore concluded that under a con
ventional degree of deoxidation the inclu s i o n f l o t a t i o n mech
anism was not applicable, in disagreement with other research-(1,12,35) ers ' '
As previously stated since there are physical l i m i t
ations and d i f f e r e n t k i n e t i c s at the electroactive interfaces
i n small ESR furnaces researchers have not been able to ap
propriately monitor a l l of the reactions i n the various com
ponents and stages of r e f i n i n g , a more complete investigation
i s required.
115
3.5 Establishment of the Proposal and Objectives Sought
Through th i s Research
A clear necessity to understand and thereby to control
the sequence of events to which inclusions and the l i q u i d
metal are subjected i n the various stages of r e f i n i n g i s re
quired, as seen from the previous review. Thus, i n order to
cover a l l the e x i s t i n g gaps and to extend our present under
standing regarding the nature of inclusions i n t h i s f i e l d a
series of four questions was addressed:
1) How are electrode inclusions removed?
2) Is the i n c l u s i o n composition controlled by
the chemistry of electrodes, slags or deoxidizers?
3) Are inclusions i n the l i q u i d pool the same as
in the ingot? and
4) Is the i n c l u s i o n size d i s t r i b u t i o n r e l a t e d to
r a d i a l distances from the centreline to the
mold wall of ESR ingots.
These questions were stated i n such a way that a l l of the
phenomena involved i n the process of r e f i n i n g , i n terms of
inclusions were addressed. Once the mechanisms which govern
the o r i g i n of inclusions (reactions) were determined then
the deoxidant, deoxidation technique and the slag chemical
composition would be selected and a comparison between re
su l t s as a function of electrode and slag compositions as
well as deoxidants could be c a r r i e d out.
116
CHAPTER IV
EXPERIMENTAL WORK AND TECHNIQUES
4.1 Experimental Procedure
Ingots were refined through several slag systems using
laboratory and semi-industrial scale ESR-furnaces des
cribed e l s e w h e r e ^ 2 2 ^ ' 2 2 7 ^ . Electrodes of several chemical
compositions and diameters were refined using several slag
systems (commercial grades) and deoxidants. Tables (VII)
and (VIII) summarize t h i s information.
Electrodes 31.75 and 44.75 mm i n diameter were melted
in the laboratory size ESR-furnace at melting rates ranging
from 1.2 to 1.5 Kg min - 1. 1020, 4340 and rotor (Ni-Cr-Mo)
steels which were 76.2, 88.9 and 114.3 mm i n diameter were
remelted i n the semi-industrial (200 mm) ESR-furnace at
melting rates of approximately 1 Kg min . Both furnaces
use l i n e frequency A.C. power. Refining was ca r r i e d out
with and without a protective atmosphere. In the f i r s t case
the system enclosed an argon gas sh i e l d and deoxidant additions
could be made at monitored rates, Figure (14). The deoxidi-
zers were granular aluminum 99.99% purity, calcium s i l i c i d e
a l l o y s and aluminum 65 wt.% S i , i n the size range of 8-32
mesh- S p e c i f i c compositions are given i n Table (IX).
Several deoxidation practices were followed, namely
i) Constant addition i n small ESR-ingots,
117
i i ) intermittent additions and
i i i ) continuously increasing.
The l a s t two practices were performed i n ingots 200 mm i n
diameter. Experiments to determine the inc l u s i o n removal
mechanisms i n electrode t i p s were carried out i n 1020 mild
s t e e l produced v i a acid e l e c t r i c furnace, 89 mm diameter
4340 Ca-Si-Al treated, and 114 mm diameter rotor (Ni-Cr-Mo)
ste e l Ca-Si-Al treated. Once r e f i n i n g experiments were com
pleted electrodes were rapidly withdrawn from slags to achieve
as f a s t a cooling rate as possible. Electrode t i p s were
sectioned and metallographic (optical) and electro-micro
scopic (SEM and EPMA) analysis were performed. Qualitative
dispersive-X-ray-spectrum analysis by the SEM and back-
scattered and electron composition maps by the EPMA as well
as quantitative analysis (by EPMA) on inclusions were car r i e d
out.
Samples from l i q u i d pools and slags were taken as
deoxidation and r e f i n i n g was taking place. Liquid metal
samples were extracted by suction from l i q u i d pools and slag
samples by means of a small copper c h i l l . Chemical analysis
of inclusions i n samples d i s c r e t e l y extracted from l i q u i d
pools as well as from s o l i d i f i e d ingots were ca r r i e d out by
EPMA. Oxygen analysis by vacuum fusion also on both types
of samples were performed.
The chemical composition of slag samples were analyzed
118
by standard spectrophotometric techniques. The chemical
composition of ingots was ca r r i e d out by spectrographic
analysis along the v e r t i c a l axis of ingots.
F i n a l l y , complementary experiments to determine
where and when inclusions were nucleated and grown were per
formed. S i l i c a tubes which contained either rare earth metals
(mischmetal) or Zirconium wires were introduced through
the slag while helium was gently blown into the l i q u i d
pool to extract l i q u i d metal by the suction technique. After
wards samples were polished and q u a l i t a t i v e and quantit
ative analysis of inclusions were performed. EPMA and SEM
studies on inclusions to determine t h e i r chemical composition
and the d i s t r i b u t i o n of the i r phases (composition maps) were
ca r r i e d out.
4.2 Analysis of Inclusions
A t r i p l e spectrometer Jeolco JXA-3A electron micro-
probe and an Etec "Autoscan" scanning electron microscope
with a dispersive X-ray analyser were used to determine the
chemical nature of inclusions.
Since the diameter of the electron beam which excites
the sample, the specimen current density and the accelerating
voltage determine the steadiness and the magnitude of the
signals (X-rays, secondary and backscattered electrons, etc.)
119
emitted they were continuously c a l i b r a t e d to give a beam
approximately 1.0 ym i n diameter when the ex c i t a t i o n v o l t
age was 25 kV and the specimen current density was about
0.08 yA . Hence, the e l e c t r i c a l - o p t i c a l conditions i n these
instruments were cal i b r a t e d i n such a manner that the maxi
mum possible current was obtained i n the smallest electron
probe.
Aluminum, calcium, s i l i c o n , s u l f u r , manganese, chromium,
titanium, and magnesium were determined at 25 kV. Oxygen and
fl u o r i n e were determined at 10 kV. The i d e n t i f i c a t i o n of
inclus i o n phases was also obtained by the v i s i b l e l i g h t prod
uced by the electron beam and photon radiati o n (cathodo-
luminescence).
Since compound standards are known to produce more ac-(228229)
curate and reproducible analysis ' , compounds
l i k e CaC0 3, A^O^, S l 0 2 ' M ( ? 0 ' Z n S a n d pure Mn were used as
standards to determine the chemical composition of the
in c l u s i o n phases.
Raw data (specimen background, accelerating voltage,
take-off angle, specimen counts, compound standard information,
background from standards, X-ray counting time, etc) were
translated into chemical composition, taking into account cor
r e c t i o n for atomic number e f f e c t s , absorption and secondary
fluoresecence, by Colby's MAGIC IV program' 3 0* with U.B.C.'s
120
AMDAHL computer. Relative accuracy of ± 7-10% was obtained.
These values are i n f a i r agreement with reports avai l a b l e i n
the l i t e r a t u r e ' 0 6 ' 2 2 8 ' 2 2 9 ! (±5-7%). I t i s also worthwhile
to mention that, as reported i n the l i t e r a t u r e ' 4 0 ' 2 1 6 ' 2 1 7 ^
the calcium to aluminum r a t i o i n inclusions i n the Ca-deoxidized
ingots gave a closer indi c a t i o n of the deoxidation sequence. These r a t i o s (Ca:Al) i n agreement with Faulring's
et a l . • s ( 2 0 9 ' 3 1 6 ) and Salter's and P i c k e r i n g • s ' 4 0 } findings
were found to approximate the corresponding stoichiometric r a t i o s
of those phases given by the pseudo-binary CaO-A^O^ diagram.
Spectrographic and oxygen analysis of samples from i n
gots and l i q u i d pool and spectrophotometric and c r y s t a l l o -
graphic analysis of inclusions (by Debye-Scherrer and d i f -
fractometer techniques) extracted from ingots corroborated (207)
these findings. Based on a reported work on calcium
aluminate inclusions a minimum of twenty and a maximum of
50 single assays were performed to obtain a representative
analysis of a sample, i . e . this represents one point on the
graphs.
The inclusions were grouped according to siz e , chemi
c a l composition, fluorescence, shape and representative quan
t i t i e s . Inclusions smaller than 3 ym i n diameter were chemi
c a l l y analyzed; they were, however only q u a l i t a t i v e l y con
sidered due to the cumbersome in t e r a c t i o n e f f e c t s of the i n
clu s i o n chemical composition and the metal matrix. Aluminum
121
deoxidized ingots contained either FeO-A^C^, pure alumina,
or calcium hexaluminate as inclusions phases which were
very small single or clustered. Since i n c l u s i o n diameters
were less than 6-8 pm inclusions smaller than 3.0 ym were
also analysed on a q u a l i t a t i v e basis. Analyses were carr i e d
out by scanning diagonally across the sample and the i n
clusions so that re s u l t s s t a t i s t i c a l l y represented the i n
clusion chemistry of the sample.
Typical inclusions of a given sample were micro-
photographed, o p t i c a l , scanning and backscattered (EPMA)
analyses and composition maps were also obtained.
4.3 Total Oxygen Analyses The t o t a l oxygen content of samples extracted from
l i q u i d pools and ingots was determined by the standard i n e r t
gas vacuum fusion technique using a Leco 537 induction f u r
nace (507-800) and a Leco oxygen analyzer (509-600). Samples
weighing 1.0 - 1.5 grams were cut, ground, and washed, u l t r a -
s o n i c a l l y cleaned and rinsed with anhydrous 1,1,1, three-
chloroethane. Since the accuracy and r e p r o d u c i b i l i t y of
t h i s analysis was strongly influenced by the weight of the
sample and i t s preparation, a meticulous procedure was
followed to obtain a minimum of three assays with a maximum of
2-5% deviation amongst them. If the deviation i n analyses
were greater than t h i s , a new set of three analyses was per-
122
formed.
To obtain an appropriate c a l i b r a t i o n of the analy
t i c a l equipment standards of known oxygen content were
analyzed. Analysis on samples containing an upper and lower
l i m i t of oxygen as well as a blank test to check the v a r i
ation i n gas (helium) flow rate were continuously performed
to maintain them constantly throughout any series of analyses.
Samples extracted from l i q u i d pools were c a r e f u l l y selected
because p o r o s i t i e s and slag entrapment were occasionally de
tected .
4.4 Inclusion Extraction Method
It i s an accepted f a c t that chemical extraction meth
ods of non-metallic inclusions followed by analysis pre
sent several advantages over chemical analysis performed
i n situ.(by microprobe or scanning E.M.). The major ad
vantages are the following:
1. A n a l y t i c a l r e s u l t s are far more representative
because a much larger sample i s taken for extraction than
for i n s i t u methods.
2. Phases can be s p e c i f i c a l l y i d e n t i f i e d after ex
t r a c t i o n .
3. The t o t a l oxygen content and the t o t a l amount
of most phases i n s t e e l can be estimated by chemical analy
s i s of the residue.
123
There are, however, several disadvantages as well.
1. Some phases cannot be quantit a t i v e l y extracted.
This f a c t i s due to either i n c l u s i o n size (< 5ym i n d i a
meter) or to the chemical nature of inclusions and re
agents (too agressive).
2. Phases containing common elements and amorphous
or/and isomorphic structures may int e r f e r e with c e r t a i n analy
s i s .
3. Since i n c l u s i o n size i n ESR-materials i s r e l a t i v e l y
small (S 10ym i n diameter) analysis by X-rays i s rather d i f
f i c u l t .
Several chemical methods of in c l u s i o n extraction were
performed. Among them bromine i n methanol, iodine i n meth
anol, bromine-ester-methanol and iodine-methyl acetate-
methanol. From a l l the above methods only the l a s t two were
suitable to thi s purpose. The iodine-methanol-methyl acetate
however, was found to be the most convenient because of i t s
accuracy and r e p r o d u c i b i l i t y .
4.4.1 Apparatus and Experimental Procedure
The apparatus used was integrated i n four units. The
f i r s t unit (I) was used to pour iodine c r y s t a l s and the
methyl acetate-methanol mixture under a protective atmo
sphere consisting of) anhydrous argon. This part of the sys
tem was also used as a di s s o l u t i o n chamber. An u l t r a
sonic cleaner or a magnetic s t i r r e r with con t r o l l a b l e temp
erature served to speed up the iodine d i s s o l u t i o n .
The second unit (II) consisted of a m i l l i p o r e f i l t e r
which contained a whatman paper No. 50 or a t e f l o n m i l l i
pore f i l t e r (0.5 ym) . The reaction chamber was the third (III)
unit. This was a glass container with four outlets which
served as a) breathing system b) reagent-level regulator,
c) vacuum c o n t r o l l e r and d) argon-flux valve. This cham
ber was inside the ultrasonic agitator which was used to
speed up the d i s s o l u t i o n of the metal sample. The fourth
unit (IV) was also equipped with another m i l l i p o r e f i l t e r
s imilar to the one used i n unit no. 2 but t h i s had a 0.5
ym diameter porous size f i l t e r (made of teflon), Figure (15) .
Since humidity i s one of the major concerns i n any
halogen-methanol-methyl acetate technique a c a r e f u l pro
cedure was followed. Reagents used such as methanol and
methyl acetate were 99.99% i n purity and iodine c r y s t a l s
were previously dried i n a dessicator which contained s i l i c a
gel and d r i e r i t e .
This extraction method was based on Rooney's and Staple (231)
ton's , i t was, however, improved i n terms of avoiding
cert a i n complexity In i t s design and minimizing the r i s k
of contaminating the apparatus by moisture and other vapours
ca r r i e d away during evacuation and heating. Combustion tube
containing s p i r a l s of copper and nickel as well as s u l f u r i c
acid and contaminated iron turnings were also avoided.
125 Experimental Procedure
20-30 grams of an ESR-steel sample free of oxide,
cleaned with 1,1,1-trichlorethane and dried with hot a i r
were introduced into the reaction chamber. Argon was
completely dehydrated by passing i t through 3 U-tubes con-
taning d r i e r i t e , s i l i c a - g e l - i n d i c a t i n g and activated carbon
i n a U-tuhe immersed i n l i q u i d nitrogen. The argon had two
functions: to sweep out the a i r and humidity i n the four
units and to pump the l i q u i d s from one unit to another. For
th i s l a s t purpose a vacuum was also used.
The whole drying operation was executed i n two hours.
Once the system was free of moisture and a i r d i s s o l u t i o n of
the predried iodine c r y s t a l s was c a r r i e d out. The next step
in t h i s technique was to pump the iodine solution either by
vacuum or by argon to the f i r s t f i l t e r i n g unit. Then the
solutio n was pumped to the reaction chamber.
When the s t e e l sample was completely dissolved a f t e r
10-24 hours, the liquor containing the extracted inclusions
was sent to the l a s t f i l t e r i n g unit. At t h i s l a s t unit,
r i n s i n g of the f i l t e r e d inclusions with 99.99% methyl alcohol
was performed several times u n t i l the f i l t e r e d alcohol was
completely c o l o r l e s s , i . e . iodine free. This l a s t operation
was performed under an argon atmosphere.
Extracted inclusions were dried and weighed before and
after s u l f i d e inclusions were eliminated. Samples from sev-
126
e r a l ESR ingots were subjected under t h i s extraction tech
nique for various purposes, namely 1) to corroborate the
EPMA inc l u s i o n chemistry by quantitative (spectrophotometry)
and q u a l i t a t i v e crystallographic (X-rays) analysis 2) to
v e r i f y the v a l i d i t y of the t o t a l oxygen analysis and 3) to
determine how inclusions were present i n the f i n a l product.
4.5 Crystallographic X-ray Analysis of Extracted Inclusions
The extracted inclusions were c r y s t a l l o g r a p h i c a l l y an
alysed by a P h i l i p s high angle diffractometer and a Debye-
Scherrer camera 114.83 mm i n diameter. The machine setting
was 40 kV and 15 uA. The scanning rate used i n the d i f f r a c t o -
meter was 1° 26/min. The camera i s designed such that 2 mm
measured on the f i l m corresponds to 1° 6. The distance along
the f i l m between the zero point and the reference end i s 180mm.
In both X-ray techniques the iron ka^ r a d i a t i o n (X = 1.9 36)
was used.
Although the amount of extracted inclusions ranged from
10 to 30 milligrams, p r i o r to t h e i r X-ray analysis, micro-
photographs and q u a l i t a t i v e SEM analysis were performed on
a portion of the sample. The crystallographic X-ray analysis
of inclusions c l e a r l y showed a sequence of A^O^ up to CaO*Al 20 3
and 12 CaO*7Al2C>3 (as given by the CaO-Al 20 3 pseudo binary phase
127
diagram) as the Ca-Si deoxidation l e v e l was increased. It
i s worthwhile to mention that i n the calcium aluminates en
riched i n calcium (CaO•2Al 20 3~CaO•A1 20 3 and CaO•Al20^-12CaO•
7Al 20 3) very wide d i f f r a c t i o n peaks and bands were observed.
This was a clear i n d i c a t i o n of t h e i r lower degree of cry-
s t a l l i n i t y . The d i f f r a c t i o n bands corresponding to the
12CaO«7Al 20 3 were very weakly traced.
Generally, the c r y s t a l patterns showed from 6 to 16
p r i n c i p a l d i f f r a c t i o n bands with various i n t e n s i t i e s . Mix
tures of at least two i n c l u s i o n c r y s t a l structures and some
n i t r i d e s and carbides were observed.
4.6 Atomic Absorption Analysis (Spectrophotometry)
A Perkin Elmer 306 spectrophotometer with a controller
model HGA-220 was used to analyze slags and extracted i n
clusions from metal matrices. The lithium metaborate fusion (232 233)
procedure available in the l i t e r a t u r e ' was followed.
The concentration of the elements of i n t e r e s t were determined
by using appropriately matched standards and blanks with the
routine procedure given i n the general information of the
Perkin-Elmer manual. Generally speaking the accuracy of the
solution analysis was Ca ± 0.2%, A l ± 0.06%, S i ± 0.04%, .
F + 0.2%, Mg ± 0.002% and Fe ± 0.001 wt %. Stoichiometric
balances were ca r r i e d out on the bases that Ca, A l , S i , Fe, Mg,
and F were present as CaO, A l 2 0 3 , S i 0 2 , FeO, MgO and CaF 2 res
pectively. Under t h i s assumption the stoichiometric calcu
l a t i o n s gave 10 0% with a minimum accuracy of + 1.0%.
128
4.7 Metallographic Analysis
Ingots were sectioned l o n g i t u d i n a l l y i n such a manner
that a s l i c e 2.5 cm i n thickness across the diameter was ob
tained. These plates were surface ground and etched with a 50%
HCl-H^O solution at 70-80°C for approximately one hour. Thus
l i q u i d pool marks made with m e t a l l i c tungsten powder to
id e n t i f y deoxidation l e v e l s and ingot structure were re
vealed. Longitudinal discrete sampling (matching the pro
gressive sampling of l i q u i d pools and slags) to perform
spectrographic, i n c l u s i o n chemical and t o t a l oxygen analysis
were thus obtained.
Samples obtained r a d i a l l y were used to determine i n
clu s i o n s i z e d i s t r i b u t i o n s and dendrite arm spacings. The
o p t i c a l analysis was performed using a Zeiss ultraphot i n
d i f f e r e n t i a l interference mode. Samples from electrode
t i p s , l i q u i d pools and r a d i a l and longitudinal specimens
from ingots were ground on emery paper and diamond polished.
Polishing was ca r r i e d out to determine incl u s i o n sizes
and t h e i r l o c ation with respect to dendrites i n samples ex
tracted from l i q u i d pool and ingots using: diamond compound
pastes (Metadi II) down to 0.25 ym diamond p a r t i c l e s i z e , tex-
met, microcloth and nylon polishing (Buehler) cloth ( o i l and
water r e s i s t a n t and alcohol and an o i l based lubricant,
(Geomet thinner Micrometallurgical MM218).
By following the above sequence removal of inclusions
from metal matrices i n any siz e range was completely avoided.
To determine the loc a t i o n of inclusions and dendrite arm
spacings standard Oberhoffer's reagent was used to l i g h t l y
etch these specimens. Four and f i v e faces of each specimen
were polished, etched and microphotographs were taken at a
magnification such that representative number of inclusions
were i n each photograph. The magnification used was very
dependent on the l e v e l of deoxidation and the chemistry of
the deoxidant.
130
CHAPTER V
RESULTS AND DISCUSSION
5.1 Mechanism by Which Electrode Inclusions are Eliminated
In the l i t e r a t u r e review on inclusions p a r t i c u l a r
emphasis has been given to the p r e c i p i t a t i o n sequence. It
has been established that inclusions according to the de
oxidation technique exhibit sequential changes i n t h e i r
chemical composition as s o l i d i f i c a t i o n takes place. These
tr a n s i t i o n s may take place at subsolidus temperatures; p a r t i
c u l a r l y where s u l f i d e phases p r e c i p i t a t e .
Thus, to f u l l y understand the mechanism by which i n
clusions are removed, electrode t i p s from 1020 M.S., 4340
and rotor (Ni-Cr-Mo) steels were studied. 1020 M.S. ingots
were o r i g i n a l l y produced by acid e l e c t r i c furnace p r a c t i c e .
The 4340 and the rotor s t e e l were calcium aluminum treated
i n the l a d l e . Subsequently, they were hot r o l l e d
into electrodes 76.2, 88.9 and 114.3 mm i n diameters res
pe c t i v e l y .
5.1.1 Behavior of Oxisulfide Inclusions i n 1020 M.S.
Electrode Tips
The most complete picture of the i n c l u s i o n d i s s o l u t i o n
and the transformation of the metal matrix i s given by the
1020 M.S. electrodes. Typical inclusions i n electrodes i n
t h i s series of experiments, as received, are shown i n
Figures (16) and (17). Their q u a l i t a t i v e (SEM) chemical
131
composition are also presented (spectrum X-ray a n a l y s i s ) .
Two well-defined phases are i d e n t i f i e d . The darker phase
i s enriched i n Si and the l i g h t phase i s enriched i n mang
anese s u l f i d e . Iron was also found i n both phases.
Metallographic studies of electrode t i p s which were sub
jected to c r i t i c a l (ESR) thermal gradients have c l e a r l y re
vealed the existence of several heat affected areas, Figures
(18) to (21). Findings from t h i s research i n q u a l i t a t i v e (8) (13 agreement with previous t h e o r e t i c a l and experimental work '
19 20 22) ' ' . show that au s t e n i t i c grain growth and changes i n
the morphology of inclusions occurs between 0.5 - 0.8 cm above
the l i q u i d f i l m . Sulfide inclusions are f i r s t spherodized
and subsequently dissolved i n t h i s region as well. The chem
i c a l composition of inclusions determine the c r i t i c a l length
at which the above changes take place.
The d r i v i n g forces to produce these changes i n order of
importance are: the heat produced by the high r e s i s t i v i t y
of the slag and hence the au s t e n i t i c grain growth of the e l e c
trode t i p . The deformation of inclusions and metal matrix which
produce sharp concentration gradients and the non-equilibrium
nature of the incl u s i o n p r e c i p i t a t i o n — a s indicated i n Figure (5)
by the single dotted l i n e — c a n also provide a d r i v i n g force for
t h i s sequence ' 6 4 ~ 1 6 6 \ The Mn depletion around
inclusions i n the metal matrix should also be considered as
another d r i v i n g f o r c e ' 6 4 166,199)^ T h e c n e m i c a - ] _ nature of
inclusions according to t h e i r appearance and thermal h i s
tory (location) can be c l a s s i f i e d as: a) Deformed double
phase o x i - s u l f i d e s and deformed s u l f i d e s , Figures (16,17).
These inclusions were commonly found i n areas where i n c i p i e n t
grain growth was observed, b) Comparatively large globular
s u l f i d e s and o x i s u l f i d e s which were located i n r e l a t i v e l y grown
au s t e n i t i c grains, Figure (22). c) Spherical single oxide phas
dark i n appearance, Figure (23) and d) Relatively small re-
p r e c i p i t a t e d complex (Ca, A l and Si) oxides. These two kinds
of inclusions were located i n p a r t i a l l y and completely l i
quid areas. The l a s t type was p r e f e r e n t i a l l y located i n the
l i q u i d f i l m and i n droplets, Figures (24,25). As shown i n
Figures (18) to (20), areas at which physical and chemical
changes take place either i n inclusions or i n the metal ma
t r i x are very well defined. The presence of the d r i v i n g forces
previously described compensate the r e l a t i v e l y short periods
of time at which the volume of electrode i s therm
a l l y affected. This promotes quasi-equilibrium thermo-chemical
conditions approaching the predicted changes given i n
Figures (4) and (5). Based on the experimental q u a l i t a t
ive (SEM) and quantitative (EPMA) information about how the
electrode t i p and inclusions are p h y s i c a l l y and chemically tran
formed as a p r i n c i p a l consequence of the thermal gradients
and the chemical composition of i n c l u s i o n s , a mechanism which
d e s c r i b e s the removal of o x y s u l f i d e i n c l u s i o n s i s proposed.
Once a g i v e n r e g i o n i n the e l e c t r o d e t i p i s a f f e c t e d
by the heat coming from the l i q u i d s l a g the i n c l u s i o n s and
metal m a t r i x s t a r t to experience c e r t a i n changes. The spher-
o i d i z a t i o n of s u l f i d e s and o x i s u l f i d e s takes p l a c e almost
s i m u l t a n e o u s l y to the growth of the a u s t e n i t i c g r a i n s of the
metal m a t r i x . E x p e r i m e n t a l l y estimated temperatures along a x i a l
p o s i t i o n s i n the e l e c t r o d e and the t h e o r e t i c a l c a l c u l a t i o n s (8)
i n d i c a t e t h a t t h i s t r a n s i t i o n (a + p e a r l i t e -»• y) s t a r t s to
occur a t about 0.5 to 0.8 cm above the l i q u i d f i l m where the
e l e c t r o d e i s a t a temperature of about 8 5 0 - 9 5 0 ° C , F i g u r e s (2,
21). T h i s f i n d i n g approximately corresponds to the alpha
i r o n p l u s p e a r l i t e to gamma i r o n t r a n s f o r m a t i o n . In the zone
where r e l a t i v e l y l a r g e g r a i n s were observed (about 1000°C to
1350°C) s u l f i d e s e n r i c h e d i n manganese are almost t o t a l l y d i s
s o l v e d , a f a c t which i s i n agreement w i t h Turkdogan's and co
workers' f i n d i n g s ^ ^ 4 166)^ F i g u r e s (4) and (5). These r e
s u l t s should a l s o be dependent on the Mn:S r a t i o as i n d i c a t e d
i n the literature'°"*' 1"'" 3* . O x i s u l f i d e i n c l u s i o n s c o n t a i n i n g
s i l i c o n , a c c o r d i n g t o Van V l a c k e t a l . ' 3 0 * , S i l v e r m a n ' ^ 9 * * r, (129,163,168) ,. ., , . . and H i l t y and C r a f t s • are d i v i d e d i n two c a t e g o r i e s ,
namely those i n which the S i : 0 r a t i o i n wt. % i s e i t h e r s m a l l e r
or g r e a t e r than u n i t y .
Low S i phases, S i : 0 r a t i o s l e s s than u n i t y q u a l i t a t i v e l y
obey Turkdogan e t a l . ' s ' * * 4 * e q u i l i b r i u m s t a b i l i t y phase
diagram. In the temperature range of 900° to 1225°C the
equilibrium phases in the Fe-Mn-S-0 system are ruled by the
univariant h i n Figure (5). This univariant i s constituted
by the gamma iron, "0"as [Fe(Mn)0], "MnS" and %^ as l i q u i d
o x i s u l f i d e . Under normal ESR-operating conditions, however,
i t i s not expected that the composition of inclusions s t r i c t
l y follows t h i s univariant (h). Instead the behavior given
by the "triple-dashed"-lines i n t h i s diagram i s expected,
Figure (5). Simultaneous with the above changes solute ac
cumulated i n grain boundaries, growth of some inclusions
and sulf u r depletion from s u l f i d e inclusions are observed.
These events f u l l y coincide with the formation of l i q u i d
o x i s u l f i d e , i^, i n Figures (4,5). It i s important to note
that the s p e c i f i c temperature at which %^ forms, i s s t r i c t l y
a function of the o r i g i n a l amount of Mn present (Mn:S ratio)
and the degree which the iron i s saturated with Mn(Fe)0 and
M n ( F e ) S ' 6 4 ~ 1 6 6 ^ . I t also has to be emphasized that with
commercial steels i n these series which have standard S
content, (0.02-0.05 wt % ) , there i s s u f f i c i e n t Mn and S i
present that no l i q u i d o x i s u l f i d e (^) i s present at temp
eratures lower than 1150°C. Above th i s temperature the l i q -(131 132)
uid o x i s u l f i d e i s expected *~' to show up as material
penetrating the "original" a u s t e n i t i c grains i a fa c t which
was indeed observed.
In areas closer to the f u l l y transformed (austenitic)
grains the Mn content i n inclusions, due mainly to the d i s s o l
ution of sulfur i n the metal matrix i s increased, Figure (4).
Although the Mn content was not s i g n i f i c a n t l y greater the
inc l u s i o n composition q u a l i t a t i v e l y obeyed the univariant h.
From approximately 1150° to 1250°C the composition of low
Si o x i s u l f i d e inclusions changes and £^ i s expected to flux
the s o l i d s u l f i d e as Silverman'*' 9* and Van Vlack et a l . ' 3 0 *
have indicated. Mn w i l l continue increasing at slower rates
than i n previous transformations as the temperature i s
increased.
Upon quenching a sample taken from areas closer to the
f u l l y ytransformed region, duplex ( r e l a t i v e l y grown) o x i -
s u l f i d e s low i n S i should be precipitated; a f a c t which i s
c l e a r l y 'seen i n Figure (22 a-b). Their major constituents
were i d e n t i f i e d as a Mn r i c h s u l f i d e , Mn(Fe)S, and a Mn
r i c h oxide, Mn(Fe)0. At temperatures higher than 1250°C the
manganese s u l f i d e enriched phase p r a c t i c a l l y disappears;
a fact which was observed i n the q u a l i t a t i v e (SEM X-ray
spectrum analysis) and semiquantitative (EPMA) analysis, F i g
ures (23 and 25). At temperatures about 1370° to 1420°C r e l a t
i v e l y large a u s t e n i t i c grains are observed. The region which
i s exposed to thi s temperature range experiences the gamma
to delta iron transformation. This t r a n s i t i o n i s i d e n t i f i e d
i n Figure (5) as the in t e r s e c t i o n of the "triple-dashed" l i n e
crossing the univariant g_ i n which delta and gamma iron, 0
136
and £^ are i n equilibrium. The major changes i n inc l u s i o n
composition w i l l s t a r t just a f t e r univariant f i s reached,
f i s a univariant equilibrium which i s constituted by delta
iron, o, ^ and &2, where &2 i s the l i q u i d metal. After
f_ i s reached the only s o l i d compounds may be the Fe(Mn)0
and a very small amount of ir o n oxide. It i s important to
r e c a l l that t h i s event takes place only i f Si content i s low • 4-u i x-o- (130,169) i n these l a t t e r stages '
Under t h i s condition, between f_ and e univariants where
delta i r o n , o and £,2 are i n equilibrium the remaining S from
inclusions goes i n solution i n delta iron. Furthermore,
the remaining inclusions w i l l be exclusively constituted by
Mn(Fe)0. This l a s t step was p a r t i c u l a r l y clear i n the range
at which the l i q u i d f r a c t i o n was about 0.5. In areas where
the l i q u i d f r a c t i o n was greater than 0.5 inclusions were
completely dissolved and reprecipitated. Since t h i s region
was i n contact with the slag some reprecipitated small i n
clusions (very few) Figures (24, 25), show the slag char
acter, i . e . , some A l and Ca.
The second category of o x i s u l f i d e inclusions which Si:0
r a t i o i s larger than unity exhibit an almost equivalent pat
tern as the previous behavior ( i . e . Si:0 < 1), except i n the
l a s t stages. Instead of p r e c i p i t a t i n g Mn(Fe)0 and FeO, S i 0 2
and MnO or 2Mn0 w i l l p r e c i p i t a t e i n the f u l l y austenitized
zone and when l i q u i d fractions are smaller than 0.5
These transformations as proposed by Silverman'** 9*
and Van Vlack et a l . ^ 1 3 ^ * can be represented by quaternary
diagrams, Figures (6) and (7). As shown i n Figures (6)
and (7) as the temperature increases Mn ( and thus 'MnO')
and the proportion of o r t h o s i l i c a t e increases. As a r e s u l t
the liquidus for the "MnS" i s decreased, thus over a s i g
n i f i c a n t range of compositions the amount of l i q u i d o x i -
s u l f i d e i s increased. Silverman'** 9* has pointed out that
i f the proportion of s i l i c a t e i s increased, as observed i n
th i s case then at temperatures above 1300°C what i s l e f t
as a s o l i d (from the previous Fe-O-S-Mn-system, FeO and
Mn(Fe)O) w i l l become almost completely l i q u i d . These
transformations as proposed by Van Vlack et a l . ' 3 0 * occur
from C to B i n Figure (8a) and C to B i n Figure (8b).
During t h i s temperature increase "FeS", "MnS" and "FeO" are d i s
solved, Figure (25). Subsequent chemical reactions of the i n c l u
sion components which take place i n the f u l l y austenitized met
a l and the p a r t i a l l y l i q u i d zones are shown i n macrophotographs
(18a-b) and (21) and photographs (23a) and 23b). These reactions
are represented by:
2MnO + S i 0 2 Z 2MnO«Si0 2 «- Mn 2Si0 4
or
2MnO + S i 0 2 X MnO«Si0 2 + MnO t Mn 0 + MnSi0 3
138 If a sample with t h i s composition and thermal history
i s r a p i d l y cooled then . single phase inclusions enriched i n
manganese and s i l i c o n must be found, a fact which i s l a t e r
corroborated with the inclusion chemical analysis. Their
f i n a l composition, according to Van Vlack et al . ' s work ' 3 °V i s dictated by the Si:0 r a t i o and the amount of manganese
present i n the electrode. Thus, i f the S i content exceeds
the amount of oxygen, as seen i n analysis from samples of _3
thi s series (0.25 wt% Si and 90 ppm = 9.0 x 10 wt!) then
a v i t r e o u s ( s i l i c e o u s ) type of in c l u s i o n i s formed i n place
of the monophasic tephroite (MnO'SiC^) or rhodonite
(2MnO'SiO^) as the temperature i s increased, Figures (23a,b).
Some electrodes which were very rapidly withdrawn from
the slag showed small s i l i c e o u s and (iron) s u l f i d e s as re
pre c i p i t a t e d inclusions exclusively in the l i q u i d f i l m .
Their q u a l i t a t i v e (SEM) and semiquantitative (EPMA) analysis, (15)
as reported, i n the l i t e r a t u r e i d e n t i f i e d them as non-
stoichiometric f a y a l i t e ^FeOSiC^) compounds.
To confirm the previously described mechanism a seq
uence of t y p i c a l analysis i s shown. The electrode chemical
(spectrographic) analysis i n wt. % as received, i s as
follows: C Mn S Si P 0.19 0.71 0.026 0.025 0.01
The average t o t a l oxygen analysis as received, was 90 ppm.,
i . e . at centreline 87 ppm., midradius 91 ppm. and edges
93 ppm. The average i n c l u s i o n chemistry i n at.% as re
ceived, was S S i Mn Fe 35.0 27.0 37.0 balance
The average chemical composition of inclusions located bet
ween the r e c r y s t a l l i z e d area and l i q u i d f r a c t i o n s smaller
than 0.5,were (in at.%) as follows
Si Mn S Fe 51.0 46.0 2.0 balance
This composition immediately suggest the formation of teph-
r o i t e ( 1 3 0 > .
Due to the number and size of inclusions found i n the
neighbourhood of the l i q u i d f i l m chemical analysis from
inclusions located i n l i q u i d f r a c t i o n s greater than 0.6
were performed only on a q u a l i t a t i v e basis. Calcium and
aluminum i n these inclusions as shown i n Figures (24)
and (25) were traced.
This p a r t i c u l a r electrode was remelted through a
(50 wt% CaF 2, 30 wt% Al 2C> 3 and 20 wt% CaO) slag which was
Ca-Si deoxidized. The chemical analysis of inclusions from
l i q u i d pool (a) and from the (ESR) ingot (b) i n at.%.
140 are as follows:
A l Ca S Mn Fe a) 49.0 30.0 20.43 0.3 balance b) 46.0 30. 5 22.80 balance
Thus, i n summary the change in inc l u s i o n chemical com
po s i t i o n i n terms of sulfides and low Si-oxysulfides and the
morphological changes of inclusions take place i n austenitic
temperature ranges i n the metal matrix. In intermediate stages
the d i s s o l u t i o n of s u l f i d e s i n the metal matrix also occurs.
These series of events represent approximately 10-30% of the
t o t a l "transformation-dissolution" mechanism and i t takes place
at about 0.4 - 0.8 cm above the l i q u i d f i l m . The next 20-
40% of the transformation of inc l u s i o n phases occur either
between the g_-f univariants, Figure (5), i n the low S i con
tent phases or i n the C-B sequence of transformations, Figure
(8a), i n the high S i content phases. The next sequence of
transformations takes place between f-e and B-A for low and
high S i content phases respectively. I t occurs between the
f u l l y austenitized region and the point where the l i q u i d f r a c
t i o n does not exceed 0.5. It represents another 20-40%. The
remaining "transformation-dissolution" takes place i n the
neighbourhood of the l i q u i d f i l m where inclusion d i s s o l u t i o n
i s the major mechanism and in c l u s i o n slag reactions (form
ation of lower melting point i n c l u s i o n phases with slag char
acter) acts as a very limited mechanism (^1.0-3.0%).
141
5.1.2 Removal of Oxide and Sulfide Inclusions i n 4340 and
Rotor Steels
Since the aim of th i s research i s to gain an extensive
understanding of the inclusion-removal mechanism; electrodes
with d i f f e r e n t matrix-inclusion compositions were also anal
yzed. 4 340-electrodes with alumina type of inclusions and
tool s t e e l electrodes with calcium-aluminum-silicates were
also included i n t h i s research program.
Macrophotographs (19) and (20) show the sequence of trans
formations i n 4340 and rotor steels respectively. One of
the f i r s t differences to notice between these two types of
steels i s that i n the l a t t e r type a u s t e n i t i z a t i o n did not
occur due to t h e i r chemical composition (electrode) and to
the chemical composition of inclusions. Another conse
quence of t h i s i s that solute penetration (between austeni
t i c grains) at subsolidus temperatures i n the rotor s t e e l
did not take place. On the other hand i n the p a r t i a l l y
molten electrode much more"segregated material," as a re
su l t of the i n c l u s i o n d i s s o l u t i o n and the in t e r a c t i o n of
the electrode with the slag, was observed at l i q u i d f r a c t i o n s
larger than 0.5.
Once the d i f f e r e n t areas were i d e n t i f i e d ( i n c i p i e n t l y
heat affected area, semi or austenitized region, l i q u i d
f i l m i n or out of the droplet) i n each type of electrode,
meticulous chemical analysis of inclusions (by EPMA) was
c a r r i e d out at every 250-300 ym. Results obtained from 4340
and rotor s t e e l electrodes are shown respectively i n Figures
(26) to (28).
Among the most important conclusions from t h i s study
are the following: a) since the au s t e n i t i c transformation
i n the rotor s t e e l was almost absent the inclusion transform
at i o n - d i s s o l u t i o n started to take place p r i n c i p a l l y i n the
p a r t i a l l y l i q u i d zone. It takes place about 0.3-0.5 cm above
the solidus. b) A l and S i n inclusions i n both types of
electrodes (4340 and rotor steel) were gradually dissolved
as the electrode experienced higher thermal gradients. These
changes, shown i n Figures (26) to (28) s t a r t to take place
i n a very discrete manner just above the solidus isotherm
for the rotor s t e e l and i n e a r l i e r (lower temperature gradi
ents) subsolidus temperatures i n the 4340 ingots. c) In
graph (27), i t can be observed that due to a strong grain
growth and thus intergranular segregation, the A l , Mn and S
compositions were s h i f t e d i n 2500-4500 ym range. These re-u . . ... . (1,17,18,129,131-133,
suits i n agreement with previous research ' ' ' ' ' 163 169) a n ( j w ^ t n p r e v i o u s findings i n the 1020-series
show that s u l f i d e s are the most thermally affected phases.
Oxide inclusions i n deformed 4340 electrodes also experi
enced gradual morphological changes. d) The largest changes
i n the i n c l u s i o n chemical composition i n both electrodes i n
a manner equivalent to the 1020 M.S. electrodes oc
curred i n the p a r t i a l l y l i q u i d region. The presence of
strong intergranular and i n t e r d e n d r i t i c (segregated) material
which contained inclusion-formers c l e a r l y indicate that i n
clusions were completely l i q u i d i n this transient zone.
Figure (19a) from 4340-electrodes and (20) from the rotor
s t e e l electrodes strongly corroborate these findings,
e) Chemical analysis i n 4340 electrodes, Figures (26,27),
suggest that Si and Mn i n inclusions, p a r t i c u l a r l y i n the
l i q u i d f i l m follow the same behavior as i n 1020 M.S., i . e .
inclusions get richer i n S i and Mn j u s t before they are
t o t a l l y dissolved.
144
5.1.2.1 Removal of Oxides and Sulfides i n 4340-electrodes
The sequence of changes i n the chemical composition
of inclusions i n these electrodes i s summarized as follows.
The spheroidization and a subsequent d i s s o l u t i o n of s u l
fides just as i n previous observations i n the 1 0 2 0 M.S.
electrodes i s the f i r s t step i n the removal mechanism. It
takes place i n subsolidus temperature ranges. Figures (19),
(26) and (27) indicate that these changes occur at about 2000-
3000 ym above where the solidus of the a l l o y was metallographi-
c a l l y i d e n t i f i e d . The most d r a s t i c changes, however, take place
in areas where complete a u s t e n i t i z a t i o n was observed.
In f u l l y austenitized areas and i n the region
where the melting started the A l and S i n inclusions de
crease while the S i , Mn and the Ca correspondingly increase.
This behavior i s accentuated as the l i q u i d f r a c t i o n i n -(9192)
creases. A mixture of feldspars ' (MO "A^O^ «2Si02) and garnets (3M0«Al20.j *3Si02) ° r e v e n c o r d i e r i t e s (2M0*
2 A I 2 O . J • 5Si02) instead of the o r i g i n a l aluminates should
p r e c i p i t a t e as the l i q u i d f r a c t i o n approaches unity. The actual stoichiometry of t h i s compound i s i r r e l e v a n t since
M i n a l l of these compounds cart be either Fe, Mn or Ca and
these series of compounds show v i r t u a l l y complete mutual (91 92)
s o l u b i l i t y ' . The most important finding, however,
i s that oxides i n t h i s matrix are suddenly transformed
not so much in subsolidus temperature ranges as occurs i n
145
the 1020 M.S. as i n the semi-liquid stage. The presence
of i n c l u s i o n r e l i c s and " l i q u i d " enriched i n i n c l u s i o n
constituents i s shown i n Figures (19c) and (2 0a) as i n t e r
d e n d r i t i c segregates.
In the l i q u i d film, about 50ym from the edge of the
electrode t i p a similar e f f e c t as that seen i n the 1020
M.S. electrodes was observed. Since t h i s volume of l i q u i d
electrode was subjected to a d i r e c t i n t e r a c t i o n with the
slag a s h i f t i n the chemical composition of inclusions was
detected. This indicates that the electrode i n c l u s i o n
composition was completely transformed. The chemical analysis
of inclusions i n the l i q u i d f i l m (a) i n the droplet or
i t are (in at.%) as follows:
A l Ca S Si Mn
a) 19.0 2.78 _ 78.0 0.22 b) 7.0 3.90 1.5 70 .20 17.40
a) 8.27 9.30 _ 56.0 26.46 b) 6.30 11.00 - 56.0 27.60
4340 (I)
4340 (II)
F i n a l l y , the chemical composition of inclusions i n either
(a') l i q u i d (pool) stage or .(b')the ingot correspond to:
434 a') Al„0_ and traces of s i l i c a + MnS II
0 (I) 2 3
b') A1 20 3 + MnS I I
4340 (II) a') Ca«Al 20 3
b') 12CaO-7Al 20 3
146
It i s important to note that 4340 (I) was Al-deoxidized
and 4340 (II) was CaSi treated. Ingot 4340 (I) was s l i g h t l y
deoxidized 0.02 kg ton-"'') and ingot 4340 (II) was heavily
CaSi deoxidized (10 Kg t o n - 1 ) both ingots were refined
through a 50wt % CaF 2, 30 wt% Al 2C> 3 and 20 wt% CaO.
If the above chemical analyses are compared i t can
c l e a r l y be seen that inc l u s i o n r e l i c s from the bulk of the
electrode, i f any, are dissolved and reprecipitated i n c l u s
ions which show the slag character. Inclusions i n the l i q u i d
f i l m are small and complex i n composition. Therefore, i n
clusions located in t h i s narrow f i l m do not represent i n
any way what happened i n the transformation-dissolution of
previous stages.
5.1.2.2 Calcium-Aluminum S i l i c a t e s i n a Rotor (Ni,Cr,Mo)
Steel
The i n c l u s i o n chemical composition was meticulously
determined at discrete locations i n electrode t i p s . These
analysis were carr i e d out at every 250-300 ym st a r t i n g from
the l i q u i d f i l m . For the sake of c l a r i t y as i n previous
studies (1020 and 4340), they were arranged according to
the s p e c i f i c area at which they belonged i . e . , l i q u i d
f i l m (in and out of droplet), p a r t i a l l y molten area at
several l i q u i d f r a c t i o n s and at subsolidus temperatures.
147
A summary of the results obtained by EPMA and t h e i r c l a s s i f i c a t i o n
according to the thermal history at which these volumes were
subjected i s shown i n Figure (28) *
Since these electrodes experienced almost no grain growth
during a u s t e n i t i z a t i o n , the composition of inclusions was exclu
s i v e l y changed i n regions close to the solidus temperature of the
a l l o y . It i s important to notice that the size of the mushy zone
i n the rotor s t e e l (> 2800 pm) i s larger than i n 1020 M.S. or the
4340 s t e e l s . The sl a g - i n c l u s i o n d i s s o l u t i o n (in only very few
remaining inclusions) took place i n a manner equivalent to a l l
of the other electrodes, i . e . l i q u i d f i l m .
T y pical analysis of inclusions from these areas i n at. %,
are as follows:
P a r t i a l l y L iquid (f^ ^ 1) A l Ca S i S Mn
ingot (1) 25.18 61.15 8.65 5.0
ingot (2) 25.88 61.36 9.40 3.5
Liquid Film
out of droplet 38.48 20.20 34. 85 6.50
i n droplet 61.16 21.63 16.38 0. 82
After ESR 62.35 2.27 3.16 0.16 1.16
148
These ingots were refined through the 50% CaF 2,
30% A^O^ and 20% CaO slag. Their deoxidation was ca r r i e d
out with the CaSi a l l o y at approximately 0.1-0.2 Kg ton 1 .
5.1.3 F i n a l Remarks About the Removal Mechanism
Inclusions i n electrode t i p s during ESR are removed by
di s s o l u t i o n i n the metal matrix in well defined steps ac
cording to the electrode i n c l u s i o n composition and location
i n the electrode t i p . The thermal gradient to which the
electrode and hence inclusions are subjected i s the main
dri v i n g force for the i r removal. This mechanism, however,
does not correspond to that previously described i n the
l i t e r a t u r e review. Gradual d i s s o l u t i o n along the heat
affected zones or s t r i c t d i s s o l u t i o n of inclusions at the
l i q u i d film, and the "washing o f f " or the mechanical re
moval of inclusions are not operative mechanisms. In
stead a mechanism based on substantial experimental and
the o r e t i c a l evidence which suggests the quasi-thermo-
chemical equilibrium of inclusions and the i r location
i s indicated.
The series of actual chemical transformations and the
di s s o l u t i o n of inclusions in electrode t i p s are s t r i c t l y
confined to a distance no greater than 0.5 - 0.8 cm above
the l i q u i d f i l m . Sulfides i n the Fe-Mn-S-0 system and o x i -
s u l f i d e s i n the Fe-O-S-Mn-Si system are p a r t i a l l y dissolved
and p a r t i a l l y transformed at subsolidus temperatures..
149
Reactions between i n c l u s i o n components and the metal matrix
leads to the solution of ce r t a i n inclusion components such
as "FeS", "FeO", "MnS", etc., which i n i t i a t e solute penetra
t i o n i n au s t e n i t i c grain boundaries. This f a c t implies that
these inclusions are a mixture of phases, namely "MnS", "FeO"
and a small amount of "FeS". The chemical reactions to which
inclusions are subjected and the i r d i s s o l u t i o n i n the metal
matrix are q u a l i t a t i v e l y predicted by using diagrams available (164 —16 6)
i n the l i t e r a t u r e . Sulfides belonging to the Fe-Mn-
S-0 system and o x i s u l f i d e s low i n s i l i c o n which belong to the
general Fe-O-S-Si-Mn system i n electrode t i p s follow a quasi-
thermochemical equilibrium dictated by the Figure (5). This
equilibrium phase diagram obtained by Turkdogan and co
workers i s n o t completely obeyed and instead a be
havior given by the " t r i p l e dashed l i n e " i s followed.
Inclusions belonging to the Fe-Si-Mn-O-S system which
have a s i l i c o n content such that the Si:0 r a t i o i s greater
than 0.5, follow the FeO-MnS-MnO-Si02 quaternary system dev
eloped by Silverman'** 9^ as indicated by the arrow i n F i g
ure (9). The behavior of these inclusions i s also complemented
by the binary MnO-Si02 as part of the Si02~MnO-FeS-MnS quat
ernary diagram developed by Van Vlack et a l . , Figures
(8a) and (8b). This proposal corresponds largely to that a l -(168)
ready suggested by H i l t y and Crafts , i . e . the pseudo
150
ternary behavior of the metal-oxide-sulfide phases. Thermo-
dynamically more stable phases such as Mn(Fe)0, MnO, Si02^
MnO«Si02A 2Mn0«Si02/ calcium aluminates, calcium s i l i c o n a l
uminates, etc., are transformed and dissolved i n the neigh
bourhood of the l i q u i d f i l m between the f u l l y austenitized
area and the f u l l y l i q u i d metal. Electrode i n c l u s i o n r e l i c s
( i f any) i n the l i q u i d f i l m react, up to a limited extent
with the slag producing complex in c l u s i o n phases.
F i n a l l y , inclusions i n ESR ingots which show the e l e c t
rode i n c l u s i o n chemical composition are found only under un
stable ESR conditions, i . e . at the s t a r t i n g and during the
"hot topping" stages.
151
5.2 The Chemical Influence of the Electrode, Slag
and Deoxidizer on the Chemical Composition of
Inclusions
5.2.1 Description of Experimental Findings
5.2.1.1 Preliminary Studies on the E f f e c t of the Slag and
the Deoxidation
4340-electrodes 31.75 and 44.75 mm i n diameter and with
d i f f e r e n t i n c l u s i o n chemical compositions were refined to 75 mm
i n diameter ingots at melting rates of about 1.3 Kg m i n - 1
through d i f f e r e n t slag systems and under a protective (argon)
atmosphere.
The components of the slag systems (CaF 2, CaO, Al-^O^
and Si0 2) were previously dried at 650°C and the "cold
s t a r t i n g procedure" was followed. Electrodes were refined
through slags which had several S i 0 2 contents. Two d i f
ferent slag systems were chosen to be deoxidized with a
CaSi a l l o y and A l , Table (X). The aluminum was i n the
form of p e l l e t s (99.9%). The CaSi a l l o y contained 62.5
152
wt. % S i , Table (IX). The deoxidation rates were constant
(̂ 2.3 Kg/ton) and they were performed when steady remelting
conditions were observed.
The purpose of these experiments were: i) to determine
the chemical composition of inclusions as an exclusive ef
fe c t of the slag composition (and electrode). i i ) to i n
vestigate what changes i n i n c l u s i o n compositions could be
achieved by using the same slag system and deoxidizers and
to compare r e s u l t s from (i) against ( i i ) and ( i i i ) to det
ermine the most appropriate slag systems to be used i n the
200 mm i n diameter ESR-furnace.
Experimental r e s u l t s are shown i n Table (X) where the
slag chemical composition i n wt. % # the incl u s i o n chemical
composition i n at. %, the chemical composition of inclusions
i n electrodes and the major in c l u s i o n phases are given. It
i s important to note that although chemical analysis of i n
clusions was performed on a large number (30-40) a wide
scatter (± 5.0 - 7.0%) i n th e i r analysis was found. The
scatter i s p r i n c i p a l l y attributed to the i n c l u s i o n size
153
d i s t r i b u t i o n s (less than 5ym i n diameter) and to the un
steady r e f i n i n g conditions due to the use of inappropriate
slag systems. As a consequence of t h i s lack of s t a b i l i t y
occasionally an uneven surface of the ingot was observed
and l i q u i d enriched i n deoxidizers and oxygen pr e c i p i t a t e d
alumina type of inclusions i n a confined volume, Figure (29).
The f i r s t set of experiments performed without a de
oxidizer showed s i l i c o n i n inclusions where SiC^ i n slags
was higher than 10 wt %. Calcium-aluminum s i l i c a t e s i n
inclusions were found above t h i s l e v e l . The presence of
more than two i n c l u s i o n phases was commonly observed i n the
same sample. Table (X).
The second series of experiments i n which deoxidation
was c a r r i e d out, s p e c i f i c a l l y ingots (7) and (9), showed
v i r t u a l l y the same behavior as ingot (1). The chemical
composition of inclusions i n ingot (1), used as a reference
showed almost exclusively calcium aluminates. Ingots (7)
and (9) also showed calcium aluminates, the Ca:Al r a t i o ,
however i s larger than i n previous cases. The s i l i c o n con
tent of inclusions i n ingots (1) and (7) were equivalent,
whereas i n ingot (9). i t was higher.
If the analysis of inclusions from electrodes used i n
ingots (1) to (7) are compared against (9) and (10) then
154
i t can be inferred that the increased S i content comes from
the electrode, Table (X). The maximum calcium content, as
calcium aluminates, was found i n ingots remelted through
r e l a t i v e l y high CaO (15-22 wt %) and r e l a t i v e l y low S i 0 2
(less than 10 wt %) slags.
The i n c l u s i o n chemical analysis performed i n ingots (2)
and (10) shows that by r e f i n i n g electrodes through the (55/
15/15/15) slag system, t h e i r composition with and without
Ca-Si deoxidation remains unaltered. On the other hand,
for electrodes refined under the same slag system and de
oxidized with aluminum; lower s i l i c o n and r e l a t i v e l y higher
calcium and hence higher aluminum i n incl u s i o n phases i s
found.
Results i n t h i s i n v e s t i g a t i o n c l e a r l y show that the i n
t r i n s i c slag e f f e c t i n the chemical composition of inclusions
follows a very well defined pattern. Slag systems i n which
the S i 0 2 content i s lower than 10 wt % as i n ingots (1), (4),
(5), (7), and (9) yielded i n c l u s i o n compositions which pre
dominantly l i e i n the CaO-Al 20 3 pseudo binary phase diagram
on the A l 2 0 3 r i c h side, i . e . alumina types and low CaO-
aluminates.
To corroborate these findings other series of ex
periments i n the 200 mm ESR-furnace were performed. Ex
perimental d e t a i l s and a summary of findings are given i n
155
Tables (VIII) and (XI). The main point to be considered
i n t h i s set of experiments i s the low l e v e l of deoxidation
(0.02 kg ton 1 ) to which the s l a g - l i q u i d pool was sub
jected and the electrode surface preparation. As seen i n
Table (XI) consistent q u a l i t a t i v e r e s u l t s are found i n
terms of the chemical composition of inclusions i n both
ESR-furnaces.
P a r t i c u l a r emphasis should be given to r e s u l t s ob
tained from ingot (11). A rotor s t e e l electrode with
chemical composition i s given i n Table (VII) and with an
average i n c l u s i o n chemical composition (in at%) as follows:
A l Ca S i S Mn Fe
28.5 24.1 42.0 2.2 2.5 balance
was remelted through a 49 wt % CaF 2, 16 wt % CaO, 17 wt %
A^O-j, 12 wt % S i 0 2 and 6 wt % MgO slag. The average chemical
composition obtained from 40 inclusions i n the (ESR) ingot,
i n at.% i s as follows:
A l Ca S i S Mn Mg + Fe 44.70 13.60 24.5 7.4 9.2 balance
Since the at.%Mn:at.%S r a t i o i s approximately one then
i t can be assumed that a MnS phase was pr e c i p i t a t e d . The
s u l f i d e phase was commonly found surrounding the oxide phase.
156
X-ray composition p r o f i l e s and maps as well as dispersive
X-ray spectrum analysis confirmed these findings. The
oxide phase was not spherical (as calcium aluminates-
calcium s u l f i d e inclusions), instead angular oxides were ob
served. If the above values are normalized then the re
maining elements can be further analyzed; thus the o v e r a l l
composition i s
A l Ca S i ^54 ^16 ^30
From these computed values the r a t i o s for A l , Ca and S i res
pectively are approximately 4:1:2. Thus, the f e a s i b l e phase
present i n these type of inclusions could be:
CaO«2Al 20 3'2Si0 2
This compound, as i n d i r e c t l y stated i n the l i t e r a t u r e review* 9 2 ^ has not been reported as an i n c l u s i o n phase; i t can be
instead a feldspar of the type M0«A1 20 3«2Si0 2 i n which MO
can be either FeO, MgO, MnO or CaO. Based on i n d i v i d u a l
analysis of inclusions; alumina r i c h phases and complex
CaO-Al 20 3 s i l i c a t e s were indeed the major phases present.
Hence, the only f e a s i b l e compounds i n t h i s ingot are the
Al 20 3«CaO•2Si0 2 (anorthite) phase i n conjunction with an
Al_0_ (corundum) enriched phase.
157
5.2.1.2 Intermittent CaSi Additions and the Reaction Scheme
Since no difference was found between deoxidizers
(Al and the Ca-Si alloy) i n small ESR-furnace, i n terms of
the chemical composition of inclusions ( i . e . p r e c i p i t a t i o n
of calcium aluminates and up to a given extent calcium
sulphides), an extension of these experiments i n the semi-
i n d u s t r i a l size ESR-furnace was ca r r i e d out. These experi
ments are l i s t e d i n Table (VIII). 50 grams of FeO and
the Ca-Si all o y were alt e r n a t e l y added at two discrete time
i n t e r v a l s during r e f i n i n g under two d i f f e r e n t slag systems;
namely 50/30/20 and 70/30/0. These figures represent the
CaF 2, A^O^, and the CaO compositions i n wt %. 1020 M.S.
electrodes whose chemical composition and i n c l u s i o n chem
i s t r y have already been described, were refined at about
1 kg min 1 , with and without (RIII-W and RII-W respectively)
an argon atmosphere shielding.
Total oxygen content, slag chemical analysis from samples
taken during r e f i n i n g , ingot chemical analysis and incl u s i o n
chemical composition as well as size d i s t r i b u t i o n s from
ingots were determined. These re s u l t s are shown i n Figures
(30) to (36). Ca, F, A l , S i , Mn and Fe and Mn, C, P, S,
S i , Mo, Cr, Al and N i were analyzed i n the slags and ingots
respectively. Elements with s i g n i f i c a n t changes i n t h e i r
composition are plotted.
Figures (32) and (33a-b) from RII-W and RIII-W, show the
158
e f f e c t of the Ca-Si a l l o y (intermittent) additions. These
graphs show that the deoxidizer produces a sharp decrement
in a l l of the slag constituents. Correspondingly, the chem
i c a l composition of ingots show a sharp increment i n Mn,
A l and S i , Figures (34,35). Other important points to be
considered from these graphs are that s u l f u r i s decreased
when the Ca a l l o y was added and iron oxide additions in the
slag did not produce as sharp changes i n the ingot composi
tion as did additions of the deoxidizer. The s i l i c o n which
comes from the electrode gradually increased i n the slag
as r e f i n i n g takes place, Figure (32) and (33b).
The response time of the system was also observed. RII-W
showed a response to the deoxidizer i n about 100-150 seconds
and RIII-W within 200-250 seconds. The e f f e c t of the deoxidizer
i n the slag and ingot decrease i n a r e l a t i v e l y slow manner.
The sudden changes i n FeO content (expressed as wt % of Fe)
i n the slag as well as the abrupt changes i n the t o t a l oxygen
content and the i n c l u s i o n chemical composition as a d i r e c t
e f f e c t of these intermittent(Ca-Si) additions are the most
important responses i n the refined ingot, Figures (30) to
(33). I t i s important to notice that there was not enough
reaction time to show the•individual Ca or FeO effects i n terms
of oxygen i n (RIII-W), Figure (30).
The difference between RII-W and RIII-W are the
s t a r t i n g slag compositions and the presence of an argon
shie l d i n g atmosphere i n the l a t t e r , Table (VIII). Thus a
159
lower oxygen content (15-20 ppm) i n RIII-W was expected
and observed, Figures (30) and (31).
The main conclusion from the above r e s u l t s i s that the
ESR-process i n terms of slags and deoxidizers i s a very com
plex reactor. The series of reactions taking place i n the
slag do not occur independently of the ones taking place i n
the l i q u i d pool and i n the ingot, i . e . the i n c l u s i o n com
po s i t i o n i s not c o n t r o l l e d by one single factor.
5.2.1.3 Refining of 1020 M.S., 200 mm Diameter Ingots
Deoxidized Continuously with Aluminum
To gain a better understanding of the above sequence of
reactions, complementary and more detailed experiments by which
reactions i n the l i q u i d slag l i q u i d pool and ingots could be
monitored were planned. The next set of experiments included the
r e f i n i n g of electrodes through equivalent slag compositions. The
purpose of these experiments was to discriminate the i n t r i n s i c
chemical e f f e c t of the electrode i t s e l f on the slag ( i . e .
without any deoxidizer).and to i d e n t i f y separately the ef
f e c t of deoxidizers on the slag during the three stages of
the r e f i n i n g process. The i n t r i n s i c electrode-slag-in
clusion chemistry of a refined (1020) ingot, without deoxid
ation and remelted through an i n i t i a l 50 wt% CaF^, 30 wt %
A^O^ / and 20 wt % CaO, was used as the basis of comparison
for other ingots, CaSi and Al-deoxidized.
160 The ingot i d e n t i f i e d as RI-Il was refined under an
argon atmosphere and i n the absence of a deoxidant. The
"FeO" i n the slag and the t o t a l oxygen content remained
approximately constant i n the i n i t i a l stages, Figure (37).
Inclusions i n samples extracted from the l i q u i d pool and from
the ingot were e s s e n t i a l l y i d e n t i f i e d as spherical single
p a r t i c l e s or as small clusters of alumina type (FeQ'A^O^
and a-A^O-j) and less frequently as f a y a l i t e type (2FeO«Si02) •
The alumina type was usually associated with manganese s u l
fides, (Fe,Mn)S and MnS I I . The gradual increase of S i 0 2
i n the slag was considerable. I t ranged from 0.75 wt.% at
the bottom and up to about 2.0 wt.% at the top of the ingot.
The s i l i c o n content in the ingot ranged from 0.095 up
to 0.125 wt.% from s t a r t to f i n i s h . The ingot chemical an
a l y s i s i s shown i n Figure (38).
Experiments which are equivalent to the small (Ca-Si
and Al) deoxidized (4340) ingots were also c a r r i e d out. 1020
M.S. electrodes were refined through equivalent slags and
samples from l i q u i d slags and pools were extracted while continu
ously increasing additions of deoxidizers to the slag were
made. Table (VIII) summarizes the d e t a i l s of t h i s set of
experiments. Refined ingots referred to as R I I - I l and
RII-I2 were aluminum treated by using two d i f f e r e n t deoxid
ation sequences. Deoxidation rates were 3.63, 6.1, 6.8 and
7.6 kg t o n - 1 and 1.21, 2.42, 3.64, 4.85, 6.06 and 12.12 kg ton - 1
161
for R I I - I l and RII-I2 respectively. RII-I2 was CaSi (50
grams) deoxidized i n the i n i t i a l r e f i n i n g stage. ..
The iron oxide content, given as wt- % iron i n Figures
(39) and (40), slowly and continuously decreased from about
0.6 wt.% down to about 0.4 wt-% as the aluminum addition
rates were increased. While high deoxidation rates produced
r e l a t i v e l y steady t o t a l oxygen content (RII-Il) and hence
equilibrium behavior i n i n c l u s i o n compositions, the low de
oxidation rates produced an o s c i l l a t i n g behavior i n both
parameters, (RII-I2), Figures (41, 42) and (43,44).
Findings i n these experiments, although not as drama
t i c as the (Ca-Si) intermittently deoxidized ingots, show
that the aluminum as a deoxidizer also produces simult
aneous exchange reactions between two l i q u i d s of the general
type (11) and reactions of the type .(12). The elements i n
volved i n these reactions, i n a manner equivalent to the i n
termittent Ca-Si additions are the S i , Mn and the A l by
i t s e l f , Figures (37-40) and (43-46).
The most sensitive parts of the system to the aluminum
deoxidation were the t o t a l oxygen content and the chemical
composition of inclusions represented by the at.%Ca:at% A l
i n Figures (43) and (44).
Figure (40) from RII-I2 shows that the aluminum was
able to slowly overcome the s i l i c o n e f f e c t from the electrode.
162
Hence, from the calcium to aluminum, r a t i o of inclusions,
the chemical composition of ingots and slag, i t i s inferred
that the major reactions which govern the chemical composition
of inclusions d e f i n i t e l y involve the CaO and A^O^ from the
slag. On the other hand the s i l i c o n - from electrodes,
although i t i s transported i n the ingot , Figures (45,46), does
not play a role i n the deoxidation scheme.
The sequence of in c l u s i o n formation was as follows:
1) i n the bottom part of these ingots, where r e f i n i n g was
unstable some inclusions (approximately 5%) contained
s i l i c o n . This was almost invariably located i n the i n
clusion core and associated with manganese and calcium.
This i s considered as a clear i n d i c a t i o n that some inclusions,
at low r e f i n i n g e f f i c i e n c i e s , come from the electrode and i n
subsequent stages they are transported d i r e c t l y into the
ingot which i s also s o l i d i f y i n g under unsteady state condi
tions. The bulk of these inclusions are, however, mainly
represented by small single or clustered type (alumina
galaxies) of inclusions, Figures (47) to (48). 2) As the
degree of deoxidation i s increased, globular and faceted
single inclusions were observed (FeO-A^O^ and a-A^O^ res
p e c t i v e l y ) , Figure (49).
These types were usually associated with a s u l f i d e
phase (MnS II) and 3) At the highest deoxidation rates,
163
a mixture of spherical and faceted alumina with hexagonal
aluminates were observed, Figure (50). This l a t t e r type had
a peripheral double, (Mn,Ca)S, s u l f i d e . These findings
are shown i n Graphs (51) and (52) where the Ca:Al r a t i o
against the S:Mn i n at.% are plotted. These figures were
obtained from inclusions i n R I I - I l and RII-I2 respectively.
R I I - I l which was a heavily deoxidized ingot showed
very highly segregated material. These segregates which
under the electron beam produced a red fluorescence occurred
i n the t h i r d deoxidation l e v e l , Figure (53) and t h e i r t y p i c a l
composition i n at %, was as follows:
A l Ca S i Mn 26.23 45.67 17.19 Balance
Composition maps shown i n Figures (53a-d) are t y p i c a l of
these segregates. Their area ranged from 10ym2 up to 65-70ym2.
This finding also confirms the "multi-exchange-reacting"
nature of the deoxidation i n the ESR-process, e.g. exchange
reactions of the type (12 a-b). These types of segregates
were commonly seen i n samples extracted from l i q u i d pools.
In these types of samples inclusions containing s i l i c o n
and occasionally f a y a l i t e type of inclusions were also
found.
Based on mass balances the "FeO" content of the slag
during r e f i n i n g changed from 0.45 wt % down to 0.27 wt %
164
i n RII-Il and from 0.6 wt % to 0.26 wt % in RII-I2. The
lowest "FeO" l e v e l i n slags was also accompanied by a change
i n the A^O^tCaO r a t i o ; whereas the C a F ^ content was only
s l i g h t l y changed (from 0.5 to 1.0 wt % ) . Thus, the minimum
l e v e l of deoxidation reached without s h i f t i n g the slag
composition i s about 0.28 - 0.30 wt % FeO for 1.3 and 1.5
CaOiA^O^ r a t i o s i n slags of RI I - I l and RII-I2 respectively,
Figures (54, 55).
5.2.1.4 1020 M.S. Ingots Deoxidized Continuously with a
CaSi Alloy
Since the e f f i c i e n c y of the Ca-Si all o y i n the small
4340 ingots and i n the large diameter (200 mm) 1020 M.S.
intermittently deoxidized was observed to be higher than *
i n the aluminum deoxidized ingots, a series of experiments
were conducted using equivalent slag systems and degrees
of deoxidation as well as melting rates (1 kg min *) with
the CaSi a l l o y . Experimental techniques used to determine
the t o t a l oxygen content from l i q u i d pool and ingot, the
slag chemical analysis, the i n c l u s i o n composition and t h e i r
size d i s t r i b u t i o n s and the chemical analysis of ingots were
the same as those used to study the 1020 M.S. ingots re
fined through the CaF 2-Al 20 3-CaO system. The Ca-Si a l l o y was *
added to the slag i n R I I I - I l and RIII-I2 i n about equivalent
* Notice that 1 gram A l i s approximately equivalent to 3.12 grams of the CaSi a l l o y ; Table (IX).
165
aluminum rates as i n RII-Il and RII-I2 namely: 5.11, 11.23,
16.83, 22.24 and p a r t i a l l y 28.0 kg t o n - 1 and 5.11, 11.23,
16.83, 22.44, 28.05, and 56.1 kg t o n - 1 , Table (VIII).
The p r i n c i p a l differences between RII I - I l and RIII-I2
are .their slag chemical composition and their deoxidation
rates. R I I I - I l and RIII-I2 were refined through 60/36/4
and 50/30/20; (CaF 2, A1 20 3 and CaO) respectively. The three
lowest (CaSi) rates were added i n shorter periods of time
at the bottom of the ingot i d e n t i f i e d as RIII-I2 and the
fourth l e v e l of additions (22.4 4 kg ton ^),was longer
than i n R I I I - I l . Due to these differences the t o t a l oxygen
analysis shown i n Figures (56) and (57) and hence the i n
clusi o n chemical compositions, Figures (58) and (59) from
RII I - I l and RIII-I2 respectively, were the most c r i t i c a l l y
affected parameters.
Their slag and ingot compositions followed equivalent
behavior, Figures (60) and (61) and Figure (62) and (63).
The S i and Ca contents i n slags gradually increased whereas
the "FeO"and the A l 2 0 3 given as Fe and A l in wt % gradually
decreased as the (Ca-Si) deoxidation rates were increased.
An important point to note i s that the iron oxide i n slags
from both ingots was reduced to about 0.2 wt% as shown i n
Figures (64) and (65) without producing strong changes i n
the composition of the slags. These graphs consistently show
166
that the aluminum and s i l i c o n gradually increased as the
l e v e l of deoxidation i s increased.
The i n c l u s i o n chemical composition p r i n c i p a l l y given
as the at.% Ca: at% A l r a t i o i n Figures (58) and (59) for
R I I I - I l and RIII-I2 respectively, followed an equivalent
pattern. This r a t i o , however, was larger in RIII-I2 due
to heavier deoxidation i n i t s fourth l e v e l . Another import
ant finding i n regard to the i n c l u s i o n composition was the
proportional changes of sulfur (as a CaS) with the Ca:Al
r a t i o s and hence with the deoxidation rates, Figures (66)
to (68). Inclusions at the bottom of ingots where low
rates of deoxidizer were used were mainly faceted and round
alumina types, (a-A^C^ and FeO*Al 20 3). At intermediate de
grees of deoxidation faceted alumina and hexagonal aluminates,
(a-A^O^ and CaO6Al 20 3) as c l u s t e r s , together with s u l f i d e s
(MnS III and (Mn,Ca)S) were found. In the high deoxidation
(rate) ranges spherical aluminates enriched i n calcium,
(CaO«2Al 20 3, CaO«Al 20 3 and 12CaO•7A1203) associated with
peripheral CaS were i d e n t i f i e d . Extreme deoxidation con
d i t i o n s , as i n RIII-I2 and p a r t i a l l y i n R I I I - I l , produced a
CaS phase with i n c i p i e n t amounts of aluminum (usually
located i n t h e i r core) and the largest segregated material
enriched i n deoxidizers (Al, S i , Ca and sometimes Mn).
These r e s u l t s indeed corroborate the q u a l i t a t i v e
findings previously described for RII-W and RIII-W as well
as for the small 4340-ingots which were Ca-Si deoxidized.
167
The above findings (RII-W, RIII W, RIII-Il and
RIII-I2) c l e a r l y indicate that the CaSi as a deoxidizer
plays by f a r a more important r o l e i n the chemical composi
ti o n of ingot and inclusions than the chemical composition
of the slag.
The low l e v e l of SiC>2 and the gradual decrement of
A^O^ i n the slag, the amounts of Si and Al which are continu
ously increasing i n the ingot and the Ca:Al r a t i o i n i n
clusions immediately suggest that the Si i s u t i l i z e d i n the
reacting (ESR) system as a c a r r i e r . I t i s also observed
that simultaneous exchange reactions between the two l i q u i d s
(slag and metal pool) d e f i n i t e l y contribute to the deoxid
ation mechanism, Figures (60-68). Further evidence that
t h i s mechanism, reactions of the type (11) and (12 a-b) ,
rules the chemistry of the melt, i s seen i n the Si content
i n inclusions at high deoxidation rates i n RIII-I2, Figure
(68). Excessive (CaSi) deoxidation performed i n t h i s ingot
induced the formation of calcium aluminates with peripheral
CaS and some s i l i c o n as well as the formation of segregates
enriched i n deoxidizers, Figure (69).
168
5.2.1.5 Corroboration and Extension of Previous Findings to a 4340 Steel CaSi (continuously) Deoxidized
Once the l i q u i d pool-slag deoxidation mechanism was
unmistakenly i d e n t i f i e d through the previous work, a fu r
ther set of experiments was ca r r i e d out to reconfirm and
extend these r e s u l t s to more complex deoxidizers and a l l o y
systems such as 4340 and a rotor (Ni-Cr-Mo) s t e e l . Re-
melting of these electrodes was performed using the equi
valent experimental conditions as i n the 4 340 electrodes
remelted by the small ESR-furnace and the 1020 M.S. e l e c t
rodes remelted by the semi-industrial ESR-furnace. The
melting rates were kept i n the 1 kg min 1 range.
The l a s t ingot included i n the f u l l y monitored set of
experiments (through the three r e f i n i n g stages) was a
4 340-electrode which was refined through a 50 wt. % CaF 2/
30 wt. % A1 20 3 and 20 wt. % CaO slag and CaSi deoxidized.
The degree of deoxidation was continuously increased from
4.17 to 41.67 kg (CaSi) t o n - 1 for almost equivalent periods
of time as i n R I I I - I l . The l a s t l e v e l of deoxidation was
also suddenly decreased from 41.67 down to 20.83 kg ton
Chemical composition of the slag and ingot followed
the same pattern as the 1020-ingots, deoxidized with the CaSi
a l l o y , Figures (70) and (71). The major difference found
i n t h i s ingot was i t s iron oxide and i t s oxygen content,
Figure (72). The ir o n oxide given i n Figure (73) changed
169
from 0.4 down to 0.15 as the rate of deoxidation was i n
creased. The average t o t a l oxygen content was about
10-20 ppm. Whereas i n the 1020 ingots either A l or CaSi
deoxidized the l e v e l ranged from 30 and up to 80 ppm.
This substantial difference i s e s s e n t i a l l y attributed to
the t i g h t Ar-atmosphere enclosure of the furnace, the de
gree of (Ca-Si) deoxidation of these ingots (50-70 ppm
of oxygen) and the chemical composition of the electrode.
The chemical composition of inclusions given i n
Figures (74 a,b), followed the general trend observed in
the equivalent 10 20 M.S. ingots. The chemical composition
of inclusions i n similar manner to previous 1020 M.S.
ingots deoxidized with the CaSi a l l o y exhibited the gradu
a l t r a n s i t i o n from aluminate to calcium aluminate phases.
Their proportional increment i n sulfur content (as CaS)
up to 25 at. % was also observed, as the deoxidation rate
was increased, Figure (75).
A very important fact to address i n these experiments
i s the r a t i o of the S i i n the CaSi-alloy even though i t
i s constituted by approximately 62.0 wt. % S i , i t has not
played a role in the deoxidation scheme previously presented,
i . e . involving the CaO and A^O^ of the slag and the Ca
and A l i n inclusions and deoxidizers . It i s also important
to emphasize that the S i from either the electrode (in
170
the 1020 electrodes) or the CaSi all o y i s v i r t u a l l y trans
ported conjointly to the A l and Ca into the ingot and
i t does not appear i n inclusions.
At t h i s point i n t h i s description, i t becomes e s s e n t i a l
to r e c a l l that although the Al as a deoxidizer seems to be
operating within the same frame of reactions as the CaSi
(slag-deoxidizer and l i q u i d pool), i t due to k i n e t i c fac
tors, does not deoxidize the ESR-melt as e f f i c i e n t l y as the
CaSi a l l o y . In the l i g h t of these findings a new set of
three experiments was planned. The major purpose was
to corroborate the conclusion that the s i l i c o n i n the de
oxidizer works exclusively as a c a r r i e r , to reconfirm the
v a l i d i t y of the proposed mechanism (reaction scheme) and
to compare the degrees of deoxidation reached through three
d i f f e r e n t deoxidizers, Table (VIII). Three rotor (Cr-Mo-V)
ste e l electrodes were refined in the 200 mm diameter ESR-
furnace using equivalent melting rates, slag system and
deoxidation rates, Table (XII). The (Si-based) deoxidizers
were CaSi, SiAlCaBa ("hypercal") and an A l S i a l l o y . Their
chemical compositions are given i n Tables (XII a-c).
Despite the f a c t that the chemical composition of the
deoxidizers i s quite d i f f e r e n t calcium aluminate phases
with peripheral calcium sulphide are precipitated. The
Ca:Al r a t i o vs. the sulfur content are plotted i n Figure (76).
171 Again i t i s shown that exchange reactions of the type
(12 a-b) play the most important role i n the deoxidation
(reaction) scheme and since the S i appears i n the ingot
but i t does not i n inclusions i t s role i s p r i n c i p a l l y as
a c a r r i e r of Ca and A l into the l i q u i d pool and ingot.
These r e s u l t s support the three previous proposals.
The chemical composition of inclusions indicate that s i l i c o n
free, calcium aluminates with peripheral calcium s u l f i d e
(i.e. others than C a O 6 A l 2 0 3 and (Mn,Ca)S) were the pre
c i p i t a t i n g phases. The s i l i c o n content i n the slag was
considerably increased and the "FeO"content was also held
i n the ranges previously described, Table (XII).
Furthermore, segregates which were found i n the ex
cessively and abruptly A l and CaSi deoxidized ingots were
the same as observed i n (size, fluorescence under the e l e c t
ron beam and t h e i r chemical composition) the A l S i deoxidized
ingot. The d i s t r i b u t i o n of elements in a segregate enriched
i n strong oxide formers, namely aluminum, s i l i c o n and p a r t i
c u l a r l y calcium, i s shown i n Figures (77a-d). Their t y p i c a l
composition was (in at.%) as follows:
Ca A l Si Mn + Fe
40.6 41.5 19.7 balance
This finding i s considered as another evidence that the pro
posed mechanism i s indeed operating.
172
5.3 Discussion of Results i n Terms of Electrode and
Slag Composition, Related to the Second Question
5.3.1 The E f f e c t of the Electrode on the Inclusion Comp
o s i t i o n of ESR-ingots
The experimental findings previously described have
been used to address the second question stated i n Chapter
III, i . e . i s the i n c l u s i o n composition controlled by the
chemical composition of electrodes, slag or deoxidizers?
This question as envisaged i n the l i t e r a t u r e review (in
terms of the complexity of the reaction scheme and the i n
cl u s i o n chemical composition of ingots i n the ESR-process)
and as shown in the previously described r e s u l t s cannot
be answered unless the reactions between the three d i f
ferent stages of r e f i n i n g and i t s components (electrode,
slag, s l a g - l i q u i d f i l m , deoxidizer, l i q u i d pool-slag and
ingot) are c a r e f u l l y monitored.
Through the f i r s t part of t h i s research i t has been
concluded that inclusions from electrodes under stable
r e f i n i n g conditions are completely dissolved i n the e l e c t
rode t i p . Thus, further discussion assumes t h i s f a c t .
The elucidation of the role played by the electrode
and the slag on the chemical composition of inclusions i n
the ESR-ingot have been shown through several experiments:
1) The small 4340 ingot .(6), Table (X), refined through
173
an alumina free slag and i n the absence of gaseous oxid
ative and deoxidant (external) sources, i . e . under argon
and without deoxidant. 2) A large diameter (200 mm)
1020 M.S. electrode refined through a CaF 2-CaO-Al 20 3
slag under the above conditions, (RI-Il), Table (VIII)
and 3) A series of 1020 M.S. electrodes refined i n the 200 mm
i n diameter ESR-furnace which were intermittently deoxidized
with CaSi and others continuously deoxidized with A l and
a CaSi a l l o y .
1. The ingot (6) which was remelted through the
31 wt.% CaF 2, 46 wt. % CaO, and 23.0 wt. % S i 0 2 has con
c l u s i v e l y shown that the electrode composition indeed plays
a dominant role i n the f i n a l i n c l u s i o n chemical composition
of the refined ingot. The alumina content i n inclusions
represented as Al in Table (VIII) with respect to s i l i c a ,
given as s i l i c o n , shows that the aluminum which came ex
c l u s i v e l y from the electrode (in solution) has co n t r o l l e d
the i n c l u s i o n chemical composition. This finding i n agree-(83)
ment with Rehak 1 s et al.'s i n Al and CaSi treated electrodes
c l e a r l y demonstrates that the chemical composition of e l e c t
rodes s p e c i f i c a l l y due to the presence of deoxidizers i n
solution, under a given slag system can play an important
r o l e in the f i n a l i n c l u s i o n composition of refined ingots,
i . e . self-deoxidation.
174
2. Results found from a 1020-ingot remelted under a
protective atmosphere without the influence of a deoxidizer
are shown i n Figures (37,38). These graphs from RI-Il c l e a r l y
i l l u s t r a t e that as the remelting of the electrode i n the
200 mm mould takes place an accumulation of s i l i c a in the
slag and a gradual change i n the alumina and calcium oxide
occurs. The SiC^ i n the slag acts exclusively as a diluent
and only contributes to a gradual s h i f t i n the CaOiA^O^ II II (38
r a t i o s and to control the i n t r i n s i c "FeO"content i n the slag
It does not, however, influence the chemical composition of
i n c l u s i o n s r A deeper discussion related to t h i s matter w i l l
be pursued i n a subsequent section. It i s pertinent to
c l a r i f y that the change i n the CaOrA^O^ r a t i o was not i n
fluenced by the formation of v o l a t i l e f l u o r i d e s (AlF^, S i F 2
or HFl) since the f l u o r i n e analysis was changed only in the
measure of the experimental error, (± 0.2%).
The continuous increment of S i 0 2 and "FeO"in the slag
produces a s h i f t in the CaOtA^O^ r a t i o i n the s l a g — g i v e n
as the t o t a l A l and Ca content i n Figure (37)--and an A l
depletion i n the ingot, Figure (38). It i s worthwhile to
point out that i n spite of the (gradual) increase i n Mn and
Si (0.65 to 0.72 wt. % and 0.09 to 0.13 wt. % respectively)
i n the ingot, only aluminates (A^O^ and A^O^'FeO) and
s u l f i d e s ((Fe,Mn)S and MnS II) were pre c i p i t a t e d .
3. A t y p i c a l a l t e r a t i o n of the otherwise continu
ously increasing content of s i l i c a i n the slag and hence
Si i n the ingot i s shown i n Figure (32). This ingot i d e n t i
f i e d as RIII-W was remelted under argon and under an equi
valent slag to R I - I l . The abrupt changes i n the S i and Mn
contents i n the slag and ingot and'the Ca:Al r a t i o i n i n
clusions are a r e s u l t from the intermittent additions of the
(CaSi) deoxidizer. Since the s i l i c o n comes from the e l e c t
rode and the deoxidizer a more appropriate analysis should
be performed on the Mn since i t comes exclusively from the
electrode. The Mn content changes from about 0.65 down to
0.45 wt. % i n the slag, Figure (32).
The behavior of the S i from the electrode becomes more
important when the 1020 M.S. electrodes are A l deoxidized.
The ingot i d e n t i f i e d as RII - I l c l e a r l y shows that as the
degree of deoxidation i s increased the l e v e l of S i and Mn
in the slag and i n g o t — F i g u r e s (39) and (45)—are held to
a constant l e v e l . This suggests that the chemical composition
of the electrode no longer plays a role i n the r e f i n i n g pro
cess. This ingot was refined, as shown i n Table (VIII),
through a Si and Mn free slag under an argon atmosphere.
Thus, the only source of S i (0.25 wt. %) and Mn (0.6 - 0.7
wt. %) either in solution or as inclusions i s the electrode.
The above r e s u l t s c l e a r l y indicate that i f an electrode
176
contains oxide forming elements i n solution or as inclusions,
which are not present as oxides in the slag and are stronger
deoxidizers than those present i n the slag, a cooperative
deoxidation takes place. The mechanism by which these re
actions take place i s within the general scheme given by
reactions (11) and (12 a-b). The most important conclusion
from these experiments i s that these reactions predominate
sol e l y i n the absence of or under i n e f f i c i e n t deoxidation.
A conventional Al-deoxidation (0.05 - 0.2 wt. %) i s able
to overcome the e f f e c t of the Si from the electrode i f the
r e f i n i n g slag i s low (< 10 wt, %) i n SiO,,.
5.3.2 Elucidation of the Ef f e c t of Slag and Deoxidizers
(Preliminary Studies)
The elucidation of the e f f e c t of slags and deoxidizers
was approached through the small 4340 ingots. Ingots (1)
to (6) which were refined through d i f f e r e n t Si0 2-containing
slags, yielded calcium aluminate and calcium-aluminum s i l i
cate phases. The former type of inclusions did not exceed
3.75 at. % Ca and they were i d e n t i f i e d only where the S i 0 2 ~
content in the slag was less than 10 wt. %. Above t h i s per
cent the l a t t e r type of phases were i d e n t i f i e d . Table (X)
summarizes the main features of these experiments.
177
I f the above analysis i s extended to both slags
and i n c l u s i o n compositions and they are plotted i n tern-(14 53 54)
ary diagrams as suggested by A l l i b e r t et a l . ' ' (207)
for ESR ingots and slags and Bruch and K i e s s l i n g and (91)
Lange for CaSi deoxidized ingots i n conventional s t e e l -
making processes then the chemical compositions of i n
clusions and t h e i r probable o r i g i n i s e a s i l y followed.
Figures (12) and (13) . Several important points should be
considered i n these diagrams: i) the slag chemical composi
ti o n i s plotted under the assumption that the CaF 2 acts as
an i n e r t d i l u e n t ( 3 3 ' 3 8 ' 4 5 ' 5 3 ' 8 8 ' 8 9 } . i i ) regarding the t e r
nary diagram which describes the i n c l u s i o n composition, (91)
Kiessling's and Lange*s studies as an extension of (207)
Bruch's have considered a replacement of either Mn, Mg or Fe by Ca as oxides i n the CaO corner. The replacement
(34) of Mn by Ca as oxides has also been reported by Holzgruber
in ESR ingots. And the two divergent l i n e s which emerge from
the A^O-j corner to the CaO-Si0 2 binary are l i n e s which rep
resent the maximum and minimum MeO:Si0 2 r a t i o s , L^ and L 2
respectively, i n inclusions. i i i ) The t r i p l e l i n e which
also emerges from the A l 2 0 3 ~ c o r n e r are Holappa et a l . ' s ' 1 0 ^
findings from Ca-treated (conventional) ingots and iv) data obtained by other researchers i n c o n v e n t i o n a l ' 4 0 ' 1 4 4 ' 15115783)
' ' and i n ESR-research i s also included i n t h i s
diagram.
178
Slags containing less than 10 wt. % S i 0 2 produce i n
clusions l y i n g along the A^O^-CaO binary and s p e c i f i c a l l y
located closer to the A l 0_-corner. These types were 2 3 J c
usually associated with s u l f i d e s (MnS II, MnS III or
(Ca, Mn)S). The type and composition of these s u l f i d e s (34)
depended on the Ca:Al r a t i o in the oxide phase. Holzgruber (14 53 54)
and A l l i b e r t et a l . ' ' who have studied the reaction
(12 v i i i ) , have also reported these i n c l u s i o n phases,
Figures (12,13).
The chemical composition of inclusions which were ob
tained from slags containing more than 10 wt. % S i 0 2 were
located along the SiC^-A^O^ axis. Ingots (2), (3),
(8), (10) and (11) were located i n an area confined i n the
20 to 85 wt. %. Al 202 range along the SiC^-A^O^ axis and
about 30 wt. % CaO. These r e s u l t s are in agreement with
Bruch's and Kiessling's and Lange's^^' and q u a l i t a t i v e l y
with A l l i b e r t ' s et a l . ' s ( 5 3 ) , Rehak's ( 8 3 ) and Holzgruber ' s ( 3 4 ) .
Findings from the l a t t e r two researchers were obtained from
ESR experiments where experimental conditions were not d i r
e c t l y concerned with the slag chemical composition.
Thus, i f CaF 2 i s s t r i c t l y considered as an i n e r t diluent (234 )
and Rein's and Chipman's data are used to substantiate
the thermodynamic behavior of s i l i c a i n ESR-slags then i f
a_.__ £ 0.01, a,,. S 0.5 and a„ . 5 0.1 , aluminates S i 0 2 AlO^ 5 CaO w i l l p r e c i p i t a t e . On the other hand i f the S i 0 2 content
of slags exceeds 10 wt. % then aluminum-silicate inclusions
with some calcium (up to about 30 wt. % CaO) w i l l p r e c i p i t a t e .
The a c t i v i t y r e l a t i o n s are as follows: a„.^ ^0.01, S i O „ '
a C a 0 < 0.10 and a A 1 Q ^ 0.5. Holappa's r e s u l t s v ' 1. 5
from Ca-treated ingots suggest that as the a c t i v i t y of Al
in the melt i s reduced and the a c t i v i t i e s of Si and Ca are i n
creased, a gradual change i n composition of the inclu s i o n phase
as indicated by the 'triple-dotted' l i n e i n Figure (12) should
be observed. This behavior i n ESR ingots (1) to (6) was not
completely followed and instead a mixture of calcium- s i l i c o n -
aluminate and aluminate phases were found, Table (X).
The best deoxidation measured as the CaO:Al 20 3 r a t i o
i n i n c l u s i o n s * 2 1 * 3 ^ was found i n the CaF 2-CaO-Al 20 3 system,
s p e c i f i c a l l y where the CaF 2 and CaO contents were 50 wt. %
and 20 wt. % respectively i n the slag. Polish researcher's
work ̂ ^ ' o n slags belonging to t h i s system have reported a
maximum of 71.0% reduction of non-metallic inclusions in
r e l a t i o n to the st a r t i n g s t e e l electrode i n exactly the
same slag composition (50/30/20) at which the largest Ca:Al
r a t i o i n inclusions was found i n t h i s research, Table (X).
Thus, through the previous description i t has been
found that the slag d e f i n i t e l y plays a ro l e i n the r e f i n i n g
process. What has not been answered yet i s how large the
180
slag e f f e c t i s i n comparison with the deoxidizer. This
question can be answered in a t r i v i a l manner by analyzing
r e s u l t s i n Table (X). By comparing the i n c l u s i o n chemical
composition of ingot (1) against (7) and (9), one can ob
serve that the Ca-content in inclusions i n the l a t t e r two
ingots i s double that i n the former. Thus, on an a p r i o r i
basis, one could e s t a b l i s h that since deoxidation rates and
slag were equivalent (2.3 kg ton ^, 50/30/20) and the Ca:Al
r a t i o s i n inclusions i s twofold i n the A l and the CaSi
deoxidized ingots (7) and (9) respectively then the de
oxidizer overcomes the slag e f f e c t at deoxidation rates
larger than 1.15 kg ton And i f these findings are ex
trapolated, i t could be estimated that since the Ca i s
almost insoluble in molten iron then a r e l a t i v e l y higher
deoxidation rate could generate aluminates riche r in calcium
and hence much lower t o t a l oxygen contents. These premises,
however, do not rest on any mechanism and consequently they
do not explain why the A l and the CaSi deoxidation produce
an almost equivalent trend i n r e s u l t s i n terms of i n c l u s i o n
compositions. I t i s important to c l a r i f y that although the
amounts of deoxidizers (2.3 kg ton 1 ) were equivalent the CaSi
a l l o y contains 62.5 wt.% S i . The e f f e c t of deoxidizers on the
i n c l u s i o n chemical composition, i n the small 4 340 ESR ingots
refine d through the 55 wt.% CaF 2, 15 wt.% A l 2 0 3 , 15 wt. %
CaO and 15 wt. % S i 0 0 slag i s shown in Table (X).
181
I f i n g o t s (8) and (10) which were A l and CaSi de
o x i d i z e d are compared a g a i n s t i n g o t (2), one can see t h a t
the CaSi d e o x i d i z e d i n g o t d i d show the i n f l u e n c e of the
s i l i c a . The S i C ^ e f f e c t from the s l a g i n i n g o t (8), how
ever, was almost completely suppressed by the d e o x i d i z e r ,
( A l ) . To c o r r o b o r a t e these f i n d i n g s a 1020-electrode
76.2 mm i n diameter was remelted under e q u i v a l e n t c o n d i t i o n s .
T h i s p a r t i c u l a r i n g o t (11), however, was A l - d e o x i d i z e d
at a c o n s t a n t r a t e (0.02 Kg/ton ). The e f f e c t of
the d e o x i d i z e r , Table (X) and i n c l u s i o n composition p r e v i
o u s l y c i t e d , was not enough to counterbalance the S i C ^ e f f e c t
o f the s l a g . The i n c l u s i o n phases were " a n o r t h i t e " and
alumina.
I t i s worthwhile to p o i n t out t h a t the s i l i c o n content
i n the i n c l u s i o n phases of i n g o t s (2) and (8) are one order
of magnitude l a r g e r than the aluminum-deoxidized i n g o t s .
And although t h e r e i s a d i f f e r e n c e i n the chemical compo
s i t i o n o f e l e c t r o d e s (1) to(8) and (9) and (10) i n terms
of t h e i r s i l i c o n c o n t e n t (SiC^ i n i n c l u s i o n s ) , the main e f
f e c t i s a t t r i b u t e d to the l a r g e q u a n t i t i e s i n the (CaSi)
deoxidant and the chemical composition of the s l a g , i . e . ,
l a r g e r than 10 wt. %. Thus, the chemical composition of
the s l a g a t which an a p p r o p r i a t e d e o x i d a t i o n i n terms of
the CaSi treatment, should be c a r r i e d out i s a t s i l i c a
c o n t ents s m a l l e r than 10 wt. %.
182
Hence, the most important conclusion from previous
findings i s that the deoxidation phenomenon i s a net re
s u l t of cooperative reactions between deoxidizers and slags.
Thus, to obtain an e f f i c i e n t deoxidation an appropriate
se l e c t i o n of these parameters i s e s s e n t i a l . A further proof
of these statements w i l l be approached in the next section.
It i s also worthwhile to c l a r i f y that although there i s
a common trend of res u l t s i n both ESR-furnaces the v a l i d i t y
of previous findings i s q u a l i t a t i v e . The lower sur
face area available for reactions, the higher thermal gradi
ents and the unsteadiness of the s o l i d i f i c a t i o n conditions
i n the small ESR-furnace are factors that must be con
sidered.
5.3.3 Preliminary Discussion on the Deoxidation Mechanism
In the l i g h t of previous discussion of findings, the
evaluation of the o r i g i n a l q u e s t i o n — I s the inclu s i o n comp
o s i t i o n c o n t r o l l e d by the chemical composition of electrodes,
slag or deoxidizers?—becomes i r r e l e v a n t and instead other
questions emerge, i . e . what i s the mechanism by which the
deoxidation (in i n d u s t r i a l slags and deoxidants) takes place?
What i s the ro l e played by the slag? and What are the condi
tions which an appropriate deoxidation i s carr i e d out under?
To s a t i s f a c t o r i l y answer th i s set of questions a summary
183
of experimental r e s u l t s correlated to the general theory
i s presented i n advance. The following section emphasizes
the r e s u l t s obtained with the 20 0 mm ESR ingots continu
ously or constantly deoxidized.
Findings from the small (4340) ESR ingots, Table (X)
suggest that the chemical composition of ingots and i n
clusions (Ca:Al ratios) i s determined solely by the CaO:
A^O^ r a t i o in the slag when the Si02 content i s lower than
10 wt. % and deoxidation i s absent. The r e s u l t s obtained
through the 200 mm ESR-furnace have confirmed these f i n d
ings and they have also contributed to the formulation
of a more self - c o n s i s t e n t deoxidation model.
The inherent reaction scheme (1-5) at the electroactive • ^ * • t v „r.T, (27,28,30, 33,34) interfaces i n the ESR-process • ' ' / t h e atmos-
n • (1,34,38) , . . phere-slag-liquid pool in t e r a c t i o n (in terms of
the oxygen transport). and the amount of scale (as a re
s u l t of the thermal history of the electrode) introduced
i n the slag are the main factors which determine the o x i
dative state of the slag and hence the oxygen po t e n t i a l
i n the l i q u i d pool.
Thus, the deoxidant, the sequence and degree of de
oxidation as well as the slag system are parameters which
should be adequately selected to optimize deoxidation
without s a c r i f i c i n g the chemical i n t e g r i t y of ESR-ingots.
The appropriate control of the '^ed1 i n the slag by the de-
184
oxidation w i l l influence the l e v e l of oxygen i n the molten
pool and consequently the p r e c i p i t a t i o n of inclusions.
The importance of the electrochemical reactions on the
"FeO" l e v e l of a melt and t h e i r influence on the chemical
composition of inclusions i s seen through ingot (14) in
Table (XI). This rotor (Cr-V-Mo) s t e e l electrode was sur
face ground, thus scaling formed during mechanical working
was removed from i t s surface (about 1 mm). This electrode
was also coated with an Al-Mg spinel painting to prevent i t s
oxidation during r e f i n i n g . The chemical analysis of i n
clusions i n t h i s (ESR)-ingot show that despite the pre
vious surface preparation, the argon atmosphere enclosure
and a s l i g h t Al-deoxidation (0.02 kg ton ^ ) , round, single
and c l u s t e r s of aluminates (mostly FeO-A^O^ and A^O^) and
iron oxides and s u l f i d e s (FeO, FeS) and (Mn,Fe)S) were de
tected instead of the a-A^O^ expected from the s l a g - l i q u i d (218)
metal i n thermodynamic equilibrium , i . e . according (234)
to Kuo Chu Kun ' s phase diagram th i s slag i s A^O^-
saturated. On the other hand, despite large deoxidation
rates (10 kg t o n - 1 ) there i s a minimum "FeO" l e v e l i n the
slag which can be achieved for a given deoxidation practice.
The e f f e c t of the atmosphere as well as the a r t i f i c i a l l y
introduced "FeO" i n the slag and hence in the l i q u i d pool
are c l e a r l y shown in Figures (32,33) and (34,35) which belong
to the ingots i d e n t i f i e d as RII W and RIII W.
185
The i n i t i a l stages of r e f i n i n g are controlled by the
slag composition which i n i t s e l f allows a given amount of
" F e O ( 1 , 3 8 ' 8 2 ) . If the "FeO" content i n the slag i s per
mitted to r i s e above this l e v e l by any of the described
mechanisms the r e s u l t i s an increased rate of oxidation
of the reactive species from the electrode or the slag, hence
moving the system towards unacceptable r e f i n i n g conditions,
i n terms of slag or ingot composition. The reaction (12-iv)
2[A1] + 3 (FeO) t ( A l ^ ) + 3Fe (12-iv)
which has contributed to produce the gradual depletion of
A l i n the ingot; i d e n t i f i e d as R I - I l , Figures (37,38),
at expense of the slag (change i n the CaOiA^O^ r a t i o i n
the slag) has also been influenced by the presence of the
gradual introduction of "FeO" (mostly as scale) into the
slag as the r e f i n i n g took place. The CaOtA^O^ r a t i o i s
controlled by
2[A1] + 3(CaO) J (A1 20 3) + 3[Ca] (20-C)
Although the extent to which t h i s reaction occurred was very
limited, i t s r e s u l t s were detected, Figures (37) and (38).
These reactions can take place only when there i s not enough
deoxidizer to suppress the continuously increasing amount of
"FeO" in the slag. Under these conditions the major pre
c i p i t a t i o n reactions are governed by the oxygen po t e n t i a l as
follows:
186
Fe + 2[A1] + 4[0] t (FeO«Al„0 ) 2 3 inclusion (22)
or 2[A1] + 3[0] t (Al o0_) 2 3 inclusion (11-i)
These p r e c i p i t a t i o n reactions are controlled by reaction
(12-iv) i n the A l deoxidized ingot. Once the oxygen pot
e n t i a l has been decreased as a r e s u l t of the low but
f i n i t e "FeO" content i n the slag then the reaction (20)
plays a very important r o l e .
Figures (45) and (46) which represent the behavior
of the Al-deoxidized ingots show that the Al as a deoxid
i z e r i s v i r t u a l l y introduced into the ingots (RII-Il and
RII-I2). I t can also be seen that the"FeO"content i n the
slag i s decreased to about 0.3 wt. %. 7 Figures (54,55). The
i n c l u s i o n composition, however,•changes gradually from
FeO«Al 20 3 to a - A l 2 0 3 and at. very high deoxidation rates the
CaO«6Al 20 3 i n c l u s i o n phase i s p r e c i p i t a t e d , Figures (47-50).
The t o t a l oxygen analysis also r e f l e c t s the deoxidation trend,
Figures (41,42). From a consideration of these r e s u l t s i t
i s i n f e r r e d that to a limited extent the "deoxidation
Phenomenon" i s also attributed to the reaction (20). This
reaction induces the p r e c i p i t a t i o n of the CaO-eA^O^ phase
and also (up to the same degree) the p a r t i a l substitution
of Mn by Ca i n the s u l f i d e phases,. Figures (50,52). As a
consequence of these reactions inclusions are precipitated
according (140,144,147,148,216) to:
187
m CaO + n A l 2 0 3 J [m(CaO) «n(A1 20 3)] i n c l u s i o n (19)
or i n a more s p e c i f i c formulation:
CaO + 6(A1 20 3) t CaO-6Al 20 3 (19-a)
Since these reaction products are i n equilibrium with
oxygen and sulfur then a s u l f i d e phase can be pre c i p i t a t e d (147, 148,197,210,215),. according ' ' ' ' to:
ic ic CaO + [S] t CaS + [0] (21)
For low CaO content phases such as the CaO«6Al 20 3, a
su l f i d e phase such as the double s u l f i d e (Mn, Ca)S should
be expected to (heterogeneously) p r e c i p i t a t e on oxides.
Although very rare when the s u l f i d e phase did not contain
Ca, i t was faceted and contained only Mn and S. Hence, i t
was c l e a r l y i d e n t i f i e d as MnS II I .
The 1020-M.S. ingots intermittently deoxidized with
the CaSi a l l o y have c l e a r l y reconfirmed that the deoxidation
reactions are not exclusive of each other. Instead a
cooperative process between the deoxidizer, slag and
l i q u i d metal takes place. At a given discrete addition
of CaSi into the slag i t s "FeO" content decreases, Figures
(32) and (33). Simultaneously to t h i s change an increment
of A l i n the ingot and a decrement of A l 2 0 ^ in. the slag
i s also observed. As a r e s u l t of the above coincidental
reactions a net change i n t o t a l oxygen content which i s
a consequence of the inc l u s i o n quantities and compositions
i s expected. A reduction from 75 down to 30 ppm was ob
it represents a peripheral phase on inclusions
188
served.
Another inter e s t i n g finding i s that the reduction of
t o t a l oxygen content was very dependent on the melting
conditions. RII-W was melted under a i r whilst RIII-W
was under an argon blanket. The in c l u s i o n phases i d e n t i f i e d
i n t h i s experiment (RII-W and RIII-W) were e s s e n t i a l l y
a-A^O^ and FeO-A^O^ as clusters of small spherical pre
c i p i t a t e s . The samples c r i t i c a l l y affected by the additions
of d e o x i d i z e r — F i g u r e (36)—showed Ca:Al r a t i o s (at. %)
which c l o s e l y correspond to the formation of the CaO«6Al 20 3
phase. This phase was metallographically i d e n t i f i e d be
cause of i t s faceted hexagonal appearance and i t s peripheral
envelope of ( C a , M n ) S ( 1 5 ' 9 4 ' 1 4 5 ' 2 0 3 ' 2 1 0 ) . The a - A l 2 0 3 and
the FeOA^O^ were observed closer to the FeO additions
and they were associated with (Mn,Fe)S or MnS II.
The series of findings from the continuously CaSi de
oxidized ingots remelted through the CaF 2-Al 20.j-CaO slag
i n the 200 mm diameter moulds can be condensed as follows.
The slag chemical analysis have shown—Figures (60) and
(61)-^that as the degree of deoxidation i s increased the
CaO content of the slag i s increased while the A l i s de
creased. Although the f l u o r i n e analysis showed a s l i g h t
decrement suggesting the formation of v o l a t i l e fluorides
by reactions (8 to 10), the major changes, however, were
189
due to the reaction between the deoxidizer and the slag.
The "FeO'1 Content of the slag was gradually reduced as the
deoxidation rates were increased. The i n i t i a l deoxidation
changes are adequately described by the following reactions:
[Ca] + (FeO) t Fe(1) + (CaO) (12-v)
2[A1] + 3(FeO) t 3Fe(l) + ( A l 2 0 3 ) (12-iv)
At t h i s degree of deoxidation of the slag alumina type of
inclusions and MnS II were v i r t u a l l y the only phases pre
c i p i t a t e d . As the l e v e l of deoxidation i n the slag was
increased Ca-aluminates with increasing CaO-content were
observed, Figures (58,59) and (66 ,67). The in c l u s i o n chem
i c a l analysis revealed the following p r e c i p i t a t i o n se
quence: a - A l 2 0 3 , CaO«6Al 20 3, CaO«2Al 20 3, CaO«Al 20 3 and
traces of 12Ca0«7Al 20 3 at extreme degrees of deoxidation
(^0.2 wt "FeO"). Beyond th i s deoxidation l e v e l the form
ation of segregates enriched i n Ca, A l , Si (and Mn) and the
formation of (Al, Ca)S were seen. These su l f i d e s showed
(EPMA analysis) that the A l ( A l 2 0 3 ) although i n very low
amounts was p r e f e r e n t i a l l y located i n the incl u s i o n core.
Coincidental to the p r e c i p i t a t i o n of the oxide phases a
s u l f i d e phase, which was enriched i n calcium proportional
to the amount of CaO i n the Ca-aluminate phase was ob
served. These analysis lead to the conclusion that the
mechanism by which the deoxidation-precipitation occurs i s
190
as follows: once the l e v e l of the "FeO" i n the slag has
reached i t s minimum the transport of aluminum and calcium
into the l i q u i d pool by:
3[Ca] + (A1 20 3) t 2[A1] + (CaO) (20-c)
takes place. Thus, the lev e l s of deoxidation are propor
t i o n a l to the amount of A l and Si in the ingot and also
to the amount of CaO i n the i n c l u s i o n phases ( i . e . Ca:Al
r a t i o s ) . Hence the p r e c i p i t a t i o n sequence, i n terms of
the degree of deoxidation i s as follows: i n the absence
of or at very low deoxidation rates i n a conventional ( i n
d u s t r i a l ) CaF 2-Al 20 3-CaO slag, the p r e c i p i t a t i o n i s gov
erned by:
2[A1] + 4[0] + Fe = (FeO«Al 20 3) in c l u s i o n (24)
2[A1] + 3[0] = (A1 20 3) i n c l u s i o n (11-ii)
The former oxide (Fe0*Al 20 3) i s usually associated wi th
(Mn,Fe)S or MnS II. With the l a t t e r oxide (a-Al 20 3) MnS II
or MnS III are usually observed. The degree of (Al) de
oxidation dictates the formation of a s p e c i f i c s u l f i d e . At
intermediate deoxidation lev e l s the reaction (20-c) st a r t s
to operate according to reaction (19) or (19a).
191
This p a r t i c u l a r oxide phase (CaO«6Al 20 3) was commonly observed
with either MnS III or (Ca,Mn)S. Hence suggesting that i n
addition to reaction (20-c) the following reaction
[Ca] + MnS t CaS + [Mn] (25)
was also taking place.
At r e l a t i v e l y high deoxidation levels, where the re
action (20-c) e n t i r e l y controls the transport of deoxidizers
i n the melt and when very low "FeO" content i n the slag i s
reached the p r e c i p i t a t i n g phases are CaO«2Al 20 3 with either
(CaMn)S or CaS and CaO«Al 20 3 and 12CaO«7Al 30 3.
These l a t t e r two oxide phases were surrounded by an
envelope of CaS. This mixture of phases (oxide and s u l
fides) suggests that the calcium oxide from the calcium
aluminates strongly interacts with the sulf u r and oxygen
i n solution according to:
(CaO)* + [S] = (CaS)* + [0] (21)
This reaction generated a CaS enriched phase wherever the
minimum oxygen content was reached (^20-30 ppm).
F i n a l l y , at extremely high deoxidation rates where the
CaO:Al 20 3 r a t i o in the slag i s d r a s t i c a l l y and suddenly
shifted, the formation of small segregates enriched i n Ca,
A l and Si can occur. Under these deoxidation conditions
192
the formation of (Al,Ca)S and a peripheral envelope of
Si-phase around the CaS (which surrounds the calcium
aluminate) were also observed. Similar r e s u l t s were obtained
with 4 340 and rotor (Cr-Mo-V) s t e e l s . This allows the
formulation of a comprehensive mechanism discussed i n the
next section.
It i s worthwhile to mention that while there are some , . (151,153, 157,214,221,235,236) . . . ̂ . . advantages • ' • ' • ' ' ' to p r e c i p i t a t i n g
aluminates enriched on calcium oxide (because of their round
shape and t h e i r CaS envelope) instead of the alumina galaxies
and the manganese su l f i d e s the coincidental transport of
aluminum into the refined ingot constitutes a p o t e n t i a l prob
lem. This behavior can be represented as i n the calcium i n -(210) , j e c t i o n processes by:
X[Ca] + Y ( A l 2 0 3 ) i n c l u s . o n i x C a O ( Y - f ) A l ^ + § X(A1) (20-b)
This reaction as Holappa* 2 1 0^ has pointed out i s equival
ent to reaction (20-c). Thus, i f an excess of deoxidizer (CaSi)
i s used A l i s introduced i n the melt and hence the p o t e n t i a l • (221-223)
to reduce the mechanical properties i s enhanced
5.3.4 Comprehensive Discussion on the Deoxidation Mechanism
When Al or Ca i s added to the ESR-slag as a deoxidant,
i t becomes part of a reaction scheme represented by:
193
[Ca] + (FeO) % Fe(l) + (CaO) (12-v)
2[A1] + 3(FeO) + Fe(l) + A l 2 0 3 (12-iv)
3[Ca] + (A1 20 3) % 2[Al] + 3(CaO) (20-C)
At high l e v e l s of "FeO", reactions (12-v) and (12-iv) w i l l pre
dominate leading to a simple deoxidation scheme i n which
the deoxidant addition appears as the appropriate slag oxide
component. At low lev e l s of "FeO" reaction (20-c) w i l l take
over (12-v) and (12-iv) and i t w i l l be observed that an ex-(46 47 52
change reaction (similar to those already reported ' ' ' 54)
for S i / A l and Ti/Al) i n which the a l l o y Ca:Al r a t i o i n
the ingot w i l l be determined by the CaO:Al20 3 r a t i o i n the
slag low i n s i l i c a , (less than 10 wt. % S i 0 2 ) , not by the
rate, form or composition of the deoxidant. In determining
the point at which reactions (12-v) and (12-iv) w i l l give
way to (20-c) i n the sense of producing ingot composition,
the l e v e l of slag FeO-activity i s evidently of prime import
ance. It i s more important than the i n t r i n s i c slag and the
electrode chemical composition. At low FeO-activity i n the
slag i f large quantities of Ca are added as a deoxidizer,
the r e s u l t i n a conventional ESR-slag composition w i l l
be a corresponding increase i n the a l l o y A l l e v e l , through
reaction (20-c).
194
In order to i l l u s t r a t e the role of reactions (12-v)
to (12-iv) the re s u l t s obtained from the 4 340 ingot were
selected as the prototype of the general behavior to
approach t h i s discussion. The deoxidation sequence i n
terms of the chemical composition of the ingot and the slag
are shown i n Figures (70) and (71). Applying a mass balance
to t h i s ingot and the slag, i t i s apparent that the s i l i c o n
addition (from the deoxidizer), appears almost quantitat
i v e l y i n the ingot with very l i t t l e of SiC^ to the slag.
I t i s also observed that the concentration of A^O^ i n
the slag decreases while the CaO content (represented as a
part of the t o t a l Ca content) increases. U t i l i z i n g t h i s
data i n conjunction with the increased A l assay of the
ingot leads to an excellent closure of a mass balance drawn
on the system using equation (20-c). In consequence, i f
the following stoichiometric r e l a t i o n s h i p * 2 1 0 ^ which re
lates the reduction of alumina by Ca i n the chemical comp
o s i t i o n of the inclusions,
X[Ca] + Y(Al o0,). , t XCaO-(Y - *-) A l o 0 , + \ X[A1] z J i n c l u s i o n J z 6 s (20-b)
i s u t i l i z e d to perform the balance, equivalent r e s u l t s and
a very close prediction of the inclu s i o n chemical composition
i s obtained. These re s u l t s are taken as a strong evidence
that the calcium component of the CaSi a l l o y addition has re
duced A^O^ from slag, following (20C) producing A l i n the
195
ingot and leading to a CaO increase i n the slag. This reduction i s stoichiometric, as would be e x p e c t e d ' 4 7 ' 1 4 8 ,
210 216)
' from an equilibrium analysis of (20) . During the
process of r e f i n i n g , as expected ' ' 2 7 ' 3 8 ' ^ 7 ' 8 2 ^ the "FeO"
l e v e l i n the slag remained at low but f i n i t e l e v e l of
0.1 - 0.2 wt. %, Figure (73).
These r e s u l t s lead to the conclusion that at an 'FeO"
a c t i v i t y corresponding to approximately 0.1 - 0.2 wt. %
in t h i s slag (50/30/20), the reaction of A l 2 0 3 to give an
increase i n the ingot w i l l take place i f calcium i s used
as a deoxidant. When either A l or aluminum s i l i c o n was
used as deoxidant at equivalent rates, i t was observed that
the slag "FeO"concentration was again held at low l e v e l s —
Figures (39, 40) and (45,46) and Tables XII and XIII (a-c)
— a n d that both A l and Si were qu a n t i t a t i v e l y transferred to ... . . . , . ,(147, 148,210,216) the ingot. This r e s u l t i s to be expected ' '
from reaction (20-C) as only a very s l i g h t degree of alum
inum reduction of Ca from CaO would arise from reaction
(20-C), producing composition changes not detectable within
the accuracy of t h i s mass balance. The behavior of the i n
clusion compositions i s a more sensitive guide than the
mass balance i n r e l a t i o n to the Ca/Al exchange reaction
i n the slag. The Ca:Al r a t i o i n the oxide phase and the
p a r t i a l substitution of Mn by Ca i n the s u l f i d e phase i n i n
clusions i n Al-deoxidized i n g o t s — F i g u r e s (43,44) and (51,
52)--as well as the Ca:Al r a t i o i n the oxide (inclusion)
196 phases and th e i r proportional amounts of CaS i n the CaSi
and i n the A l - S i based deoxidized i n g o t s — F i g u r e s (58,59),
(66,67), (71,75) and Figure (76)—indeed monitor the magni
tude and d i r e c t i o n of the equilibrium dictated by reactions
(20-b,c). Figure (74) which shows the behavior of the
4340 ingot shows that the Ca:Al r a t i o i n the oxide i n
clusions r i s e s r a p i d l y to a constant value which i s approxi
mately equal to a composition CaD'A^O^. These r e s u l t s
may be compared to those reported for calcium i n j e c t i o n (144 148 154 157) processes ' ' ' and for the basic e l e c t r i c arc
steelmaking p r o c e s s ' 4 0 , 2 0 6 ^ p a r t i c u l a r l y those shown i n (178)
Figure (78) where an equivalently low-sulfur s t e e l shows
the same behavior. Faulring and Ramalingam* 2 1^ have
studied the r e l a t i o n s beween CaO and A^O^ i n generating
oxide inclusions i n t h i s system. Their conclusions are
represented by the p r e c i p i t a t i o n (equilibrium) phase diagram
shown i n Figure (11). When the Ca a c t i v i t y i s estimated
i n the ESR slag/metal system following reaction (20-c), (234 ) (19 8) using the data of Rein et a l . and Sponseller and F l i n n
and assuming that CaF 2 acts as an i n e r t diluent, an a c t i -— 8 v i t y of h = 6.7 x 10 at 0.1 wt.% A l i s obtained. This (_a
conclusion would indicate from Faulring's data, Figure (11),
that one should observe an i n c l u s i o n composition close to
CaO-A^O^/ which i s indeed the case. It i s c l e a r , there
fore, that in spite of an excessively high addition of
197
calcium ('vlO kg/ton) , the in c l u s i o n composition i s
controlled e n t i r e l y by reaction (20-c) and that i n c l u s i o n
calcium contents w i l l not r i s e above those permitted by
reaction (20-c), despite the calcium addition. It i s i n
teresting to note also that at th i s l e v e l of calcium ad
d i t i o n , the s u l f i d e inclusion surrounding the calcium a l
uminates were exclusively composed of CaS not MnS. This
finding can also be compared to those reported i n (Ca,CaO)
i n j e c t i o n processes in Al deoxidized melts of r e l a t i v e l y
low oxygen a c t i v i t y , where i n c i p i e n t amounts of Ca induce
the p r e c i p i t a t i o n of either MnS m ' ^ " ' ^ ^ o r 4 - n e pre
c i p i t a t i o n of aluminates (low i n CaO content) with a p e r i
l s , , u i /„ „ ^ ( 1 4 0 , 144, 146,153, 159, 160,
pheral double s u l f i d e , (Mn,Ca)S ' 19 7 212)
' '. This t r a n s i t i o n i s obviously dictated by:
MnS + [Ca] t CaS + [Mn] (25)
and i t i s expected to occur when the Ca i n the melt i s ap
proximately 5-10 ppm. At higher lev e l s of Ca, the s u l -. N • . ,(140, 144, 146,147, 153,157,211,216) . f i d e phase i s expected • ' • ' ' ' • to
be ruled by:
* * CaO + [S] t CaS + [0] (21)
and hence p r e c i p i t a t i n g a peripheral s u l f i d e , namely (Mn,Ca)S
or pure CaS. In the case of deoxidation with the A l S i a l l o y ,
the observed changes in slag composition were almost equi-
198
valent to the CaSi, the only change being a s l i g h t increase
i n both SiC^ and Al^O^ at low "FeO" levels i n the slag,
Table (XII). The observed i n c l u s i o n composition was that
of Ca-aluminates containing approximately 20-30% CaO and
about 5-7% S as CaS i n t h e i r periphery, Figure (76). These
findings indicate that since the equilibrium p o s i t i o n of
reaction (20-c) re s u l t s i n reduction of CaO by A l , the
t o t a l mass of Ca ca r r i e d into the metal by t h i s means with
the AISi a l l o y i s s i m i l a r to that i n the CaSi and the
"hypercal" case, Figure (76). The equilibrium r a t i o s of
Ca:Al reported by Faulring and Ramalingam^ 2 l^ r hence are
attained by the CaSi, "hypercal" and the AISi a l l o y . The
chemical r e s u l t of deoxidation by AISi and the "hypercal"
a l l o y i n respect of ingot composition i s therefore
v i r t u a l l y i d e n t i c a l with that obtained by conventional
CaSi deoxidation at about the same rate. I t i s i n t e r e s t
ing to note that aluminum deoxidation follows a pattern
f a m i l i a r from e l e c t r i c furnace prac t i c e . In the case of
aluminum deoxidation at the present l e v e l s (10 kg/ton), a
range of A l content i s achieved i n the ingot as shown i n
Figure (71). The corresponding l i q u i d and s o l i d ingot
oxygen analysis are shown i n Figure (72 a-b) where i t can
be seen that the minimum oxygen content i s at approximately
0.1 wt% A l . This minimum i s at the composition expected, (175 177)
Figure (10) ' , from a consideration of 2 [Al] + 3 [0] t (Alo0-,) . i . (11-ii) 1 J 1 J ' 2 3 i n c l u s i o n
using the compiled information by Gustaffson and Melberg
199
(175)
A l Figure (10), for e Q at an equilibrium temperature of ap
proximately 1600°C.
It can therefore, be assumed that the deoxidation and
p r e c i p i t a t i o n of inclusions are c l o s e l y equivalent to those
established i n the e l e c t r i c f u r n a c e ' 4 0 , 2 0 6 ^ .
5.3.5 F i n a l Remarks
It i s important to emphasize that an inadequate de
oxidation of a ESR-melt with A l or Ca-bearing deoxidants
becomes very important where ever the aluminate-CaS i s not
properly adjusted. The A l deoxidation or the lack of
CaSi deoxidation seen as generators of alumina galaxies or as (133)
inducers of "burning" or the excess of A l , A l S i or CaSi a l l o y s , because of the excess of A l i n ingots introduced through reaction (15 a-b) constitute a p o t e n t i a l source of s u l fides and n i t r i d e s and hence to a degradation of mechanical
(221-223)
properties of the refined ingots.
Hence, the f i n a l remarks, which at the same time answer
the l a t t e r set of questions to be drawn are:
i) ESR-deoxidation with Ca w i l l follow the reaction
(20-c) at low oxygen po t e n t i a l s , i . e . slag "FeO" contents
below 0.2 wt.%;
i i ) At intermediate oxygen potentials (slag "FeO" content
ranging between 0.4-0.6 wt,%), the deoxidation process using
either A l or Ca i s equivalent and serves only to control
the slag composition. This l a t t e r factor w i l l therefore de
termine the choice of deoxidant;
i i i ) Deoxidation with A l w i l l also follow the reaction
(20-c) but w i l l not reach the predicted ingot Ca:Al r a t i o
due to unfavorable k i n e t i c factors, unless Si i s added as
a c a r r i e r for Ca;
iv) The maximum SiC^ content i n the slag for e f f i c i e n t
deoxidation through reaction (12-v), (12-iv), and (20-c); i s
less than 10 wt. %;
v) The chemistry of the electrode does not play a
rol e i n the deoxidation (reaction) scheme unless inappropriate
(low) deoxidation rates are used, i . e . s a c r i f i c i a l e l e c t
rode deoxidation which leads to change the ESR slag or ingot
composition;
vi) Excessive and/or abrupt deoxidation can lead to
deleterious mechanical properties, i . e . i n introduction of
high l e v e l s of A l or deoxidizers (as small segregates) i n
the ESR ingot; and
v i i ) The shape of inclusions depends upon the type and
degree of deoxidation: 1) i n A l deoxidized ingots spherical
single or cl u s t e r s of alumina phases ( a - A l ^ ^ and FeO'A^O^)
associated with manganese s u l f i d e s are found at r e l a t i v e l y low
deoxidation l e v e l s and faceted aluminates and low calcium alum
inates (CaO-eA^O-j) as single or clust e r s at r e l a t i v e l y high
deoxidation rates. 2) i n CaSi, "hypercal" or AISi deoxidized
201
ingots, faceted ( a - A l 2 0 3 and CaO6Al 20 3) and single spherical
calcium aluminates (CaO*2Al 20 3, CaO«Al 20 3 and 12CaO«7Al 20 3)
with peripheral s u l f i d e s are found at r e l a t i v e l y low and high
deoxidation rates respectively.
202
5.4 Findings and Discussion Related to the Third
Question
5.4.1 Description of Experimental Results
5.4.1.1 The Inclusion Mean Diameter
Tables (XIV a-b) show the standard information drawn
from the in c l u s i o n (EPMA) analysis. As previously described,
a minimum of twenty and a maximum of 40 single analyses were
performed i n each sample. Solid ingot samples 2.5 x 2.5 cm2
i n area and l i q u i d pool samples approximately 75% of this
area were systematically analyzed. Analyses were performed
i n longitudinal and transversal directions i n both types
of samples.
The mean inc l u s i o n diameter was s t a t i s t i c a l l y obtained.
Figures (79 a,b) show an example of each type of sample.
The l i n e a r i t y shown i n these graphs c l e a r l y indicates that
the size of inclusions i s represented by a normal d i s t r i b u t i o n .
The c o r r e l a t i o n c o e f f i c i e n t s ranged from 0.97 to 0.99 which
are excellent for the p a r t i c l e size found. Based on thi s
information plots of mean in c l u s i o n diameter, given as the
50% of the cumulative frequency, against ingot height (or
deoxidation levels) were obtained. To correlate t h i s inform
ation to the deoxidation behavior, these findings are also
plotted along with the t o t a l oxygen analysis from both types
of samples.
203
5.4.1.2 Findings from Individual Experiments
The Figure ( 36 ) which corresponds to ingot l a b e l l e d
as RII-W i n which the CaSi was d i s c r e t e l y added, shows that
the i n c l u s i o n average size depends strongly on the deoxidation
practice. This p l o t c l e a r l y shows that coincidental to the
largest Ca:Al r a t i o s i n inclusions the smallest mean i n
clusi o n s i z e i s found.
The behavior corresponding to RII-Il i s shown i n
Figure (41). This shows that the mean inc l u s i o n size varies
with the t o t a l oxygen content. This graph also shows that
at low deoxidation rates (3.60 and 6.1 kg ton ^ ) , the i n
clusion size of samples from the l i q u i d pool are smaller than
the ones from the ingot. At moderate and high deoxidation
l e v e l s , however, the opposite behavior i s observed. The d i f
ference i n average size i s approximately 1 pm. V a r i
ations i n the Ca:Al r a t i o s i n i n c l u s i o n s — F i g u r e ( 4 3 ) — a l s o
r e f l e c t t h i s behavior.
Findings from RII-I2 are shown i n Figure (42).
Since t h i s ingot was deoxidized by lower Al-addition rates
than R I I - I l the mean in c l u s i o n diameter i n samples from
ingot and l i q u i d pool were almost equivalent. The average
Ca:Al r a t i o s of inclusions i n t h i s ingot also changed i n an
equivalent manner, Figure (44).
The ingot i d e n t i f i e d as R I I I - I l , which was CaSi de-
oxidized, i n some respect behaved as RII-I2. Variations in
the Ca:Al r a t i o s i n inclusions and oxygen contents are
also r e f l e c t e d in the mean size of inclusions. It i s
important to note that the CaSi addition rates were almost
equivalent from one to the other and gradually increased
during r e f i n i n g , Figures (56 a-b) and (58).
RIII-I2 used an equivalent deoxidation procedure to
R I I I - I l . RIII-I2, however, was refined under d i f f e r e n t de
oxidation regimes. The three lowest CaSi additions were
added i n shorter periods of time at the bottom of RIII-I2,
while the fourth l e v e l of additions (22.4 4 kg ton ̂ ) was
much longer than i n R I I I - I l . In t h i s experiment, i t i s
unmistakenly shown that inclusions from l i q u i d pool, at
moderate and higher CaSi additions, are larger i n size than
the ones from the ingot. The Ca:Al r a t i o s , the t o t a l oxygen
content as well as the mean i n c l u s i o n diameters behaved i n
exactly the same way as R I I - I l , Figures 57(a-b) and (59).
F i n a l l y , Ingot 4340 which was CaSi deoxidized shows
the same pattern, i n terms of t o t a l oxygen analysis, Ca:Al
r a t i o s and p a r t i c l e sizes, as RII-I2 and R I I I - I l , Figures
(72 a,b) and (74a,b).
205
5.4.1.3 Complementary Studies
To gain a better understanding about i n c l u s i o n form
ation and growth and to elucidate whether the f l o t a t i o n
mechanism operates under the ESR-conditions one more set
of experiments was performed. Samples were sucked from
the l i q u i d pool into s i l i c a tubes which contained either a
mishmetal or Zr wire. Refining was carried out under argon
and at a constant deoxidation rate (10 kg/ton of either
AISi or CaSi). Three major areas, i n terms of the composition
of inclusions were i d e n t i f i e d i n these samples. Regions
where inclusions were mainly constituted by either rare earth
or zirconium enriched phases. A region where the Zr or the
rare earth phases were mixed with complex C a - A l - s i l i c a t e s
and a region where a mixture of pure Ca-aluminates and very
few inclusions with peripheral rare earth or zirconium
s u l f i d e phases were i d e n t i f i e d . This l a t t e r type of i n
clusions i s shown i n Figures (80,81) and (82,83) where
the composition maps for A l , Ca,Ce, La, S and Zr are given. It
i s very important to emphasize that t h i s type accounted for
as much as 7-10% of the t o t a l amount of inclusions analyzed
(40-50) i n each sample.
Metallographic analysis on specimens obtained from
l i q u i d pools (1020 and 4340)show that a considerable amount
206 of inclusions was found close to the wall of the s i l i c a tube.
Most of them, however, were located in i n t e r d e n d r i t i c regions
and only approximately 5-7% were trapped by primary dendrites,
Figures (85) and (86).
207 5.4.1.4 Summary of Experimental Findings
In order to approach the answer (discussion ) to t h i s
question a summary of findings, in terms of 1) i n c l u s i o n mean
siz e , 2) i n c l u s i o n composition and 3) t o t a l oxygen analysis,
i s presented.
1) - Inclusion size d i s t r i b u t i o n s obey the normal d i s t r i
bution (the c o r r e l a t i o n c o e f f i c i e n t ranged from
0.96 - 0.99) .
The mean i n c l u s i o n diameter was approximately 6-8 ym.
The average incl u s i o n size increases approximately
1 ym i n diameter i n both types of samples (ingot
and l i q u i d pool) as the deoxidation rate i s i n
creased.
In ingot heads (CaSi deoxidized) some inclusions were
seen as large as 30 ym i n samples extracted from
l i q u i d pools and t h i s size was very r a r e l y seen i n
the ingot.
The i n c l u s i o n density, number of inclusions per area,
i s gradually increased from the center to the mould
wall. This fa c t was accentuated i n CaSi deoxidized
ingots.
2) - Calcium aluminates were almost always observed to
contain a core enriched i n alumina ( i . e . A l by EPMA).
208 The Ca:Al r a t i o s i n inclusions i s almost always smaller
i n samples extracted from l i q u i d pools than from ingots
The peripheral S content as CaS i s proportional to
the Ca:Al r a t i o s in the calcium enriched aluminate
phases.
At r e l a t i v e l y large deoxidation rates (^10-12 kg/ton)
the formation of segregates enriched i n deoxidizers
(Al, Ca and Si) was observed. It i s important to
r e c a l l that these segregates were commonly seen i n
samples extracted from the l i q u i d pool and very r a r e l y
i n ingots. They also contained some Mn.
3) - The average difference i n t o t a l oxygen content between
samples extracted from l i q u i d pool and ingot i s approxi
mately 20 ppm.
5.4.2 P r e c i p i t a t i o n of Inclusions i n the Fe-Al-Ca-O-S(Mn) System
By using Faulring's and Ramalingham's d a t a ^ 2 1 ^ f given i n
Tables (IV) to (VI), to construct the Fe-Al-Ca-0 p r e c i p i
t a t i o n diagram, conjointly with observations of other i n v e s t i
gators, previously described i n sections 2.3.5, 2.3.7.2 and
5.3.4, the p r e d i c t i o n of the p r e c i p i t a t i o n of the Ca-aluminate/
Ca-sulfide phases can be pursued a l i t t l e further. If the
Fe-Al-Ca-0 system i s now approached by superimposing the
(CaS) + [0] t CaO + [S] (21)
equilibrium to the Fe-Al-Ca-0 system, as the Fe-Al-Ca-O-S
system, and care i s taken to consider the phase r u l e , a
diagram representing the p r e c i p i t a t i o n of Ca-aluminates
and t h e i r corresponding s u l f i d e phases was constructed.
The procedure used was equivalent to that used by Wilson
et a l . ( 2 3 9^ and Faulring et a l . . The a c t i v i t y co
e f f i c i e n t s for CaS i n equilibrium with l i q u i d [CaO-Al 20
(liquid) - CaS(liquid)] and [CaO-CaS]solid were estimated N (240 241)
from Sharma's and Richardsons 1s investigations ' ,
Table (XV). This diagram based on Henrian a c t i v i t i e s i s
isothermal (1550° C) and was developed for l e v e l s of A l
( h A l = 0.001, 0.01 and 0.1) i n the range of major in t e r e s t .
The data generated from these e q u i l i b r i a i s summarized i n
Table (XV). The major reactions are given i n Appendix (I).
Although the Fe-Al-Ca-O-S system was exclusively de
veloped as a four f o l d component e q u i l i b r i a , the MnS i n
equilibrium with A^O^ and the double [(Ca,Mn)S] s u l f i d e i s • 4- • • n -A -,(140,146,210) , , . i n t r i n s i c a l l y considered ••' . The gradual s u b s t i t ution of Mn by Ca i n the MnS phase i s governed by the f o l lowing equilibrium:
(MnS)*+ [Ca] t (CaS)*+ [Mn ] (25)
The presence of Ca i n solution i n the melt generates several
t r a n s i t i o n s . Incipient amounts of Ca originate the
210 (91 92 140 144) t r a n s i t i o n of MnS II to MnS
Relatively higher Ca contents i n the melt induce the
double s u l f i d e s , e.g. (Ca,Mn)S. If the amount of Ca i n t r o
duced i n the melt i s increased a m i s c i b i l i t y gap between the
CaS and MnS appears' 4* 5^. This t r a n s i t i o n occurs somewhere
between the formation of the CaO ' e A^O^ and the CaO^A^O^.
The s u l f i d e phase, as reported by several researchers i s
heterogeneously p r e c i p i t a t e d on the oxide phases (previously
described) and i t i s very dependent on the a c t i v i t y of the A l .
In previous discussions i t was acknowledged that as the
Ca(Al) deoxidation l e v e l i s increased, the amount of Ca as
CaO i n the Ca-aluminate phase i s increased by reaction (20)
and hence the amount of CaS phase increases by reactions (25)
and/or (26). The fact that the i n j e c t i o n of CaO slags do not
produce pure CaS i s also considered. As Saxena and cowork-(147 148 214)
ers •' ' have reported, t h i s t r a n s i t i o n w i l l take place (144 153
a f t e r Ca-aluminates have formed. As several researchers ' ' 154,156 .185,197.211) . _ . . . . . . _ _
• • • ' work suggest, the p r e c i p i t a t i o n of pure CaS i s expected to occur once the Ca-content i n the melt (or the
oxygen l e v e l i s approximately 10-40 ppm) i s such that a comp
o s i t i o n i n the double s u l f i d e , (Mn,Ca)S i s greater than 4 3.0
to 50.0 wt. % (140,146)^ This t r a n s i t i o n i s reached once the (202)
CaO-A^O^ stoichiometric phase i s formed . Ototani's and Kataura's r e s u l t s * 2 1 5 ^ confirm Kiesslings and Westman 1s* 1 4^^
(140) (159) Salter and P i c k e r i n g ' s v / and Church's ' findings. They
report a "pure" CaS phase after the " A l 2 0 3 " content i n the
Ca-aluminates i s reduced by Ca to approximately 40.0%. It
i s also r e p o r t e d ' 1 ^ that once t h i s percent of CaO (40.0%)
i s reached a sharp increment in the CaS i s noted.
A schematic representation of the above description i s
shown i n Figure (87). This Figure (87) i s necessarily i s o
thermal (1823K) and at a f i x e d A l a c t i v i t y , h f t l = 0.1. To
represent the s t a b i l i t y of these composite phases over the
ranges of i n t e r e s t (h g = h A l = 0.001, 0.01 and 0.1) i n a com
prehensive manner the h ^ t h ^ r a t i o s are plotted against either
h A ^ or hg i n logarithmic scales i n Figures (88) and (89).
These Figures (88,89) summarize the behavior of the A^O^/
MnS I I - I I I , the Ca-aluminates (C•6A,C•2A)/(Mn,Ca)S and the
CaO-Al 20 3(liq) and CaO/CaS e q u i l i b r i a .
5.4.3 Discussion of Results
5.4.3.1 Nucleation, Growth and F l o t a t i o n o f I n c l u s i o n s
The difference in the mean diameter (1-3 ym) of inclusions
from l i q u i d pool and from ingots indicates that the f l o t a t i o n
mechanism i n the ESR-conditions operate. The conditions
under which th i s behavior was displayed were: a) when the
difference i n t o t a l oxygen content between the l i q u i d pool
and ingots i s greater than 20 ppm and b) Where smooth,
gradual and equivalent changes i n the Ca:Al r a t i o s of i n
clusions and i n the t o t a l oxygen content i n samples from
l i q u i d pool and ingot were observed. Case (a) was speci-
212
f i c a l l y observed i n the Al-deoxidized ingots, i . e . R I I - I l
and p a r t i a l l y i n RII-I2. These two ingots exhibit t h i s d i f
ference in oxygen content when the highest deoxidation
rate with A l was applied. I t i s important to note that
because of the changes in oxygen analysis i n the l i q u i d pool
and ingot (RII-I2), a difference greater than 20 ppm
i s observed i n t o t a l oxygen. This behavior i s also r e f l e c t e d
i n the Ca:Al r a t i o s i n inclusions. Case (b) i s found i n RIII-
12 and p a r t i a l l y i n RII-Il where the deoxidation sequence,
given by the behavior of the t o t a l oxygen content and the Ca:
A l r a t i o s i n inclusions, Figures (57,59) and (43,44), shows
smooth and p a r a l l e l changes.
The s o l i d i f i c a t i o n conditions i n samples from ingots
(center part) are less d r a s t i c than in samples extracted from the
l i q u i d pool. The secondary dendrite arm spacing (DAS 1 1 ) i n
ingots where the samples were obtained was about 250-300 ym
whilst i n samples from the l i q u i d pool 30-50 ym. A larger
D A S 1 1 , as acknowledged i n the l i t e r a t u r e ( " ' 1 0 1 ' 1 0 3 - 1 0 6 >
provides more advantageous conditions for the nucleation and
growth of inclusions. In spite o f . t h i s inclusions i n samples
extracted from the l i q u i d pool, under the described conditions,
showed a larger mean diameter.
It i s also important to note that the difference i n i n c l u s i o n
mean size was 1 ym, above where the faceted (a-A^O^) a l -
213
umina was observed. At lower concentrations of aluminum round iron aluminates (FeO'A^O^) were i d e n t i f i e d whereas at
higher deoxidation rates the aluminates with some calcium
and some angular alumina were i d e n t i f i e d . These findings (99 183
strongly agree with Turpin's and E l l i o t ' s and others ' ' 184) (99)
observations. These researchers who have studied
the nucleation phenomenon under sub-cooled conditions, have
suggested that the angular alumina phase was nucleated i n
the melt at equilibrium temperature and i t simply grew at sub^-liquidus temperatures. Coincidentally to t h i s ob-
(99)
servation , i t was also reported that i n very early stages
of sub-cooling a scum was formed on the surface of t h e i r melts.
Thus, in d i c a t i n g that at the beginning of sub-cooling some
inclusions simply floated to the surface.
To determine the extent at which the f l o t a t i o n mechanism
i s allowed i n the ESR-liquid pool a more elaborate (SEM and
EPMA) study, through the extracted samples containing either
RE or Zr, was c a r r i e d out. The peripheral RE and Zr as o x i -
s u l f i d e s enclosing the Ca-aluminates have c l e a r l y revealed
that the l a t t e r phases were already present i n the l i q u i d
pool. These experiments also suggest that inclusions are i n
a l i q u i d - s o l i d s t a t e ( 9 2 ' 9 4 ' l 2 l ) , Figures (80) to (84). These
r e s u l t s which are also i n agreement with the metallography
observations, show that the f l o t a t i o n of inclusions can occur
214
i n as much as 7-10% out of the t o t a l i n c l u s i o n content
i n the ingot. These r e s u l t s , however, do not rep
resent the amount of inclusions removed from the s o l i d i f y i n g
ingot. I f an analysis of Figures (79a) and (79b) i s made
one can see that i n order to account for the di f f e r e n c e i n
size (1.0 - 1.5 ym i n diameter) a displacement i n the cumul
ative frequency from 50 to 70% produces the expected d i f f e r
ence. This indicates that an elimination of i n c l u s i o n s of
approximately 20% i s c a r r i e d out by f l o t a t i o n .
Although these experiments do not c l e a r l y r e v e a l the
nature of saturation, i t i s believed that i t i s reached by
the three mechanisms' 0 0^ namely by cooling, additions of de-
oxi d i z e r s and during s o l i d i f i c a t i o n . The highest degrees
of deoxidation which r e s u l t from the introduction of large
amounts of A l in the melt eith e r from the deoxidizer or
through rea c t i o n (20a-c) and the high c r y s t a l l i n e character
of the alumina phase lead to the b e l i e f that t h i s phase
i s uniformly nucleated i n early stages of undercooling at
the beginning of s o l i d i f i c a t i o n . The presence of segregates
enriched i n deoxidizers i n some samples from l i q u i d pool and
ingot also suggest that " l o c a l supersaturation" (by additions)
can be achieved.
It i s i n f e r r e d that i f growth of inclusions r e s u l t s from
a mechanism other than d i f f u s i o n and p r e c i p i t a t i o n , s p e c i
f i c a l l y growth due to c o l l i s i o n coalescence between i n
clusions of d i f f e r e n t sizes and there i s s u b s t a n t i a l con-
215
vective mixing i n the ingot pool then t h i s phenomena should
be r e f l e c t e d i n inclusions i n the ESR-ingot. The s i z e , shape
and arrangement of inclusions extracted by the iodine-methyl
acetate-methanol method from A l deoxidized ingots i s shown
i n Figures (91) and (92). These (SEM) photographs reveal
that the growth by c o l l i s i o n and coalescence of A^O^ and
CaC"6Al2C>2 i n the l i q u i d pool indeed has taken place. 4. A u i u (38, 121,242,243) As suggested by several researchers ' ,
since there i s not a complete assimilation of inclusions by
the slag some inclusions are c a r r i e d back into the s o l i d i f y i n g
ingot. The type of inclusions, c l u s t e r s of aluminates,
i d e n t i f i e d i n t h i s research very much resemble those reported
i n conventional mechanically, thermally, or electromagneti-, , . . , (38,120,183, 189,193-195) . . . . c a l l y s t i r r e d melts ' ' ' ' . Thus, although
the growth of inclusions i n ESR-ingots can be accounted for
by the simultaneous d i f f u s i o n - p r e c i p i t a t i o n mechanism, the
difference i n size found in l i q u i d pool and ingot cannot be
explained by a mechanism other than the f l o t a t i o n . It i s
also r e a l i s t i c to suggest that the i n c l u s i o n size d i s t r i b u t i o n
seen by the s o l i d i f y i n g interface i s not s t r i c t l y controlled
simply by buoyance considerations but instead by o v e r a l l (244 )
bath hydrodynamics as suggested by Engh and Lmskog (245)
and Linder . From t h i s discussion, i t i s evident that
the i n c l u s i o n f l o t a t i o n mechanism cannot be approached by
216
st r a i g h t Stokes 1 Law unless the appropriate corrections to
this equation (13) are considered, Table (I).
The second important conclusion from these r e s u l t s i s
that at deoxidation rates which produce an A l content of
0.1 - 0.15 wt. % i n (ESR) ingots, inclusions are exclusively
nucleated and grown i n i n t e r d e n d r i t i c spaces during s o l i d i
f i c a t i o n . These findings i n agreement with A l deoxidized 4 . • 4 . - n 4 . i , • (90, 172,183-187) ingots i n conventional steelmakmg practice
indicate that the nucleation phenomenon at these deoxidation
leve l s i s c o n t r o l l e d by the formation of the FeO«Al 20 3 i n
clu s i o n phase, e.g. l o c a l supersaturation during s o l i d i
f i c a t i o n . Recent s t u d i e s * 2 4 6 ^ i n u n i d i r e c t i o n a l l y s o l i d i f i e d
ingots deoxidized with 0.1 and 2.0 % A l which c l o s e l y re
semble the Al(ESR) deoxidized ingots, have shown that the
FeO«Al 20 3 phase i s p r e c i p i t a t e d at a s o l i d f r a c t i o n of 0.65
i n the low A l (0.1%) content and at 0.89 i n the other. These
r e s u l t s suggest that the FeO«Al 20 3 i s the i n c l u s i o n f i r s t
phase pr e c i p i t a t e d and therefore as suggested by other re-
s e a r c h e r s ( 1 8 3 , 1 8 4 , 1 8 6 _ 1 8 8 ) t h i s phase i s transformed to a l
umina as the s o l i d i f i c a t i o n proceeds.
217
5.4.3.2 Comparison Between Theoretical and Experimental
Results
The t o t a l oxygen content of samples from the l i q u i d pool
and ingot under an appropriate deoxidation sequence,
Figures (41) and (57), have c l e a r l y shown that there i s a
difference between the samples of approximately 20 ppm. This (63)
behavior i s expected from an equilibrium s i t u a t i o n i n
Figure (10). The average Ca:Al r a t i o s in inclusions from
both types of samples ( l i q u i d pool and ingot) also indicate
that under a gradual deoxidation sequence with either an
A l S i , CaSi or hypercal a l l o y the calcium which remains i n
solution p r e c i p i t a t e s on calcium aluminates during s o l i d i
f i c a t i o n . This, acts to rai s e the Ca:Al r a t i o s by allowing
the reaction
(CaO)*+ [S] X (CaS)*+ [0] (18) , , , ,. ^. (147,148) ^ .
to take place i n the forward d i r e c t i o n ' . It i s important to emphasize that despite the 20 ppm of oxygen i n solu t i o n i n the l i q u i d pool, the equilibrium i s achieved i n the d i r e c t i o n indicated by the reaction (18). As the temperature decreases the calcium, oxygen and sulfur.prec i p i t a t e as peripheral oxide and s u l f i d e , i . e . CaO and CaS. The t r a n s i t i o n of these phases i s presented i n Figures (92
* a-c) where the CaS/CaO interphase i s c l e a r l y revealed.
This behavior i s observed when the l e v e l of "FeO" i n the slag
i s such that the calcium aluminates are either CaO«2Al nO_
218
or 12CaO«7Al 0_. At higher levels of "FeO" where the Al o0_ 2 3 3 2 3 i s transformed to CaO-GA^O^, the reaction which controls
the p r e c i p i t a t i o n of s u l f i d e i s :
MnS + [Ca] t CaS + [Mn] (19)
This reaction enables the p r e c i p i t a t i o n of double s u l f i d e s ,
i . e . (Ca,Mn)S, Figures (51) and (52).
The most important fa c t to point out i s that the sec
ondary p r e c i p i t a t i o n which i s heterogeneous i n nature i s
s t r i c t l y c o ntrolled by the Ca:Al r a t i o i n the i n c l u s i o n phases.
The e f f e c t of the temperature on the p r e c i p i t a t i o n sequence,
p a r t i c u l a r l y i n the Ca:Al r a t i o s where aluminates enriched i n
CaO are i n equilibrium i s equivalent to an increment i n the
aluminum content i n the melt i n the Ca-Al-0 system, Figure (11)
This behavior i s also expected from the Ca0-Al 20.j pseudo
binary equilibrium diagram. This indicates that as the CaO
content increases in the aluminates their s t a b i l i t y i n terms
of temperature decreases and a more stable compound i s formed.
The completion of the s u l f i d e p r e c i p i t a t i o n reactions (18)
and (19) i s e x p e c t e d ' 4 ^ to occur at approximately 1000°C
where the m i s c i b i l i t y gap i n the MnS-CaS binary diagram d i s
appears. I t i s also important to mention that during s o l i d i
f i c a t i o n some iron or Cr can be i n solution with the . (92, 140,159) s u l f i d e phase ' '
Another point to be considered i n t h i s analysis i s that
where the aluminate phase (A^O^ or CaO'GA^O^) i s stable
the s u l f i d e phase i n equilibrium with i t i s only the MnS i n
219
any of i t s shapes, i . e . MnS I, II or III which are also
dependent on the chemistry of the melt. The o v e r a l l be
havior of the aluminate-sulfide t r a n s i t i o n i s condensed i n F i g
ures (88,89) which are an extension of results shown in Figure
(87). Figure (87) also indicates that the p r e c i p i t a t i o n
of "pure" CaS cannot occur unless lower oxygen potentials,
than those required to p r e c i p i t a t e CaOiA^O^ and 12CaO • 7AI2O.J
are reached.
If these findings are compared against studies on Ca-
i n j e c t i o n processes then i t can be seen that the simultane
ous deoxidation-desulfurization mechanism i s also r e f l e c t e d • 4.1. ̂ 4.- J 4. (140,147,148,153,197, 211,214) in the deoxidation products ' ' ' ' ' .
(14 7 This diagram i n agreement with Saxena et al.'s work ' 148 214)
' c l e a r l y reveals that Ca either as a CaSi or as
CaO does not d i r e c t l y contribute to the CaS p r e c i p i t a t e
i n inclusions unless the alumina i s f i r s t transformed into
Ca-aluminates.
While calcium aluminates with peripheral Zr or RE
oxides s u l f i d e s , i n the samples extracted from the l i q u i d
pool by the s i l i c a tube containing either Zr or mischmetal,
were not commonly found, a l l of the inclusions which con
tained these elements (Zr or Ce and La) homogeneously d i s
tributed were p r e f e r e n t i a l l y composed of phases enriched i n
calcium. Inclusions generally show that among the elements
220
traced by the X-ray spectrum (SEM) analysis (Al, Zr, Ca
and Si as oxide-sulfide and almost pure CaS) calcium i s one
of the main constituents of these phases. This finding i s
taken as one more evidence that calcium, due to i t s low solu-(198 199)
b i l i t y i n iron ' . i s gradually rejected and hence i t
contributes to increase the Ca:Al r a t i o i n inclusions, by
reaction (18) as the s o l i d i f i c a t i o n progresses.
In previous discussion of r e s u l t s (section 5.4.1.4,
5.4.2 and 5.4.3.1), i t was established that the p r e c i p i
t a t i o n of inclusions i s ruled by the reaction scheme (12-iv),
(12-v), and (20-C). Reaction (20-C) and (19) dictate the
chemistry of the oxide phase, i . e . the gradual t r a n s i t i o n
of A1 20 3 to 12CaO«7Al 20 3. The reactions (21) and (25),
which occur simultaneously to the above ones determine the
s u l f i d e t r a n s i t i o n , i . e . MnS II MnS III (Ca,Mn)S -»-
CaS. Figures (87) and (88) reveal t h i s behavior. Results
given i n Tables (XVI) and (XVII) to a certain extent des
cribe these r e s u l t s .
The equilibrium, isothermal calculations shown i n Table
(XVI) were performed by assuming constant ( f i r s t order)
i n t e r a c t i o n c o e f f i c i e n t s for Al-O, Ca-0 and Ca-S. The values
used were - 5 . 2 5 ( 1 7 5 ) , - 6 2 . 0 ( 1 7 5 ) and -40 respectively. The
l a s t value for the Ca-S was assumed on the basis that since
the free energy of the CaO i s approximately 1 1/3 larger than
that for CaS, Table (XV), then the r a t i o of t h e i r f i r s t
order i n t e r a c t i o n parameters could be equivalent. Hence,
i f S-analyses from the 1020 M.S. electrode are taken as
the s t a r t i n g point i n early (low) deoxidation stages
and they are compared against those given i n Table (XVI)
for the Al 20 3-CaO«6Al 20 3-CaS equilibrium, i t i s noted that
i n terms of s u l f u r , very good agreement i s observed. If
these values are compared i n terms of Ca- and S- contents
and t h e i r respective oxide phases against those reported (144)
by H i l t y and Popp given i n Figure (78), good
agreement i s found. On the other hand, predicted values for
oxygen are overestimated, Figure (72).
Since, Gustafsson and Melberg' 7^? suggest the use of
variable f i r s t order i n t e r a c t i o n c o e f f i c i e n t s then a second
set of calculations was performed. Under these conditions,
r e s u l t s shown i n Table (XVII) were computed. The f i r s t
order i n t e r a c t i o n c o e f f i c i e n t s w e r e ; - 5 3 5 " ^ , -400, -300,
-350 and -200 for the Ca-O, -62.0 ( 1 7 5 ) for Al-0 and -110 ( 2 1 1 )
for Ca-S, for the Al 20 3-CaO•6Al 20 3~CaS, CaO•6Al 20 3~Ca0•2Al 20 3
CaS, CaO«2Al 20 3-CaO«Al 20 3-CaS, CaO'Al^-CaO + A l ^ - C a S
and Ca0^ sy - CaO + Al 20 3~CaS e q u i l i b r i a , Figure (87) and
Table (XV). While the Ca, A l and 0 are predictable by f o l
lowing t h i s approach the sulfur i s not. The sulfur content
i s underestimated (10 6 - 10 ^ wt.%).
Regarding the independence of reaction (20-C) on the
222
oxygen pote n t i a l t h i s i s c l e a r l y revealed through these
ca l c u l a t i o n s . Table (XVII). The oxygen content i n the melt
ranges between 10-40 ppm regardless of the amount of A l
and Ca i n solution.
I t i s also important to note that the predictions stated
i n section 5.3.4 i n terms of the expected in c l u s i o n compos
i t i o n which were based on Faulrings et a l . ' s ^ 2 l 6 ^ findings
are also i n agreement with the r e s u l t s presented i n Figure
(88) .
F i n a l l y , two important facts are worthwhile to mention:
F i r s t , the Ca-0 int e r a c t i o n parameters are very important i n
the Al-Ca-O-S p r e c i p i t a t i o n and second a confident pred i c t i o n
of the p r e c i p i t a t i o n sequence cannot be f u l l y r e l i a b l e unless
the i n t e r a c t i o n ( f i r s t and second order) c o e f f i c i e n t s are
appropriately determined.
223
CHAPTER VI
THE RADIAL DISTRIBUTION OF INCLUSIONS IN CaSi AND Al DE
OXIDIZED INGOTS
6.1 Experimental Details and Techniques
The l a s t part of t h i s i n v e s t i g a t i o n was undertaken to
elucidate how inclusions are d i s t r i b u t e d , i n terms of s t a t i s
t i c a l l y determined sizes, i n ingots. For t h i s purpose a
series of samples from several 1020 M. S. and one 4340
ESR ingots deoxidized with A l and the CaSi a l l o y , were
r a d i a l l y s l i c e d at known deoxidation l e v e l s and samples
were polished and s l i g h t l y etched on four and sometimes
f i v e of t h e i r faces. Prior to performing measurements,
microprobe analysis and X-ray spectrum analysis were car
r i e d out to i d e n t i f y the major i n c l u s i o n phases.
Measurements of mean diameter of inclusions or second
ary dendrite arm spacing were performed almost invariably
at every half centimeter on each face. Since inclusions i n
A l deoxidized ingots were very small and complex i n shape
(alumina galaxies associated with manganese s u l f i d e s ) ,
several approaches to determine the mean size of inclusions
were followed, i . e . normal d i s t r i b u t i o n s and "the f i v e
largest inclusions technique."
224
6.2 Experimental Findings
Results of t h i s research are shown in Figures (9 3) to
(95) and (96) to (99) for secondary dendrite arm spacing
and i n c l u s i o n mean sizes respectively. The A l deoxidized
ingots, as previously described showed alumina galaxies
and manganese s u l f i d e . Only traces of calcium were found.
The morphology of inclusions i n the 1020 M.S. A l -
deoxidized ingots varied from globular single and double
phase (spherical aluminates and manganese sulfide) at the
ingot core, to elongated with double phase (aluminates and
manganese sulfide) at midradius and almost exclusively
small alumina galaxies associated with manganese s u l f i d e
at the mould wa l l .
The 1020 M.S. CaSi deoxidized ingots showed calcium a l
uminates and single and double calcium s u l f i d e s , i . e . , CaS
and (Ca,Mn)S. The deoxidation l e v e l was such that calcium
aluminates of the type CaO«6Al 20 3 and CaO•2Al 20 3 were found
as the p r i n c i p a l i n c l u s i o n phases. The s u l f i d e phase en
closed the oxide phase. The 4340 CaSi deoxidized ingot
showed v i r t u a l l y the same types of inclu s i o n phases as the
1020 M.S. ingots deoxidized with the CaSi a l l o y . I t i s im
portant to mention that the amount of the peripheral CaS
phase was proportional to the Ca:Al r a t i o i n the oxide phase
and i t was not necessarily dependent on the size of the i n
clusions. F i n a l l y , the i n c l u s i o n density (number of i n -
elusions/per unit area) i n a l l of the ingots was increased
considerably with the radius.
6.3 Discussion of Results
In the discussion of the previous r e s u l t s , i t has been
emphasized that some f l o t a t i o n of inclusions (about 10-20%)
w i l l take place. This was determined to originate during
s o l i d i f i c a t i o n i n early stages of cooling.
This statement, however, i s true only when the required
supersaturation r a t i o for the p r e c i p i t a t i o n of aluminates
i s achieved, i . e . above 0.1 to 0.15 wt. % A l i n the ingot .
The previous discussion has also established that i n
clusions i n samples from l i q u i d pool were larger i n d i a
meter (under an appropriate deoxidation sequence) than i n
ingots although the s o l i d i f i c a t i o n conditions (given by
t h e i r corresponding secondary DAS) were more d r a s t i c i n the
former samples. Consequently, i t was concluded that the
i n c l u s i o n growth was almost e n t i r e l y controlled by the d i f
f u s i o n - p r e c i p i t a t i o n of solutes on prenucleated phases.
On the other hand, r e s u l t s obtained from the r a d i a l i n
cl u s i o n size d i s t r i b u t i o n s show that- the p r e c i p i t a t i o n i n
ingots i s e s s e n t i a l l y c a r r i e d out during s o l i d i f i c a t i o n .
Hence, i t i s very dependent on the l o c a l thermodynamic ,... (99,100,101,104,10 5,10 6,121,143,184,186,189) conditions ' ' ' ' ' ' • ' ' •
Since the i n c l u s i o n mean size ranges r a d i a l l y from 6.0 -
5.0 ym i n the ingot centreline to 3.0 - 2.0 ym i n the mould
wall and the secondary DAS equivalently varies from 20 0-250
to 60-100 ym then the p r e c i p i t a t i o n of inclusions, p a r t i
c u l a r l y i n ingots deoxidized with the CaSi a l l o y i s almost
e n t i r e l y controlled by the r e j e c t i o n of s o l u t e s ' 0 1 , 1 0 2 - 1 0 6 ' 121 243)
' , e.g. by the l o c a l thermodynamic conditions of the
i n t e r d e n d r i t i c micropools formed as the s o l i d i f i c a t i o n pro
ceeds .
Thus, although the l o c a l s o l i d i f i c a t i o n time given b y ' 4 7 ^ : DAS X 1 * = a t£ = b(GR)" n = 707.946 ( G R ) " 0 ' 3 8 2 5 (26)
(65 225) i s very short, about 5-30 seconds ' i n locations close
(1-1.5 cm) to the ESR-mould wall; t h i s i s s u f f i c i e n t to grow
inclusions on prenucleated phases^" ' 1 0 0 ' 1 0 6 ' 1 1 5 ^ , by the
d i f f u s i o n - p r e c i p i t a t i o n mechanism up to even larger
* DAS 1 1 = secondary dendrite arm spacing, i n ym
a and b = constants
n = exponent which ranges from 1/2 to 1/3
t^ = l o c a l s o l i d i f i c a t i o n time, i n seconds
GR = product of qrowth rate by the thermal gradient,
i n °C/min.
s i z e s ' 2 1 ^ . Thus, the c o n t r o l l i n g steps are either the
nucleation of inclusions where the supersaturation r a t i o . - . , - , . ( 9 9 , 1 0 0 , 1 0 1 ) i s not reached i n early stages of cooling or
the r e j e c t i o n of solutes (oxygen and deoxidizers) which are o ^ o.u -i • ^ • • ( 1 0 2 - 1 0 6 ,
gradually b u i l d i n g up as the s o l i d i f i c a t i o n progresses 1 2 1 , 1 8 5 , 1 2 1 , 2 2 7 , 2 4 3 )
I t i s also important to emphasize that the growth of i n
clusions during cooling i s very dependent on the density J • * ^ • • -i • i, 4- ( 9 1 , 1 0 1 , 1 2 1 ) T + T and size of the o r i g i n a l phases present . I f
5 7
these parameters are 1 0 - 1 0 inclusions/cm 2 and 1 - 1 0 um
i n radius, as expected i n ESR-melts, only a very s l i g h t growth
should be e x p e c t e d ( 1 0 1 ' 1 2 1 , 2 4 3 ) , Table (XVIII).
It can also be elucidated that since "oxygen segregation"
does not take place along r a d i a l d i r e c t i o n s i n i n d u s t r i a l size
i n g o t s ' 4 8 2 5 ° ) f i t i s expected to observe a gradual change i n
the i n c l u s i o n density, a f a c t which agrees with the observations
i n t h i s research. Hence the i n c l u s i o n r a d i a l size d i s t r i b u t i o n
should be inversely proportional to the i n c l u s i o n density.
228
CHAPTER VII
* 7.0 Conclusions
7.1 Inclusions from the electrode are p h y s i c a l l y and chemi
c a l l y transformed i n the electrode t i p by the thermal
gradients. Inclusions are chemically altered by the
presence of the l i q u i d slag at the l i q u i d f i l m and
they are almost e n t i r e l y removed (by d i s s o l u t i o n -
reactions) when the droplet i s completely formed.
Thus, electrode inclusions only play a r o l e i n the
f i n a l ESR ingot insofar as t h e i r solution product
enters into the slag-metal reactions experienced
during processing.
7.2 Inclusions i n ESR-ingots are more strongly influenced
by the deoxidation practice than by the electrode
composition and/or the slag system used i n low
SiC^ slags. I t i s important to comment, however,
that under low deoxidation rates the i n t r i n s i c
chemistry of the slag predominates, i . e . at high
slag oxygen po t e n t i a l s .
7.3 As a consequence of the above conclusion, i t has been
confirmed that the p r e c i p i t a t i o n of complex Al-Ca-
s i l i c a t e inclusions i s predictable i n high s i l i c a
slags where t h e i r o r i g i n i s s t r i c t l y given by the
slag chemistry, i . e . i f wt. % SiO„ > 10.0.
For ease of reference, the nomenclature of reactions i n previous texts are rewritten i n t h i s chapter.
229
7.4 The most appropriate slag system i n which to perform
an e f f i c i e n t deoxidation i s the CaF2~CaO-Al203
system at 50, 30 and 20 wt. % respectively, i . e .
the highest Ca:Al r a t i o i n inclusions i n the absence
of deoxidation.
7.5 The inc l u s i o n chemistry expected, from the most common
slag system (CaF 2-CaO-Al 20 3) used i n the ESR-process,
i s controlled by the following e q u i l i b r i a :
2[A1] + 3(FeO) t (CaO) + Fe (7.5.i)
[Ca] + (FeO) t ( A ^ O ^ + 3Fe (7.5.ii)
(A1 20 3) + 3[Ca] t 3(CaO) + 2[Al] ( 7 . 5 . i i i )
This reaction scheme depends on the type and degree
of deoxidation. Hence the p r e c i p i t a t i o n of i n
clusions i s dictated by:
mCaO + n ( A l 2 0 3 ) J mCaO-nA^O.^ (7.5.iv)
or more appropriately by:
X[Ca] + Y(Alo0-.) ? 'XCaO'(Y - *-) Al o0_ + \ X[A1] *• J i n c l u s i o n J A J J
(7.5.v)
7.6 From the above e q u i l i b r i a , i t can be seen that i f
an excess of deoxidizer i s added to the slag (in
the forward d i r e c t i o n of reactions 7 . 5 . i i i or 7.5.v)
230
undesirable composition w i l l r e s u l t i n the ingot, i . e .
high Al contents, i n which case there exists a
pote n t i a l problem of p r e c i p i t a t i n g Al n i t r i d e s or
s u l f i d e s .
7.7 Deoxidation of slags during r e f i n i n g by using A l S i ,
CaSi, CaAlSi and CaSiBaAl alloys i s more e f f i c i e n t
than by using Al p e l l e t s alone. The s i l i c o n i n a l l
of the above alloys under the appropriate slag
system (7.3), acts exclusively as a c a r r i e r of the
deoxidant into the metal l i q u i d pool.
7.8 A l and A l S i a l l o y are e f f i c i e n t deoxidizers, however,
they do not control the shape of the deoxidation prod
ucts, i . e . A l generates alumina galaxies and MnS I I .
Although the A l S i a l l o y does produce spherical Ca-
aluminate inclusions i t does not so strongly induce
the p r e c i p i t a t i o n of CaS at the periphery of the oxides.
The CaSi, CaAlSi and CaSiBaAl alloys are very strong
i n c l u s i o n shape c o n t r o l l e r s . At low CaSi or high
Al or A l S i deoxidation rates MnS II-III or (Ca,Mn)S
are formed.
7.9 Inclusion p r e c i p i t a t i o n can be explained by a detailed
Henrian-precipitation diagram i n which superimposing
the reaction:
231
(CaO)* + [S] t (CaS)* + [0] (7.5.vi)
on the previously reported (Ca-Al-0) diagram and
using i n t e r a c t i o n c o e f f i c i e n t s from the l i t e r a t u r e
a r e a l i s t i c p r e d i c t i o n can be made. This diagram
i n t r i n s i c a l l y includes the tr a n s i t i o n s of s u l f i d e s ,
ruled by:
7.10 F l o t a t i o n of inclusions to some extent (10-20%)
occurs i n moderate or highly deoxidized melts.
At low deoxidation rates, as expected from 7 . 5 . i i
and 7.5.v uniform nucleation of inclusions i n
dendriti c spaces during s o l i d i f i c a t i o n takes place.
7.11 If electrode inclusions, slag system, deoxidant
and deoxidation rates are known, by using a
s i m p l i f i e d ternary - Si02, (Ca,M)0 and A^O^-
diagram, the f i n a l i n c l u s i o n composition can be
estimated.
7.12 Inclusion s i z e , as expected from (7.10) i s a
function of the l o c a l s o l i d i f i c a t i o n time and
hence on the l o c a l (interdendritic) thermochemical
conditions.
* * [Ca] + (MnS) [Mn] + (CaS)
232
SUGGESTIONS FOR FUTURE WORK
Based on research c a r r i e d out i n the past, i n terms
of inclusions i n conventional steelmaking and i n ESR and
the type of reactions studied i n t h i s research, several
immediate research proposals are suggested:
1. To determine with a higher degree of accuracy the
t r a n s i t i o n point between reactions which involve the oxygen
pot e n t i a l i n the s l a g - l i q u i d metal such as:
2[A1] + 3 (FeO) t ( A l ^ ) + Fe
and
[Ca] + (FeO) 1 (CaO) + Fe
and the exchange reactions (between two l i q u i d s ) , such as:
3[Ca] + (A1 20 3) X 3(CaO) + 2[A1]
2. With exactly the same purpose as above, to de
termine the series of tr a n s i t i o n s i n the s u l f i d e i n c l u s i o n
phases: MnS II -y MnS III (Ca,Mn)S CaS
by designing s p e c i f i c experiments where various oxygen potent
i a l s (several amounts of A l and Ca) i n the melt should be
involved.
233
3. As an extension of the r e s u l t s found through t h i s
research, i t i s suggested to perform experiments with ex
ac t l y the same techniques and purposes as i n t h i s work
i. e . to determine the possible deoxidation and slag best
combination i n terms of the following reaction schemes
[Si] + 2 (FeO) t (Si0 2) + Fe
[Ca] + FeO t (CaO) + Fe
and
[Si] + 2(CaO) Z s i 02 + t C a l
and
[Mn] + (FeO) % MnO + Fe
and
[Si] + 2(FeO) J S i 0 2 + Fe
or
[Ca] + (FeO) J CaO + Fe
and either
[Si] + 2(MnO) t s i 02
+ 2 [ M n J
or
[Ca] + (MnO) t 2(CaO) + [Mn]
These reactions can be compared to those already reported
for A l 2 0 2 / A l / S i / S i 0 2 and for T i 0 2 / T i . It i s worthwhile
to study these reactions since the excess of A l i n ingots
can cause deleterious mechanical properties.
234
4. I t becomes obvious that the tr a n s i t i o n s s p e c i f i e d
i n point (1) w i l l be also necessary for any other set of
e q u i l i b r i a i n proposal (3).
5. I t also becomes apparent that once these conditions
are f u l l y c o ntrolled the best slag/deoxidation and hence
the most appropriate mechanical properties, as a r e s u l t
of the ingot and in c l u s i o n chemistries can be selected.
Thus, t h e i r evaluation i n terms of mechanical and chemical
resistance as a function of these parameters should be
evaluated.
6. It i s also very i n t e r e s t i n g to note that since the
Ca and the Al a c t i v i t i e s are c o n t r o l l i n g parameters, i n 8a A l and eo which have
been reported with a great deal of scatter should be once
and for a l l appropriately determined.
7. Experimental and t h e o r e t i c a l work along the same
l i n e s as those proposed i n points (1) and (2) could be per
formed; i n t h i s case, i t i s suggested to use Ce as (RE)
deoxidizer i n the presence of a CaO-CaF2 (A^O^) or a RE-
enriched slag. The reaction scheme might be as follows:
[RE] + (FeO) t RE-O + Fe
and/or
Ca + (FeO) t (CaO) + Fe
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185. H i l t y , D.C. and F a r r e l , J.W.: 13th Annual Conf. of Metallurgy, Toronto. Aug. 27, 19 74.
186. Hammar, 0.: "Progress Through Research Sandvik." Steel Research Centre Lab. Sandviken-Sweden.
187. Waudby, P.E. and Salter, W.J.M.: JISI, July 1971, 518-522.
188. Morgan, E.L. et a l : JISI, 1968, 206, 987.
189. Cremer, P. and Driole, J . : Met. Trans. B., 13B., March 1982, 45-51.
190. Plockinger, E.: Stahl Eisen, 1956, 76, 810 ( i b i d . , 1957, 77/ 701, i b i d . , 1960, 8_0, 656).
191. Straube, H. et a l . : Arch. Eisenhtlt. July 1967, 509 ( i b i d . , 1967/ 38, 607).
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193. Braun, T.B., E l l i o t , J.F. and Flemings, M.C.: Metall.. Trans. B.,1979, Vol. 10B., 171-184.
194. Okohira, K. et a l . : Trans. ISIJ., Vol. 14, 1974, 102-109.
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247
19 6. Waudby, P.E. and Wilson, F.G.: Proc. I n t l . Symp. Chemical M e t a l l u r g y o f Ir o n and S t e e l , S h e f f i e l d , J u l y 1971, 195-197.
197. G a t e l l i e r , C. e t a l . : ABM. 3_3 , No. 234 , Maio, 1977, 275-283.
198. S p o n s e l l e r , D.L. and F l i n n , R . A . : Trans. AIME, 230, 1964, 876.
199. Shurmann, E.B. e t a l . : A r c h . E i s e n h t l t t . 45_, Nr. 7, J u l i 1974, 433-436.
i b i d . , 46, Nr. 10, Oktober 1975, 619-622.
i b i d . , 46, Nr. 8, August 1975, 473-476.
i b i d . , 4_5, Nr. 6, J u n i 1974 , 336-371.
i b i d . , 46, Nr. 12, Dez. 1975, 767-771.
200. Kepka, M. : Neue Htltte, V o l . 21(11), Nov. 1976, 645-652,
201. Lindon, P.H. and B i l l i n g t o n , J.C : J I S I , March 1969, 340-347.
202. Jaeger, H. and Holzgruber, W. : Proc. I n t l . Symp. Chemical M e t a l l u r g y o f I r o n and S t e e l , S h e f f i e l d , J u l y 1971, 195-197.
203. B o r i s , M.J. : AIME, E l e c . Furn. S t e e l Conf. P r o c , 1970, 28, 89.
204. Dunn, E . J . : i b i d . , 1971, 29, 117.
205. Koch, W.: Stahl und E i s e n , 8 1 , 1961, 1592.
206. P i c k e r i n g , F.B.: P r o d u c t i o n and A p p l i c a t i o n o f C l e a n S t e e l , 1972, London, 75-95.
207. Bruch, J..: Mikrochim. A c t a , S u p l . 5., J a n . 1974, S p r i n g e r V e r l a g , 137-146. (Bruch, J . e t a l . : A r c h i v . E i s e n h t l t t , H e l f t 11, Nov. 1965, 799-807) .
208. Lin d o n , P.H. and B i l l i n g t o n , J . C : J I S I , March 1969, 340-347.
248
209. Faulring, G.M. et a l . : Iron and Steel Maker, 1980, 14-20.
210. Holappa, L.E.K. (ref. 94), 19-34.
211. Sanbongi, K.: Trans. ISIJ., Vol. 19, 1979, 1-10.
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(Kobayashi, S., Sanbongi, K., et a l . : Trans. ISIJ. 1961, 198, 260-269.)
212. R i k a t a l l i o , P.: Paper 13, Ref. 97, Scaninject I.
213. Usui, T.: Paper 12, Ref. 98, Scaninject I I .
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215. Ototani, T. and Kataura, Y.: Trans. ISIJ, Vol. 12, 1972, 334-342.
216. Faulring, G.M. and Ramalingam, S.: Met. Trans. B. 11B, March 1980, 125-130.
217. Faulring, G.M. and Ramalingam, S.: Met. Trans. A. 10A, Nov. 1979, 1781-1787.
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219. M i t c h e l l , A. and B e l l , M.M. : Can. Met. Quart. 2, 1972, 363-369.
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221. Vaccari, J.A.: Design Engineer, May 1980, 57-60.
222. Viswanathan, R. and Beck, G.: Met. Trans. A., 6A, Nov. 1975, 1997-2003.
223. R a t l i f f , J.L. and Brown, R.M.: Trans. ASM. 1967, V. 60, 176.
224. Kelly, T.N. et a l . , Proc. of the 3rd I n t l . Conf. on ESR and other Spec. Melting, Techs., Mellon Inst. P i t t s burgh, Pa., June 1971* 125-140.
249
225. Ballantyne, A.S. and M i t c h e l l , A.: Proc. of an I n t l . Conf. on S o l i d i f i c a t i o n . S h e f f i e l d Metall, and E g. Assn. and U. of S h e f f i e l d . July 1977, 363-370.
226. Etienne, M.: Ph.D. th e s i s , Dept. of Metallurgy, U.B.C., 1971.
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229. Malm, S.: Scand. J. of Metallurgy, 4_, 1975, 231-237.
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231. Rooney, T.E. and Stapleton, A.G.: JISI, Vol. 131. 1935, 249-254.
232. Yule, J.W. and Swanson, G.A.: At. Absorption Newsl e t t e r , 8, 1969, 30.
233. Ingamels, CO.: Anal. Chim. Acta' 5_2, 1970, 323.
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236 . Kuo C-K. and Yen T-S. : Acta Chim. Sinica, 30_, 1964, 381.
237. Nakai, Y. et a l . : Trans. ISIJ., Vol. 19, 1979, 401-410.
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250
242. Iyengar, R.K. and Philbrook, W.O.: Met. Trans. Vol. 3 July 1972, 1823-1830.
243. Lindskog, L. and Sanberg, H.: Scand. J. Metallurgy, 1973, 2, 71-78.
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250. Kadose, M. et a l . : i b i d . 246.
251
Figure(1) -Schematic i l l u s t r a t i o n of an ESR system.
252
Axial length (cm)
Figure (2) - Predicted and measured temperature p r o f i l e s for a 1018 M.S. electrode, 25 mm i n diameter.
253
Figure (3) - Manganese content of the metal for univariant equilibrium gamma iro n + "MnO" + "MnS" + l i q u i d (1) for the Fe-Mn-S-0 system and univar iant equi l ibr ium gamma iron + "MnS" + l i q u i d su l f i de for Fe-Mn-S system.
800 1000 1200 1400 1600
Temperature (°C)
Figure (4) Univariant e q u i l i b r i a i n Fe-Mn-S-0 system the presence of gamma iron and Mn(Fe) 0
(164) phases
255
>i •P > •H 4-> O — < CD
CO CD (0 CO .£ CD ft c C i—I (0 O S co
|Fe-Mn-0
ToFe-5-0'S>
1100 1300 1500 1700 Temperature (°C)
—•>• actual conditions (cooling)
equilibrium conditions (cooling) s t a r t i n g composition
~ — heating conditions
Figure (5) f 16 6> ) - Univariant e q u i l i b r i a involving s o l i d metal
and Mn(Fe)0 i n Fe-Mn-S-0 system bonded with ternary Fe-Mn-0 and Fe-S-0 terminal-phase f i e l d s ; (e) 6, 'O', 1 2; (p) f S , l 1 ,1 2; (f) 6, 'o',
2 ; (n) 6, 'o\ 1 1, 1 2; (g) 6, y, 'o'; 1 ^
(h) Y# 'o'; 'MnS', 1 1 .
256
MiS
.MO-&0,
(a) MiS
MnO
F i g u r e ( 7 ) * 1 6 6 ) - E q u i l i b r i u m phases i n three planes o f the FeO-MnO-MnS-Si0 2 system, a) MnS-FeO-2MnSiOj b) MnS-2FeO'Si0 2-2MnO«Si0 2 and c) MnS-FeO-MnO,
257
(95) Figure (8) a) MnO-Si02 binary phase diagr
(T-Mn_SiO. and R-MnSiO-.)
b) Schematic i l l u s t r a t i o n of l i q u i d compositions versus Mn/Si/O r a t i o s .
(b) i s a section of the Fe-Mn-Si-S-0 system. ABC are s i m p l i f i e d l i q u i d compositions. A'B'C* are l i q u i d compositions saturated with s o l i d s u l f i d e at 1315°C.
258
gure (9) - Schematic i l l u s t r a t i o n of changes i n inclusion composition (enriched i n Si and Mn) i n a 1020 MS electrode.
260
F i g u r e (11) - Isothermal. Fe-Al-Ca-0 p r e c i p i t a t i o n (Henrian
a c t i v i t i e s ) d i a g r a m ' 1 * ^ .
to
263
Figure (14) - Schematic i l l u s t r a t i o n of the used i n this investigation.
ESR arrangement
I II & 01 IV
Figure (15) - Schematic illustration of the "inclusion extractor".
265
Figure (16) - Inclusions from 1020-steel used as electrode l i g h t microscope. 4 30 X.
Spectrum X-ray analysis are aiven i n Figure 17 (a-b) .
X-ray energy (KeV)
F i g u r e (17) Deformed i n c l u s i o n i n a 1020 M.S. (a) Spectrum X-ray analyses of dark phase. (b) Spectrum X-ray analyses of l i g h t phase.
267
F i g u r e (18) - M a c r o s t r u c t u r e of a 1020-electrode t i p . (a) unetched s u r f a c e (125X). (b) m a c r o s t r u c t u r e where l i q u i d f i l m ,
s e m i l i q u i d r e g i o n and y - g r a i n growth, areas are shown.
268
(c) (d)
Figure (19) - Macrostructures from a 4340-electrode t i p . (a), (c), and (d) show the l i q u i d f i l m , p a r t i a l l y l i q u i d region and the f u l l y and p a r t i a l l y austenitized zones- (b), (c) and (e) 30X. (a) shows a droplet i n process of forming (b) 6.6 X.
269
270
F i g u r e (21) Schematic i l l u s t r a t i o n o f a 1020 e l e c t r o d e t i p s u b j e c t e d to ESR thermal g r a d i e n t s .
271
Figure (22) - Multiphase ( r e l a t i v e l y grown) inclusions i n a 1020 electrode. 400 X.
272
Figure (23) - Single phase inclusions i n p a r t i a l l y and f u l l y molten regions i n a 10 20 electrode t i p . (a) 45 X and (b) 400 X.
F i g u r e (24) - C o m p l e x ( C a , A l , S i , Mn) i n c l u s i o n s f o u n d i n t h e l i q u i d f i l m a n d d r o p l e t s o f 1020 e l e c t r o d e s ( a ) , ( c ) , (d) - 1 .8 x 1 0 2 X a n d (b) 3 . 6 x 1 0 2 X .
274
X-ray energy (KeV)
F i g u r e (25) - (a) T y p i c a l complex (Ca, A l , S i , Mn) i n c l u s i o n i n the l i q u i d f i l m and d r o p l e t of 1020 e l e c t r o d e s (2.4 x 10 3X) (b) Spectrum X-ray a n a l y s i s .
275
Composition i n at. %
Figure (26) - Changes i n inclu s i o n chemical composition i n a 4340(1) electrode t i p subjected to (ESR) thermal gradients.
276
Figure (27) - Changes i n in c l u s i o n chemical composition in a 4340 electrode t i p with strong r e c r y s t a l l i z a t i o n .
277
E
•H
fr, •a cr
•rl
£ O 4-1
CD U C fO 4 J w •H Q
12000
8000
4000
20 40
• A! ACa
o & • Mn
spherodization of sulfide inclusions
•f 5500iim Incipient heat affected zone
solidus " -|- 285Cym Liquid-solid region
— i $- 350Mm 60 80 liqiidus
Composition ( at. % x)
Figure (28) - Behavior of oxide i n c l u s i o n s i n an electrode t i p of a rotor s t e e l subjected to ESR-thermal g r a d i ents.
F i g u r e (29) - A l - s i l i c a t e i n c l u s i o n s i n a 4340 ESR i n g o t , 75 mm i n diameter. Deep etched sample by i o d i n e methyl a c e t a t e methanol (Ingot 3) (a) 1000 X and (b) 2000 X.
279
RIII-W
20 40 60 80 100 T o t a l Oxygen Content (ppm)
Figure (30) - Influence of CaSi and FeO i n t e r m i t t e n t additions on the oxygen content i n a 1020 M.S.
280
RII-W
25
20
g 3 15 4->
2 io o U l c
Ca, 2ndaddJ
FeO, 2 nd add.
'Ca', 1st add.
FeO, 1st add.
40 60 80 100
T o t a l Oxygen Content (ppm)
F i g u r e (31) - Changes i n t o t a l oxygen content r e s u l t i n g i n CaSi and FeO i n t e r m i t t e n t a d d i t i o n s d u r i n g r e f i n i n g of a 1020 M s t e e l .
281
Figure (32) - Changes i n slag chemical composition i n a 1020 (RIII-W) s t e e l r e s u l t i n g from CaSi and FeO intermittent additions.
282
RII-W
01 1 1 1 U I | | L 0 0.2 0.4 2 3 4 5
(wt.% Mn) (wt.% Fe X 10 )
Figure (33a) - Changes i n slag chemical composition as a r e s u l t of intermittent additions of CaSi and FeO i n slag during r e f i n i n g .
283
RII-W
Slag Chemical Composition (wt.%)
Figure (33 b) - Changes i n S i , A l and Ca as a r e s u l t of d i s crete additions of CaSi and FeO i n the slag during r e f i n i n g .
284
RIII-W
1 i—I—I I—I—i—i—I—r
i— i— i— i—I I—i—i—i—I I i i i i i | I • . . . I 0 6 0.75 0 B 5 0.1 02 0 0 1 0 .03 0 0 5 0 0 7 0 2 0 0 2 4 0 .28
Ingot Chemical Composition (wt.%)
F i g u r e (34) - Changes i n i n g o t chemical composition as a r e s u l t of CaSi and FeO a d d i t i o n s i n s l a g .
285
Figure (35) - E f f e c t of CaSi and FeO additions in the slag on the chemical composition of a 1020 MS ESR-ingot.
286
R I I I - W
+J x: Cn •H (1) BC
JJ O Cn C
251
20
15
10
5 h
~i i 1 1 r o inclusion size x 10>m
power off
2nd Ca-addition
2nd FeO addition
20 40 60 80 100 120 140 160
a t . % Ca 1 n 4 a t . % A l X 1 0
F i g u r e (36) a,b - Changes i n i n c l u s i o n composition (a) and mean s i z e (b) i n a 1020 MS i n g o t as a r e s u l t o f i n t e r m i t t e n t o x i d a t i o n and d e o x i d a t i o n o f the s l a g .
287
Figure (37) - Chemical analysis of slag samples in RI-H
288
0.1 0.2 0.1 0.2 0.6 0.7 0.8
Ingot Composition (wt. %)
Figure (38) - Ingot chemical analysis in R l - I l . (Ingot used as a reference).
289
R I I - I l
Slag Composition (wt. %)
Figure (39) - Slag chemical analysis (wt.%) i n R I I - I l .
290
4-> tn •H cu
K
4-1 o Cn c
H
0.2 0.6 1.0 28 30
Slag Composition (wt. % X)
Figure (40) - Slag chemical analysis (wt.%) i n RII-I2.
291
R I I - I l
30h
25h
20
15
10
i—r i—I—r
\
V
o i l i I i o
1 —
• Solid O Liquid
o I t o 4th add.
V 3rd add.
A 1 P 2nd add.J
J L st add.
i I 2 3 4 5 6 7 8 9 60 80 I00
I n c l u s i o n Mean Diameter (ym)
T o t a l Oxygen Content (ppm)
Fi g u r e (41) - I n c l u s i o n mean diameter and t o t a l oxygen content i n R I I - I l .
292
30
25
-i 20
4-» x: tn
•H 0) K 4-> o tn CJ
15
10 r
or
T 1 r
()
V o Liquid \
pool •Solid /
ingot I
\ /
j
T r
5 th odd
oi: ~o—-~
4th add
3rd odd
2Qd.Qq!d
Ca addition * \ 1
1 *A L
° Liquid pool
• Solid ingot 1st add.
* Calculated total oxygen content from
t extracted inclusions 2 4 6 8 30 40 50 60 70 80 90 100
Inclusion Mean Diameter (ym)
Total Oxygen Content (ppm)
Figure (4 2) - Total oxygen content and inclusion mean d i a meter i n RII-I2.
293
T 1 1 1 1 1 1 1 1 r
,* 20 40 , 6 0 , 8 9 liguidpod, 10 20 30 40 50 60 70 80 ingot
,at.% Ca, 1 Q 2
'at.% A1 J
Figure (4 3) - Inclusion chemical composition (at.%) as a function of continuously increasing deoxidation rates (ingot height) i n ingot RII - I l .
294
30
25
20 e £ 15 4-1
Cn
i 10 4-1 o in H 5
r -— 3 » 0
o-=^-~ /
o
o ' /
Ca addition / ̂ ^ o u • solid ingot
g j_ ^ / ° liquid pool
«/ . , , 50 100
Figure (44) Inclusion chemical composition as a function of the ingot height (or continuously increasing deoxidation rates) i n RII-I2.
295
R I I - I l
Ingot Chemical Composition (wt.%)
Figure (45) - Ingot chemical composition against ingot height (or deoxidation r a t e ) .
296
Ingot Composition (wt.%)
Fi g u r e (46) - Ingot chemical composition vs. ingot h e i g h t (or d eoxidation rate) i n RII-I2.
2 9 7
298
(c) (d)
Figure (4 8) - "Alumina galaxies* associated with MnS II i n an A l deoxidized ingot, ( R I I - I l ) . 2000 X. (a) BE photograph, (b) A l (c) Mn and (d) S maps.
299
X - r a y E n e r g y (KeV)
F i g u r e (49) - a - A l ^ O ^ ( c o r u n d u m ) i n c l u s i o n s i n A l d e o x i d i z e d i n g o t s . (a) u n e t c h e d s u r f a c e , ^ 1000 X; (b) a n d (c ) a r e d e e p ( i o d i n e m e t h y l a c e t a t e ) e t c h e d s a m p l e s , * 3000 X; (d) s p e c t r u m a n a l y s i s o f (a) a n d (b) r e s p e c t i v e l y ; •v 2000 c o u n t s .
300
(c) (d)
Figure (50) - Calcium-aluminate i n c l u s i o n from a heavily A l -deoxidized ingot. (a) backscattered electron photograph taken at 4000 X. Reverse p o l a r i t y . (b) , (c), and (d) are A l , Ca and S maps. RIII-I2-S8-SLD.
301
R I I - I l
o o
X
to u < 0\° 0\°
• - p +J f0
8
6 h
1 -•• —"1 1 1 ' /
/ / / —
try
Rat
io / /
/ /
' / / / / —
S - S
toich
iom
e
/
/ / •/ / /
4
/« /
•
»
•
c s
/ /
/
/ / •/ / /
•
/ r
/*! ' i i
i i i 0.4 0.6 0.8 1.0 1.2 1.4 1.6
.at. L a t . Mn"
F i g u r e (51) - Composition dependence of s u l f i d e phases on the Ca-aluminate i n c l u s i o n phases i n R I I - I l .
Figure (52) - Composition dependence of sul f i d e phases the Ca-aluminate inclusion phases in RII
(a) (b)
(c) (d)
'igure ( 5 3 ) - Segregated material i n an Al-deoxidized ingot, (a), (b) , (c) and (d) are A l , Ca, Si and Mn maps. -v 1 0 0 0 X.
3 0 4
Figure (54) - Dependence of "FeO" content on the ( — ^ 3 , CaO r a t i o i n slag i n a continuously Al-deoxidized mgot, (RII-H). y ueoxiaizea
305
u 1.3 1.2 1.1 A 1 2 ° 3
A l 0 Figure (55) - Dependence of the "FeO" on the ( —-) r a t i o
, CsO i n slag i n a continuously Al-deoxidized incrot. ( R I I - I 2 ) . y '
306
R I I I - I l
Figure (56) - Total oxygen content and i n c l u s i o n mean diameter in a CaSi-deoxidized ingot (RIII-Il) .
307
RIII-I2
T — i — i — i — I | 1 ' 1 1 1 r
0 4 8 3 0 4 0 6 0 8 0
Inclusion Total Oxygen Content (ppm) Mean Diameter (ym)
Figure (57) - Inclusion mean diameter and t o t a l oxygen content in RIII-I2.
308
025 0.5 0.75
r a.t • % Ca i L a t . % A1 J
F i g u r e (58) - I n c l u s i o n chemical composition (at. % ) , as a f u n c t i o n of d e o x i d a t i o n r a t e s i n R I I I - I l .
309
Figure (59) - Inclusion chemical composition as a function of deoxidation rates i n RIII-I2.
310
R I I I - I l
F i g u r e (60) - Changes i n the s l a g composition i n a c o n t i n u o u s l y Ca-Si d e o x i d i z e d i n g o t ( R I I I - I l ) .
RIII-I2
1 r T 1 1 r 30
25 ^ 6 th addition
- • \
J
" 20 -P si tn
•H CD
-P O tn C H
15
10 \ c / 3 r d add..
/ \ 2nd add. • Fe J
<
/ ^ \ o Si
\ 1st add. J I I ' I L
0.2 0.6 1.0 1.4
r t t
A Al \
II 13 15
Slag Composition (wt. %)
Figure (61) - Changes i n slag composition i n a continu ously (CaSi) deoxidized ingot, (RIII-I2)
312
Figure (62) - Changes in Al and Si i n ingot (RIII-Il) as CaSi deoxidation i s increased.
313
RIII-I2
o Si T r ~ o — i r
I
r 1
7
6th addition
5 th addition
4th add.
0.2 0.6 0.7 0.8
3rd add.
/ _i _o 1
2nd add.
6
i
1st
i i
add.
i ) 1 2 3 4
Wt.% X (Al,Mn,Si) i n ingot
Figure (63) - Changes i n chemical composition i n a continuously CaSi deoxidized ingot, (RIII-I2).
314
R I I I - I l
Figure (64) - Dependence of "FeO" contents i n the slag on the deoxidation rates i n R I I I - I l .
315
R H I - 1 2
0 . 8 I ,
2 . 5 3 .0 3 . 5
Wt. % Co Wt. % Al
] . slag
Figure (65) - Dependence of "FeO" contents i n the slag the deoxidation rate i n RIII-I2.
316
RII I - I l
Figure (66) - Sulfur content in inclusions as a function of the Ca:Al r a t i o i n the Ca-aluminate phases (RIII- I l ) .
317
R1I-I2 (Ca-Si deoxidized ingot) ~i r- 1 r~w
( A t % S ). , . i n c l u s i o n
Figure (67) - Inclusion composition in samples from pool and ingot as deoxidation rate i s l i q u i d
increased.
0
I n g o t H e i g h t (cm)
3 o rt H
C
3 P-0 n 3
0) I o a o i- g c t> g o H - 01
01 ~~ 01 cr
— 01
3 m Ul » c
H (-• M rr M I o M rt\
& (t O X H "
a 01
01 3
a
ro O
' O
cn * o
OD O
a a. a.
ro
a.
o a. a.
ro o
• 1 —
OJ
o a. a.
zr
\ a Q.
_1_ 2
• V
ro cn
o o.
o - I —
cn
Q a. a.
A t . % Ca 1 0 0 i n s a m p i e s from l i q u i d p o o l A t . % A l
ro cn oo
> r o
cn
ro o
>>. *\ \
1 a R
r ^ 0
— \ * — s \ \ \ \ \
x - \ 0) X \
-v.
x - \ 0) X \
-v. 1 1 J — 1
8 T £
319
F i g u r e (69) - Segregate e n r i c h e d i n (a) A l , (c) S i and (d) Mn. * 5000 X.
(b) Ca,
320
R-4340(1)
35
30
25
~ 20
15
10
F L 5.
5 h
T — r
I.. ... ...
M o Fe
Mn J L
7̂ t /
/
4J I f
A S i
-I I 1 I u 0 01 02 03 0.7 OS 11 13 15 17 % 16 38 AO
Slag Composition (wt. %)
Figure (70) - Slag chemical analysis of a 4340 ingot continuously deoxidized with a CaSi a l l o y ([R-4340 (1)].
321
Figure (71) - Ingot chemical composition of a 4340-ingot continuously deoxidized with a CaSi a l l o y . [R-4340 (1)].
322
Figure (72) a - Variation i n mean inclusion s i z e .
b - Oxygen analysis i n a 4340-ingot continuously deoxidized with a CaSi a l l o y , [R-4340(1)].
323
0.1 25 26 2.7 2.8 29 3.0 3.1
(wt. % A l } S l a ^
Figure (73) - Changes i n slag composition as a r e s u l t of continuously increasing CaSi deoxidation rates, R-4340 (1).
324
Figure (74) - Inclusion chemical composition, i n (a) ingot and (b) l i q u i d pool, as a r e s u l t of continuously increasing CaSi deoxidation rates in a 4340 ingot R-4340(1).
325
R-4340 (I)
l i q u i d ingot pool
140
120
100 |At.%Caxl00
At.% Al 80
60
40
20
1-700
600
h500
400
,o o'
/
/ /
/ /
/
/ /
/ /
/ /
/
300 / / / / / /
200 / p
.00//
o LIQUID POOL
• SOLID
10 (At. % S )
20
Figure (75) - Inclusion composition i n terms of the Ca:Al r a t i o and sulfur content (as CaS) in R-4340(1).
326
10 c o CO
r—I u c •H
CO U
-P cd
0.8
0.6
0.4
0.2 • (§) Ca-Si , solid and liquid * ® Hypercal,
(§) A l - S i , '» »» »«
2.0 4.0 6.0 80 10.0 12.0
Sulfur Composition i n Inclusions (at. S)
Figure (76) - Inclusion chemical composition (oxide and s u l f i d e phases) i n a rotor steel deoxidized with three deoxidizers; R-RS-I, R-RS-II and R-RS-III.
327
(c ) (d)
F i g u r e (77) - S e g r e g a t e e n r i c h e d i n A l (40 a t . %) , C a (41 a t . %) , S i (17 a t . % ) t a n d Mn ( b a l a n c e ) i n t h e r o t o r s t e e l d e o x i d i z e d w i t h A l - 6 5 w t . % S i . (a) A l , (b) C a , ( c ) S i a n d (d) M n , 250 X.
328
4 0 1 — i — i — i — i — i — i — i — i — r — r
Calcium conlent of steel ppm
Figure (78) - Inclusion"precipitation sequence"in a s t e e l containing two levels of s u l f u r .
329
Figure (79) S t a t i s t i c a l determination of the mean inclusion diameter (ym). (a) sample from an ingot, (b) sample from l i q u i d pool.
330
Ca
A l
Ce
Figure (80) - A l , Ca and Ce d i s t r i b u t i o n i n an i n c l u s i o n of a sample extracted from the l i q u i d pool by a quartz tube containing a RE-wire. (a) BE photograph, 4000 X (b) A l , Ca and Ce d i s t r i b u t i o n across the
i n c l u s i o n .
331
Figure (81) - (a) - BE photograph = 6COO X, and A l , Ca, Ce and La d i s t r i b u t i o n s i n a l i q u i d pool CaSi deoxidized. La and Ce come from a RE-wire p r e v i ously located i n the s i l i c a t e tube.
332
Figure (82) - (a) - BE photograph, 4000 X and A l Ca and Zr d i s t r i b u t i o n s i n an i n c l u s i o n of a sample extracted from a l i q u i d pool deoxidized with "hypercal". The Zr was previously located i n the s i l i c a tube.
333
Figure (83) - Composition p r o f i l e s and maps of an i n c l u s i o n i n a sample extracted from a (ESR) l i q u i d pool deoxidized with "hypercal". (b), (c), (d) and (e) are A l , Ca, S, and Zr.
334
(e) (f)
Figure (84) - BE photograph (a) * 4000 X and composition maps from an i n c l u s i o n extracted from a l i q u i d pool deoxidized with "hypercal"; (b) Ca, (c) A l , (d) S, (e) Si and (f) Zr.
335
F i g u r e (85) - I n c l u s i o n d i s t r i b u t i o n i n a d e n d r i t i c s t r u c t u r e of 1020-steel samples taken from l i q u i d p o o l d u r i n g r e f i n i n g . ( a), (b) and (c) show commonly found i n c l u s i o n s i n an A l - d e o x i d i z e d i n g o t . (a) 50 X, (b) and (c) 170 X.
3 3 6
337
Figure (87) - Isothermal (1823°K) p r e c i p i t a t i o n (Fe-Al-Ca-O-S) diagram at 0.1 a c t i v i t y of aluminum.
338
Figure (88, - E f f e c t of the a c t i v i t y of A l (h = 0.001
Symbols: A - A1 20 3
S - MnS (11,111) C-6A - CaO-6Al 20 3
C-2A I ?a"2A? 0 ^ " ' ( C a ' M n ) S ° r C a S
"2 3 C-A - CaO.Al 20 3
C - CaO A ( l ) - A 1 2 ° 3
( 1 )
339
Figure (89) - E f f e c t of the a c t i v i t y of S (h = 0.1, 0.01 and 0.001) on the " p r e c i p i t a t i o n sequence" of calcium aluminates.
Symbols are defined i n Figure (88).
340
(b)
Figure (90) - Inclusions extracted from a Ca-Si-deoxidized ingot by the Iodine-Methyl Acetate-methyl alcohol method. (RIII-Il-Sl-SLD). Photographs were taken at: (a) 3000; (a 1) and (b) 8000 X.
341
(a) (b)
(c)
Figure (91) - Inclusions extracted from a Ca-Si-deoxidized ingot by the Iodine-methyl acetate-methyl alcohol"method. (RIII-I1-S3-SLD). (a) and (b) 4000 X and (c) ^ 1000 X. (calcium aluminates)
3 4 2
F i g u r e (92a) - C a l c i u m a l u m i n a t e / c a l c i u m s u l f i d e i n t e r f a c e s o f i n c l u s i o n s i n C a S i d e o x i d i z e d i n g o t s , (a) and (b) are SEM and EPMA photographs 1.2 x 10 3 and 6.0 x 10 3 X. (b) a l s o i n c l u d e s Ca and S a n a l y s i s . (c) are t y p i c a l compo s i t i o n s o f core and p e r i p h e r y of i n c l u s i o n s , r e s p e c t i v e l y .
343
Figure (92b) : Spherical calcium aluminate (core) v/ith ( p e r i pheral) s u l f i d e phases i n CaSi deoxidized ingots (a) and (b) EPMA photographs. (c), (d) and (e) are A l , Ca and S maps, respe c t i v e l y .
3 4 4
RII-I2
Figure ( 9 3 ) - Secondary dendrite arm spacing i n a round 1 0 2 0 MS — ESR ingot.
345
R I I I - I l
i n g o t r a d i u s (cm) m ° U l d W a l 1
F i g u r e (94) - Secondary d e n d r i t e arm spacing i n a round (200 i n diameter) ESR i n g o t . 1020 MS.
346
R-4340 (1)
wall
Ingot Radius (cm)'
(95) - Secondary dendrite arm spacing in a round (200mm in diameter) ESR-ingot (4340)'.
347
RII-I2
8 9 mold wall
Ingot radius (cm)
Figure (96) - Radial size d i s t r i b u t i o n of inclusions in an Al deoxidized ingot.
348
R I I I - I l
u cn -u 0) e (0 •H Q C O •H tfi
i H C J
c
3 o o R
o o
o o o o
o o o o
o 8 § 8
A 3 measurements same points • average large dia. inclusions measurements o average value from one picture
1 i i i i ' I
o o
4 5 6
Ingot Radius (cm)
-I I
8 9 mould wall
Figure (97) - Radial inclusion size d i s t r i b u t i o n i n a low Ca-deoxidized ingot (1020 MS).
349
R I I I - I l
F i g u r e (98) - R a d i a l i n c l u s i o n s i z e d i s t r i b u t i o n i n a 200 mm ESR i n g o t . ('5-biggest i n c l u s i o n s technique')
350
R 4340 (1)
-> r
CD +J CD E nJ
• H Q C O
-H CO
H U C
CD tn rd S-i CD >
5.0
4.0
3.0
2.0
1.0
8 mold wall
Ingot Radius (cm)
F i g u r e (99) - R a d i a l i n c l u s i o n s i z e d i s t r i b u t i o n i n a 4340 i n g o t (200 mm i n diameter) d e o x i d i z e d with Ca-65% S i A l l o y .
351
TABLE I
M o d i f i c a t i o n t o S t o k e s ' Law f o r D e v i a t i o n from I d e a l i t y
I d e a l c o n d i t i o n U = 2 no 2
9 — 9 r = U s t
C o r r e c t i o n due t o c r u c i b l e w a l l
u = u s t (1 + bJL)
d i s t a n c e o f p a r t i c l e c e n t e r from w a l l
b s 0. 5 t o 2
P r e s e n c e o f o t h e r p a r t i c l e s U = U . / (1 + K* /,) s t 3
* = c o n c e n t r a t i o n o f p a r t i c l e s by volume
K = 1.3 t o 1.9
I n e r t i a l e f f e c t U = U s t / ( 1 + I Re)
Re = R e y n o l d s number o f p a r t i c l e
L i q u i d p a r t i c l e
P r e s e n c e o f s u r f a c e a c t i v e a g e n t s on l i q u i d p a r t i c l e
U = 0 ^ 3 s t 2-j + 3p'
v i s c o s i t y o f l i q u i d p a r t i c l e
v + u' + Y n
s t 2u + 3 p 1 + 3y.
= r e t a r d a t i o n c o e f f i c i e n t
S l i p a t the p a r t i c l e - f l u i d i n t e r f a c e . s t Br + 2 L ;
c o e f f . o f s l i d i n g f r i c t i o n
D • Ust< l i-H'
E = 6 ' (exp'a'S (Wk - K ) / k T ) - l )
= s l i p f a c t o r
N o n - s p h e r i c a l p a r t i c l e s
S l i g h t l y deformed i n c l u s i o n
9 v X
r g ^ = e q u i v a l e n t r a d i u s
X = dynamic shape f a c t o r
R = , 7 X R J eq sed
r , = s e d i m e n t a t i o n r a d i u s s e a
x = f ( x B )
X S = c o e f . o f s p h e r i c i t y
v = 2 Ap_ g r ; 1 3 u g 3
(3 + | Re + j i - We)
2 r p U Re = R e y n o l d s number
2 r p , U J
We = Weber number * o
r = i n c l u s i o n r a d i u s
= d e n s i t y o f l i q u i d m e t a l
o = s u r f a c e t e n s i o n
TABLE II-A
Data for Invariant E q u i l i b r i a i n Fe-S-0 System
-RT i n p0 2 -RT i n p g
Invariant E q u i l i b r i a pO^i atm P S 2 / atm k c a l k c a l 2
Iron, wustite I. 560°C magnetite, 4.8 X 10~ 2 7 5.5 X l O - 1 " 100.3 50.5
p y r r h o t i t e , gas
Iron, wustite I I . 915°C p y r r h o t i t e , 3.2 X 10~ 1 7 2.2 X l o " 8 89.6 41.6
l i q u i d (1), gas
Wustite, magnetite, I I I . 942°C p y r r h o t i t e , 1.1 X l o " 1 " * 5.4 X 10~ 6* 77.6 29.3
l i q u i d (1), gas
Composition of at 915°C: N^ e = 0.50, N Q = 0.19, N g = 0.31.
Composition of l j at 942°C: N F e = 0.49, N Q = 0.19, N g = 0.32.
a Fe = 0.09.
LO
354
TABLE I I - B
E s t i m a t e d Data f o r I n v a r i a n t E q u i l i b r i a i n Fe-Mn-0,
Fe-Mn-S and Mn-S-0 T e r n a r y Systems
1527°C
Fe-Mn-0 t e r n a r y system
6 - i r o n :
L i q u i d i r o n :
S o l i d o x i d e :
L i q u i d o x i d e :
Gas :
5 80 ppm Mn
800 ppm Mn
a =0.65 FeO
a F e O - ° - 7 3
52 ppm 0
1130 ppm 0
= 0.35 aMnO
o MnO
0.27
1.2 X 10 ~ 9 atm
Fe-Mn-S t e r n a r y system
Gamma i r o n :
S o l i d (Fe) s u l f i d e :
980°C S o l i d (Mn) s u l f i d e :
L i q u i d s u l f i d e :
Gas :
15 ppm Mn
a F e S £ 1
83 ppm S
aMnS ° ' 3 7
2 1 wt. p e t . Mn
p„ = 1.0 X 10~ 7 atm
= 30 wt. p e t . S
B 1230°C
1225°C
Mn-S-0 t e r n a r y system
L i q u i d manganese:
S o l i d o x i d e :
S o l i d s u l f i d e :
L i q u i d o x y s u l f i d e :
Gas:
S o l i d manganese: L i q u i d manganese: S o l i d o x i d e :
S o l i d s u l f i d e :
Gas:
T r a c e s o f S and 0
*MnO S ° - 9 8
>MnS ~' ° ' 9 8
E 30% Mn, s 35%S, s 351 0
p„ = 7.6 X 1 0 _ 2 0 a t m "2
atm
T r a c e s o f S and O T r a c e s o f S and 0
s 0.98
p_ = 1.5 X 10" b2
MnO
MnS = 0.98
w 2
atm
= 5.8 X 1 0 _ 2 0 a t m , p = 1.2 X 10 _ i 2
TABLE III
Estimated Data for Invariant E q u i l i b r i a i n
Fe-Mn-S-0 Quaternary System
y- i r o n = 10 ppm. Mn > 1 ppm. 0
S o l i d (Mn) s u l f i d e : a M = 0.4 MnS
=900°C S o l i d (Fe) s u l f i d e : aFeS s 1
Fe(Mn)0 oxide: a ^ g 0.5 a F g 0 = 0.5
L i q u i d (1) o x y s u l f i d e : s 26 pet. FeO, 54 pet. FeS, 15 pet. MnS and 5 pet. MnO by weight
Gas: p0 2 s 3.8 X 10 _ 1 8atm, pS 2 = 1.4 X 10" 8 atm
S o l i d Fe/Mn:
"MnS"
I I . =1225°C "MnO"
L i q u i d (1):
L i q u i d (2):
90 pet. Mn
MnS
MnO
s 1
= 1
s 0.1 pet. FeO, 0.3 pet. FeS, 65.2 pet. Mns and 34.4 pet. MnO by weight
(pet. O/pct. S) for 1 2 ( m e t a l l i c ) < (pet. O/pct. S) for l j (oxysulfide)
Ul
356
TABLE IV
C a l c u l a t e d and P u b l i s h e d F r e e E n e r g y D a t a
f o r F e-O-Ca-Al System a t 1823 K ( 1 5 5 0 ° c / 2 1 6 '
E q u a t i o n J/kg 3 Atom Ref.
Sigworth"''
Sigworth'''
E l l i o t t 2
JANAF 3
JANAF 3
Ca(g) = C a ( l wt. %) 50 574 .68 (-39 481 . 52 + 49.4T)
A l ( l ) = A l (1 wt. %) -114 137 . 1 (-63 220 .68 - 27.93T)
1/2 0,(g) = 0 ( 1 wt. %) -122 498 .9 ( -117 230 .4 -• 2.89T)
2 A l + 372 0 2 = A 1 2 ° 3 -1 085 230 . 1
C a + 1/2 0 2 = CaO -437 996, .22
2 A l + 3 0 = A l 2°3 -489 480. .46
Ca + 0 = CaO -366 081. .65
CaO + A l 2 ° 3 = C a O - A l 2 0 3 -45 845 . .46
CaO + 2 M 2 0 3 = Ca0'2 A 1 2 0 3 -50 869 . ,62
CaO + 6 A 1 2 0 3 = CaO-6 A 1 2 0 3 -60 708. ,60
Ca + 2 A l + 4 0 = C a O - A l 2 0 3 -901 407 . ,57
Ca + 4 A l + 7 0 = CaO-2 A 1 2 0 3 -1 395 912. 20
Ca + 12 A l + : 19 0 = CaO-6 A 1 2 0 3 -3 363 673. 03
Ca + S = CaS -257 659 . 0
K i r e e v ^
T a y l o r 5
1. G.K. S i g w o r t h and J . F . E l l i o t t : Met• S c i . , 1974, v o l . 8, pp. 298-310.
2. J . F . E l l i o t t and M. G l e i s e r : T h e r m o c h e m i s t r y f o r S t e e l m a k i n g , A d d i s o n - W e s l e y Pub. Co., 1960.
3. JANAF T h e r m o c h e m i c a l T a b l e s , 2nd e d . , N a t i o n a l S t a n d a r d R e f . E d i t i o n , 1971.
4. V.A. K i r e e v : Sb. T r . Mosk. I n z h - S t r o i t , I n s t . , 1971, v o l . 69, pp 3-18.
5. J . T a y l o r : P r o c . B r i t . Ceram. S o c , 1967, v o l . 8, pp. 115-23.
TABLE V (216)
Equilibrium Constants for Deoxidation Reactions
Compound A c t i v i t y Product K (1823 K)
CaO 1 / ( hC a
X h 0 ) 3 - 0 5 X l o l °
CaO-Al 20 3 l / ( h C a X h A l X X h 0 } 6 , 4 6 X 1 q 2 5
CaO-2 A1 20 3 1 / ( h
C a X h A l X h 0 ) 9.46 X 10 3 9
CaO-6 A1 20 3 1 / ( h
C a X h A l X ^ > ) 2 ' 1 3 X 1 q 9 6
A1 20 3 1 / ( h A l X h 0 ) 1 ' 0 4 X 1 q 1 4
TABLE VI
E q u a t i o n s o f L i n e s Between t h e I n d i c a t e d Phases
A l 2 0 3 - C a O - 6 A l 2 0 3
h C a X h Q = 6.01 X 1 0 ~ 1 3
h A l 3 X h O = 2.13 X 1 0 " 5
h C a / h l ( 3 = 2.82 X l O " 8
C a O - 6 A l 2 0 3 - C a O - 2 A l 2 0 3
h C a X h Q = 1.59 X 1 0 " 1 2
h A l 3 X h O = 2 , 0 1 X 1 0 " 5
h C a / h 2 { 3 = 7.90 X 1 0 " 8
C a O - 2 A l 2 0 3 - O a O - A l ^
h C a X h Q = 2.24 X 1 0 " 1 2
• , 2 / 3 Y h a A l X h 0 h 2 { 3 X h „ = 1.90 X 10 5
h „ / h 2 { 3 = 1.18 X 1 0 " 7
C a O - A l 2 0 3 ( s o l i d ) ^ 0 3 0 ( 4 2 p e t . ) + A 1 2 0 3 (58 p e t . ) ( l i q u i d )
h C a X h Q = 2.27 X 1 0 1 2
h A l 3 X h 0 = 1.89 X 1 0 " 5
h C a / h 2 { 3 = 1.20 X l O " 7
CaO ( s o l i d ) -CaO (57.4 p e t . ) + A 1 2 0 3 (42.6 p e t . ) ( l i q u i d )
h C a X h Q = 3.29 X 1 0 - 1 1
h A l 3 X h 0 = 5 - 7 7 X 1 0 ~ 6
h c a / h 2 { 3 = 5.70 X l O " 6
TABLE VII
Chemical Analysis of Electrodes Used i n t h i s Research: Electrode Chemical Composition in wt.%
1020 M Steel 4340 - Steels Rotor Steel
(1) (1) (2) (1) (2) c 0 . 19 0 . 415 0.422 0.213 0 . 202
p 0 . 099 0 . 014 0.014 0 . 007 0.007
s 0.026 0.016 0.015 0.005 0 .006
Mn 0 . 709 0.697 0.696 0.673 0 . 673
Cu - 0.094 0.091 0 .049 0.048
Ni - 1.882 0 . 184 0 .357 0.0362
Cr - 0 . 873 0. 867 1. 151 1.1150
Si 0 . 25 0.357 0.353 0 . 246 0 . 244
V - 0. 005 0 .005 0 . 246 0 . 244
Mo - 0. 189 0.188 0.940* 0.940*
A l - 0 . 029 0.029 0.006 0 .006
Sn 0.005 0 .005 0.005** 0.005*
where (*) stands for more than 0.940 wt.%
and (**) indicates less than 0.005 wt.%.
TABLE V I I I - L i s t of E x p e r i m e n t s
I n i t i a l S l a g Run Type o f E l e c t r o d e C o m p o s i t i o n Type o f No. o f A d d i t i o n s Type o f D e o x i d a t i o n r a t e s No. E l e c t r o d e Diameter (mm) C a F 2 / A l 2 O 3 / C a O / S i O 2 / M g 0 D e o x i d i z e r Rates A d d i t i o n (deox.) (kg t o n - 1 )
1 4340(I) 31.75 50 30 20 - - N i l - - -2 •' " 55 15 15 15 - - - - -3 •• 40 20 20 20 - - - - -4 40 22 23 5 - - - - -5 » 55 15 22 8 - - - -6 31 0 46 23 - - - - _ i 7 •• 50 30 20 - - A l a l o n g r e m e l t i n g c o n s t a n t 2.3 kg t o n
8 •• " 55 15 15 15 - A l "
9 4340(II) 44 .75 50 30 20 - - C a - S i
10 4340 ( I I ) " 55 15 15 15 - C a - S i
11 r o t o r s t e e l 114.3 49 16 17 12 6 A l " c o n s t a n t ^ 0.0 2 kg ton
12 1020 M.S. 76. 2 70 0 30 - - A l " "
13 r o t o r s t e e l 114 ,3 70 15 15 - - A l "
14 " 70 0 30 - - A l II II "
15 " •• 50 20 30 - - A l "
RII-W 1020 M.S. 76 . 2 50 30 20 - - C a - S i 2 i n t e r m i t t e n t 50 grams (each)
RIII-W 1020 M.S. 76.2 70 30 - - - C a - S i 2 50 grams (each)
R I - I l 1020 M.S. 76.2 50 30 20 - - N i l - - -R I I - I l 50 30 20 - - A l 4 c o n t i n u o u s l y
i n c r e a s i n g 3.63, 6.1 and 3 7.6 kg t o n
RII-I2 » 50 30 20 - - A l 5 1.21, 2.42, 3.6' 4.85,6.06 and 12.1/
R I I I - I l " •• 60 36 4 - - C a - S i 4 . 5 " 5.61,11.23,16.83,2; and p a r t i a l l y 28.0J
R I I I - I 2 '• 50 30 20 - - C a - S i 6 5.61,11.23,16.83,2; 28.05 and 56.10 .
R-4340 4340 88.9 50 30 20 - - C a - S i 6 4.17,8.35,12.5,16. 20.85,41.7, and 20
R-RSI r o t o r s t e e l 114 . 3 50 30 20 - - C a - S i a l o n g r e m e l t i n g c o n s t a n t 36.0 kg t o n - 1
R-RSII 50 30 20 - - A l - S i c o n s t a n t 33.0
R-RSI 11 50 30 20 - - Hyperca1 c o n s t a n t 36.0
oo
o
361
TABLE IX
Chemical Composition of D e o x i d i z e r s
C a l c i u m - S i l i c o n Hypercal A l - S i
Calcium 29 . . 50 10 , . 50 -
S i l i c o n 6 2 . . 50 39 , . 00 65
Iron 4 , . 50 18 , .00 -Barium 0. . 50 10 , . 30 -Aluminum 1. . 20 20 , . 00 35
Manganese 0 , . 2 5 0 . . 30
Carbon 0 . . 55 0 , . 50
Chromium 0 . . 10 0 , . 0 3
Copper 0 , . 01 0 . . 03
N i t r o g e n 0 . . 0 3 0 , . 0 5
N i c k e l 0 . . 01 0 , . 02
Oxygen 0 . . 50 0 , .70
Phosphorous 0 . . 01 0 , . 02
S u l f u r o , . 0 5 5 0 , . 12
Titanium 0 , . 08 0 , . 06
Bulk D e n s i t y 110 l b . / c u . f t . 95 l b . / c u . f t .
TABLE X
I n c l u s i o n C h e m i c a l C o m p o s i t i o n as a F u n c t i o n o f S l a g and
D e o x i d i z e r i n 4340 ( s m a l l d i a m e t e r ) ESR I n g o t s
a ) I n c l u s i o n C h e m i c a l C o m p o s i t i o n as a F u n c t i o n o f S l a g System
No. Nominal S l a g System (wt.%) Atom P e r c e n t Types o f I n c l u s i o n s
C a F 2 A 1 2 0 3 CaO S i 0 2 A l Ca S i
1 50 30 20 - 96.1 3.733 0.170 C a l c i u m a l u m i n a t e s , Manganese s u l f i d e s
2 55 15 15 15 75.86 1.8470 22.28 A l u m i n o - S i l i c a t e s , Manganese s u l f i d e s , and " F a y a l i t e . " *
3- 40 20 20 20 85.33 2.470 13.160 C a l c i u m , Aluminum-s i l i c a t e s , Manganese s u l f i d e s and " F a y a l i t e " *
4 40 22 33 5 99.20 0.19 0.60 "Alumina", Manganese s u l f i d e s
5 55 15 22 8 96.27 3.462 0.265 "Alumina" and Ca-a l u m i n a t e s , " F a y a l i t e "
6 31 0 46 23 84.88 4.38 10.74 C a l c i u m - a l u m i n o s i l i c a t e s . Manganese s u l f i d e s and " f a y a l i t e " . *
b) S l a g - D e o x i d a n t E f f e c t on F i n a l I n c l u s i o n C h e m i s t r y
wt. % (X) at. % (X) +
No. CaF 2 A1 20 3 CaO S i 0 2 Deox. A l Ca S i
7 50 30 20 - A l 92.42 7.11 0.170 C a l c i u m a l u m i n a t e s -C a l c i u m s u l f i d e s
8 55 15 15 15 A l 89.20 7.85 2.940 A l u m i n a t e s , C a l c i u m Aluminum s i l i c a t e s and " F a y a l i t e " . *
9 50 30 20 - C a - S i 91.00 7.20 1-80 C a l c i u m a l u m i n a t e s , (*) Manganese s u l f i d e s
10 55 15 15 15 C a - S i 76.217 2.143 21.64 Aluminum c a l c i u m s i l i c a t e s , (*) f a y a l i t e * and Manganese
s u l f i d e s
c) I n c l u s i o n C h e m i s t r y o f E l e c t r o d e s
A l Ca S i Electrodes f o r 1 to 8 (31.75 im) 89.00 10.00 balance C a l c i u m A l u m i n a t e s -
c a l c i u m s u l f i d e s Electrodes!*) for 9 and 10 (44.75 nm) 81.08 5.237 13.685 Aluminum C a l c i u m s i l i c a t e s
and Manganese s u l f i d e s
Remarks: 1. Melting rates 1.2 - 1.5 Kg min. - 1, 2. deoxidation rate"^2.3 Kg t o n - 1
3. Fay a l i t e * was not always observed to follow the theoretical stoichiometry.
TABLE XI
C h e m i c a l E f f e c t o f S l a g and E l e c t r o d e S u r f a c e P r e p a r a t i o n on I n c l u s i o n C o m p o s i t i o n ( E x t e n s i o n o f R e s u l t s Found i n 4340 S m a l l and L a r g e E S R - i n g o t s , and
1020 S t e e l s t o a Cr-V-Mo R o t o r S t e e l *
I n g o t E l e c t r o d e Type Nominal S l a g C o m p o s i t i o n I n c l u s i o n Type and Shape
C a F 2 CaO A 1 2 ° 3 s i 0 2 M g 0
* 11 r o t o r s t e e l 49 16 17 12 6 A l - C a s i l i c a t e s and a l u m i n a t e s . Semiround
t y p e s ; t h e y were o c c a s i o n a l l y seen w i t h p e r i p h e r a l MnS. Mg t r a c e s were a l s o det e c t e d .
12 r o t o r s t e e l 70 15 15 - - A l u m i n a t e s o f t h e t y p e F e 0 - A l 2 0 3 and A 1 2 0 3 . Round, e l o n g a t e d ( F e O - A l 2 0 3 ) and c l u s t e r s and a n g u l a r A 1 2 0 3 .
* 13 r o t o r s t e e l 70 30 - - A l u m i n a t e s , s i n g l e r o u n d and c l u s t e r s .
I r o n o x i d e s and some i r o n s u l f i d e s were a l s o o b s e r v e d .
* 14 r o t o r s t e e l 50 20 30 - - A l u m i n a t e s , s i n g l e r o u n d and c l u s t e r s and
a minor amount o f r o u n d C a - a l u m i n a t e s and MnS I I .
15 1020 70 - 30 - - A l u m i n a t e s g e n e r a l l y as c l u s t e r s and MnS I I .
Remarks 1. These E S R - i n g o t s were s l i g h t l y d e o x i d i z e d w i t h A l a t a c o n s t a n t r a t e , 0.02 Kg t o n ^. 2. E l e c t r o d e s 11, 12, 13 and 14 were s u r f a c e g r o u n d and c o a t e d w i t h an Al-Mg s p i n e l p a i n t i n g t o
p r e v e n t s c a l e ("FeO") o x i d e f o r m a t i o n d u r i n g r e m e l t i n g . 3. E l e c t r o d e and E S R - i n g o t c o m p o s i t i o n a r e g i v e n i n T a b l e s (VII) and ( V I I I ) . 4. R e f i n i n g c o n d i t i o n s were e q u i v a l e n t f o r a l l e x p e r i m e n t , A r - a t m o s p h e r e and m e l t i n g r a t e s a b o ut
1 Kg min
TABLE XII
Slag Chemical A n a l y s i s from (Ni-Cr-Mo) Rotor ESR-ingots *
Deoxidized with CaSi, A l S i and Ca-Al-Ba-Si A l l o y s
CaF 2 CaO A 1 2 0 3 S i 0 2 "FeO"
Nominal ( i n i t i a l ) s l a g composition (wt.%) 50 20 30
C a S i * * 47.33 20.15 32.2 0.175 0.14
A l S i 43.32 18.27 38.11 0.141 . 0.147
H y p e r c a l * (Ca, A l , Ba, S i a l l o y ) 43.51 18.19 37.88 0.25 0.165
* * Remarks: Deoxidation r a t e used i n t h i s experiment was s l i g h t l y lower than i n
the other two experiments, i . e . ^ 33 grams/min and ^ 36 grams/min r e s p e c t i v e l y .
Remelting c o n d i t i o n s were approximately constant f o r the above runs, i . e . , m e l t i n g r a t e s were ^ Kg/min. Experiments were c a r r i e d out under a p r o t e c t i v e atmosphere (argon).
cn
TABLE XIII - A
Chemical Analysis (wt.%) of a Cr-Mo-Mn-Ni-V-Steel
Deoxidized with Al-65.0 wt. % Si
C P s Mn Cu Ni Cr Si V Mo Al Sn
1 0 . , 261 0 . .008 0 . 003 0 . .707 0. ,050 0 , . 353 1. 176 0 . ,594 0 . 239 +0 , .94 + 0 .250 -0 .005
2 0 . . 252 0 . .007 0. ,003 0. , 700 0. .051 0, . 369 1. 165 0. , 536 0 . 246 + 0, .94 + 0, .250 -0 .005
3 0 . . 255 0. .007 0. .003 0 . .695 0 . .050 0 . 350 1. , 160 0. , 532 0 . 242 + 0 .94 + 0 .250 -0.005
4 0. . 253 0. .007 0. .003 0. .695 0. ,050 0 . 348 1. 160 0. . 536 0. 242 +0 .94 + 0 .250 -0.005
5 0. . 251 ' 0 . .007 0. .003 .0 , . 689 0. .051 0 . 362 1. . 147 0 . . 536 0 . 245 + 0 .94 + 0 .250 -0 .005
6 0 .249 0. .007 0. .003 0, .691 0. .049 0 . 355 1. . 155 0 , .534 0 . 241 + 0 .94 + 0 .250 -0.005
7 0 . 255 0, .007 0, .003 0 .693 0, .051 0 . 354 1. . 155 1 0 , . 538 0 . 243 + 0 .94 + 0 .250 -0.005
8 0 . 250 0 .007 0 .003 0 .695 0, .051 0 . 359 1. . 160 0 . 538 0 . 243 + 0 .94 + 0 .250 -.0.05
9 0 . 260 0 .008 0 .003 0 .701 0, .052 0 . 361 1, . 161 0 . 539 0. 245 + 0 .94 + 0 .250 -0 .005
10 0 .254 0 .007 0 .003 0 . 694 0 .052 0 .361 1 . 154 • 0 . 543 0 . 246 + 0 .94 + 0 .250 -0 .005
TABLE X I I I - B
C h e m i c a l A n a l y s i s (wt.%) o f a Cr-Mo-Mn-Hi-V S t e e l
D e o x i d i z e d w i t h a C a - S i A l l o y
p i e No. C P c Mn Cu N i C r S i V Mo A l Sn 1 0 . 262 0 .007 0. .002 0, .691 0 .049 0 .337 1.103 0 .627 0 . 247 +0 .94 0. . 157 -0 .005
2 0 .271 0. .007 0 .002 0 .696 0 .048 0 . 334 1. 116 0 .674 0 . 236 + 0, .94 0. . 150 -0 .005
3 0 .242 0 .007 0 .002 0, . 673 0 .046 0 . 321 1.091 0 .646 0 . 235 + 0. .94 0. . 147 -0 .005
4 0 . 258 0, .007 0, .002 0, .688 0 .045 0 . 324 1.118 0 .650 0 .235 +0, .94 0 . .117 -0 .005
5 0 .266 0. .007 0. .002 0. .691 0 .04 8 0 . 343 1.115 0 .648 0 .244 + 0. .94 0 . .126 -0 .005
6 0 . 226 0, .007 0. .002 0. .690 0 .047 0 . 340 1. 119 0 .654 0 . 242 +0. .94 0 . . 126 -0 .005
7 0 . 273 0. .007 0. .002 0 . . 687 0 .047 0 .341 1. 116 0 .652 0 .652 + 0. .94 0 . .125 -0 .005
8 0 .264 0. .007 0. .002 0. . 680 0 .046 0 . 337 1. 1031 0 . 645 0 . 240 + 0. .94 0 . , 121 -0 .005
9 0 . 273 0. ,007 0 . .002 0. . 685 0 .048 0 .351 1. 114 0 .660 0 . 246 + 0 . .94 0. , 121 -0 .005
10 0 . 276 0. .007 0. .002 0. , 687 0 .047 0 .332 1. 118 0 . 652 0 . 241 + 0. .94 0 . , 119 -0 .005
11 0 . 228 0. .007 0. .002 0. , 700 0 .049 0 . 355 1. 125 0 . 665 0 . 245 + 0. .94 0 . , 122 -0 .005
(+) - more t h a n a c e r t a i n c a l i b r a t i o n
(-) - l e s s t h a n a c e r t a i n c a l i b r a t i o n
cn
TABLE X I I I - C
C h e m i c a l A n a l y s i s o f a C r - M o - M n - N i - V - S t e e l
D e o x i d i z e d w i t h a C a - S i - A l - B a A l l o y
%C 9 p % s %Mn %Cu ? N i % C r S i V %Mo % A l Sn
1 0 . 253 0 008 0. 004 0.706 0.056 0 366 1. 126 0 815 0 229 +0 .94 + 0 .250 -0 005 2 0. 255 0 007 0. 003 0.702 0.053 0 366 1. 146 0 772 0 241 + 0 94 + 0 250 -0 005 3 0. 253 0 008 0. 004 0. 704 0.051 0 34 7 1. 160 0 757 0 234 + 0 94 + 0 250 -0 005 4 0. 267 0 008 0. 004 0.718 0.052 0 356 1. 174 0 774 0 236 + 0 25 + 0 250 -0 005 5 0.263 0 008 0. 003 0.714 0. 056 0 382 1. 164 0 793 0. 248 + 0 94 + 0 250 -0 005
U l CTi
TABLE XIV-A
Example o f i n c l u s i o n s i z e d i s t r i b u t i o n i n R l l l - I l - L Q D - S a m p l e 1.
Sample I n c l u s i o n Shape F l o r e s c e n c e D i a m e t e r No. No. under t h e i n pm
e l e c t r o n beam
1 r o u n d b l u e 4 . 5
2 e l o n g a t e d b l u e 5.0
3 s e m i r o u n d b l u e 6.0
4 e l o n g a t e d b l u e 7.0
5 r o u n d b l u e 4.5
6 S-shape b l u e 6.0
7 r o u n d b l u e - g r e e n 6.0
8 a n g u l a r b l u e 4 . 5
9 d u p l e x - r o u n d 2 - b l u e 5.0
10 r o u n d b l u e 4.0
11 t r i a n g l e b l u e 5.5
12 d u p l e x b l u e 8.5
13 e l o n g a t e d b l u e 5.5
14 e l o n g a t e d b l u e 5.5
15 t r i a n g l e b l u e 5.5
16 a n g u l a r b l u e 4.5
17 e l o n g a t e d b l u e 6.5
18 r o u n d l i g h t - b l u e 8.0
19 d u p l e x 2 - b l u e 8.0
20 i r r e g u l a r b l u e 6 . 0
21 r o u n d b l u e 8.0
22 r o u n d b l u e 6.0
369
TABLE XIV-B Example o f I n c l u s i o n S i z e D i s t r i b u t i o n i n R I I I - I l - S L D - S l
Sample No.
L u s i o n No. Shape F l u o r e s c e n c e
D i a m e t e r i n ym
1 s e m i - r o u n d b l u e 4.0
2 e l o n g a t e d b l u e 6.0
3 r o u n d v i o l e t 5.5
4 e l o n g a t e d b l u e 4 . 5
5 r o u n d b l u e 4 . 5
6 i r r e g u l a r b l u e 5.5
7 e l o n g a t e d b l u e 5.5
8 r o u n d b l u e ^ 8.0
9 i r r e g u l a r b l u e 6.0
10 e l o n g a t e d b l u e 5.0
11 r o u n d b l u e 6.0
12 i n a c l u s t e r -r o u n d
b l u e - g r e e n 5.5
13 i n a c l u s t e r -r o u n d
b l u e 6.5
14 e l o n g a t e d b l u e 5.0
15 r o u n d b l u e 5.0
16 h a l f - m o o n shape
b l u e 7.5
17 e l o n g a t e d -i r r e g u l a r
b l u e 5 .0
18 e l o n g a t e d b l u e 5.0
19 e l o n g a t e d b l u e 4 .0
20 e l o n g a t e d b l u e 5 .0
TABLE XV
Data o f P l o t F i g u r e s (87 - 89)
E q u i l i b r i a
A l 2 0 3 / C a O - 6 A l 2 0 3 / C a S
6 A l 2 0 3 - C a O / C a O - 2 A l 2 0 3 / C a S
C a O - 2 A l 2 O 3 / C a 0 - A l 2 0 3 / C a S
+ C a O - A l 2 0 3 / ( C a O ) * + ( A l ^ W C a S
+ + C a 0 s o l i d / ( C a 0 ) t + ( A l 2 0 3 ) + / C a s
V h S
1.4155 x 10'
1.995 x 10"
Ca S
-3 0.758 x 10
4 x 1 0 ~ 3
^ 1.35 x 10"
2.3155 x 10
1.83 x 1 0 ~ 9
3.360 x 1 0 - !
6.87 x 1 0 ~ 9
^ 1.35 x 10
-10
-8
A l S
1.4228 x 10
7.85 x 1 0 ~ 4
1.2457 x 10
2.5 x 1 0 " 3
-4
-3
^ 3.84 x 10 -3
h A l h O
-5 2-014 x 10
! - 5 6 7 x 1 0 " 5
1.2156 x l 0 - 5
1-0 x l o " 5
% 9 -61 x 1 0 ~ 6
* t S l a g c o m p o s i t i o n s a c c o r d i n g t o the C a O - A l ^ p s e u d o - b i n a r y d i a g r a m a t 1827° K (1550°C)
0.7, a, + 3 C a S = ° - 0 3 " , * A l 2 o 3
+ + a C a S = °- 9*8 -1.0, a A l a
CaO
0.1, a
2 U 3 0.0625
CaO
(238,239)
0.8 - 0.9
CaS ~ f r o m Sharma and R i c h a r d s o n @ x = 0 568 C a 0 ' Y C a S ( s o l i d )
They s u g g e s t v /< Y C a S ( s o l i d ) / J = Y C a S ( l i
65
i q u i d ) and X = 1 x 10 3 - 2 x 1 0 ~ 3 . T h i s i s a l s o an a v e r a g e v a l u e s i n c e @ 1650°C X
CaS 6.3 x 10 and a v = l^ f i f l r C a S ( 1 6 5 0 ° C ) ' i b 6 B
371
T A B L E XVI
E q u i l i b r i u m ( i n v a r i a n t )
and e A 1 =-5.25, e^a = -62, and «? = - 4 0
%Ca %0 %A1 %S
(1) 1 x 10" 3 2.65 x 10~ 2 2.35 x 10" 3 7.4 x 10" (2) 2.5 x 10~ 3 2.6 x 10~ 2 5 x 10~ 2 5.35 x 10 (3) .5 x 10~ 3 2.35 x 10~ 2 1 x 10~ 2 3.95 x 10 (4) 8 x 10" 3 2.27 x 10" 2 1.2 x 10~ 2 2.75 x 10 (5) 1 x 10" 2 2.15 x 10" 2 1.89 x 10" 2 1.56 x 10'
2
2
-2
2
TABLE XVII
Computed Compositions by Using Data i n Table XV
Interaction Parameter Composition (ppm)
e£ a Ca A l 0
Equilibrium (invariant) (i) -535 10 15 20
II (2) -400 25 38 32 (3) -300 65 97 34
ti (4) -250 80 120 36 ii (5) -200 100 150 42
The in t e r a c t i o n parameter for the C a - 0 was assumed variable and the A l - 0 and Ca-S were:
e A l = -62
and
-110
TABLE X V I I I
E f f e c t o f I n i t i a l Number o f I n c l u s i o n s on
Growth D u r i n g C o o l i n g o f L i q u i d M e t a l
Number o f I n c l u s i o n s I n i t i a l F i n a l Growth Time I n i t i a l l y R a d i u s R a d i u s (Lowe r L i m i
1 0 3 / c c 1 ym 40 72 um 279. 5 s e e s
» 2 40 72 268. 0
5 40 75 238 . 9
9 40 87 208 . 3
10 40 92 201. 7
1 0 4 / c c 1 18 90 57. 37
2 18 91 52. 79
5 19 02 42. 40
" 9 19 56 32. 87
" 10 19 79 30. 98
1 0 5 / c c 1 8 78 11. 23
2 8 81 9 . 58
5 9 28 6 . 38
9 11 2 4 . 15
10 11 88 3. 79
1 0 6 / c c 1 4 09 2. 02
2 4 23 1. 5
" 5 5 77 0. 76
9 9 27 0 . 44
10 10 22 0. 39
1 0 7 / c c 1 1 98 0. 31
It 2 2 45 0. 19
" 9 9 03 0. 04
10 10 02 0. 0397
374
APPENDIX
Thermodynamic r e l a t i o n s h i p s developed to generate the sur
faces of s t a b i l i t y f o r the Fe-Ca-Al-O-S system, u s i n g data
from the l i t e r a t u r e ( 1 4 7 ' 1 4 8 ' 2 1 6 ' 2 3 2 ' 2 3 8 ' 2 3 9 > .
E q u i l i b r i a ( I ) : Al 20 3/CaO/6Al 20 3-CaO/CaS
1(1): 6 A 1 2 ° 3 + 2 C a 0 + [ S ] = 6Al 20 3-CaO + CaS + [0]
A G ° = A G o ^ + A G O A S = B A G - ^ - 2 A G ° a Q = R T l n K I ( 1 )
l f a 6 A l 2 0 3 - C a O ~ a"CaS" ~ a A l 2 0 3 ~ aCaO ~ 1
3 h 0 7.082 X 10 = -RT l n [^] h S
-1 h O l n K I ( l ) = " 1 - 9 5 5 K i ( l ) = i ^ i s s x 10 = — •
S
h Q = 1.4155 X 1 0 _ 1 h g A-I (1)
1(2): 6 A 1 2 ° 3 + 2 f C a ^ + + [S] = 6Al 20 3-CaO + CaS
-RT InK = -RT l n [ - 5 - ^ ] = -1.6781 X 10 5
h C a h O h S
1 ?n l n K I ( 2 ) = 4 - 6 3 2 7 x 1 0 t h u s K i ( 2 ) = 1 - 3 1 6 8 x 1 0
h C a h O h S = 7 ' 5 9 4 2 x i O " 2 1 / b Y s u b s t i t u t i n g A - I ( l )
h C a h s = 2.316 X 1 0 " 1 0 A-I(2)
375
1 ( 3 ) : 12 [Al] + 18[0] + CaO + [S] = 6 A l 2 0 3 « C a O + CaS
-7.8183 X 10 5 = -RT l n K l ( 3 ) -> l n K I ( 3 ) = 2.158 X 10 2
9 3 K I ( 3 ) = 5.46657 X 10 and by s u b s t i t u t i n g A-I(2)
h A l h O = 2 ' 0 1 4 X 1 0 5 A-I(3)
E q u i l i b r i a ( I I ) : 6Al 20 3•CaO/CaO/2Al 20 3«CaO/CaS
11(1): |(6A1 20 3-CaO) + y(CaO) + [S] = 2 A l 2 0 3 « C a O + CaS + [0]
AG° = AG° + A G ° A S - | ( A G ° A 0 ) - | ( A G ) = 2 D
- R T k n K I I ( 1 )
± f aCaS " a C A 0 " aCaO " a C A c " 1
2 O
- R T l n K I I ^ 1 j = 1.4257 X 10 4 + l n K ^ ^ j = - 3.9359
K I I ( D = 1 ' 9 5 3 X 1 0 ~ 3
h 0 -2 thus y- = 1.953 X 10 A - I I ( I ) h s
11(2): j ( 6 A l 2 0 3 - C a O ) + |[Ca] + |[0] + [S] =
2A1 20 3 «CaO + CaS
- R T l n K I l ( 2 ) = 1. 3146 X 10 5 -»• l n K I l ( 2 )
3.629 X 10 1
5 2 K I I ( 2 ) = 4 - 3 1 1 2 x 1 q 1 5 * h c a h O h S = 2 - 3 1 9 5 x i O " 1 6
by s u b s t i t u t i n g A - I I ( l )
h C a h S £ 1 , 6 0 8 X 1 0 ~ 9 A - I K 2 )
11(3): 4[A1] + 6[0] + [Ca] + [S] = 2Al2<D3-CaO + CaS
- R T l n K ( l I ( 3 ) = - 3.11824 X 10 5 -+
l n K I I ( 3 ) = 8 ' 6 0 8 X l o 1 •* K n ( 3 ) = 2 - 4 3 2 4 X 1 0 3 7
by s u b s t i t u t i n g A-II(1)
h A l h O = 1- 7 0 4 X 1 0 ~ 5 A - I K 3 )
E q u i l b r i a ( I I I ) : 2Al 20 3-CaO/CaO/Al 20 3•CaO/CaS
1 3 I I I ( l ) : 2(2A1 20 3-CaO) + j CaO + [S] = A l ^ - C a O + CaS + [O]
377
AG° = A G ° A + A G C a S = | ( A G ° A 2 ) - § ( A G ° a 0 ) =
- R T l n K I I I ( 1 )
l f aAl 20 3.CaO " aCaS ~~ aCA 2 ~ aCaO " 1
= R T l n K I I l ( 1 ) = 1 . 6 7 6 9 7 X 1 0 4 + L N KI I I ( 1 ) = - 4 . 6 3
h Q = 9 . 7 5 8 X 1 0 " 3 h g A - I I I ( l )
111(2): y(2Al 20 3-CaO) + |(CaO) + [Ca] + [S] = A l ^ - C a O + CaS
- R T l n K I I l ( 2 ) = 7.0675 X 10 4 l n K I I l ( 2 ) =
1.95111 X 10 1 -»• K I I I ( 2 ) = 2.97546 X 10 8
by substituting A - I I I ( l )
hCa hS = 3 , 3 6 X 1 0 ~ 9 A-IIK2)
111(3): 2[A1] + 3[0] + [Ca] + [ s ] + CaO = A l ^ - C a O + CaS
5
- R T l n K I I I ( 3 ) = -1.9366 X 10 ^ l n K I I l ( 3 ) =
5.346 X 10 1 KI I I ( 3 ) = 1-6568 X 10 2 3
378
by s u b s t i t u t i n g A - I I I (2) 2
h A l h O = i - 2 1 5 6 x !0~ 5 A - I I I (3)
E q u i l i b r i a ( I V ) :
A1 20 3-CaO/CaO(42 .0 wt.%) + A12C>3 58.0 wt. %
( l i q u i d ) / C a S
I V ( 1 ) : 2CaO + A l 2 0 3 + [S] = C a O - A l ^ + CaS + [0]
- R T l n K I V ( 1 ) = 1.06873 x 10 4 ->
l n K I V ( l ) = " 2 - 9 5 8 6 + KI V ( 1 ) = "5.18915 X 10~ 2
u 2
0 a
h ~ = CaO a A 1 0 X 5.18915 X l O - 2
S aCaS 2 3
l f aCaO - 0 . 0 6 2 5 ( 2 1 6 ' 2 3 2 ) , a 5 - 0 . 7 ( 2 3 2 ) and
a . C a S = 0 . 0 3 5 5 ( 2 3 8 ' 2 3 3 >
then h Q = 4.0 X 10~ 3 h g A-IV (1)
I V ( 2 ) : 2[Ca] + 2[A1] + 4 [0] + [S] = C a O - A l ^ + CaS
-RTlnK = -2.8111 X 10 5 ->
K I V ( 2 ) = 5 ' 0 5 3 3 4 x 1 q 3 3
by s u b s t i t u t i n g a = 0.0355 ( 2 3 8 ' 2 3 9 ^ and A-IV (1) (-3 O
h A l h O = 1 X 1 0 5 A _ I V ( 2)
I V ( 3 ) : CaO + 2[Al] + 3[0] + [Ca] + [S] = C a O - A l ^ + CaS
A Gt° = A G C A + A G C a S " A GCaO = - R T l n K I V ( 3 )
i f aCaO = A l 2 ° 3 = 1
KTT7,,> = 1.65287 X 1 0 2 3 = C a S [ IV(3) " — - - i 2 .3. T~-aCaO h A l h O h C a h S
by s u b s t i t u t i n g a , a and A-IV (2) L 3 b (_,civj
h C a h S = 6 , 8 7 X 1 0 _ 9 A - I V ( 3 )
E q u i l i b r i a (V):
CaO ( s )/CaO(57.4 wt.%) + A l ^ (42.6 wt.%) ( l i q u i d )
/ C a S ( s )
380
± f aCaS B 0.988 - 1 . 0 ( 2 3 8 ' 2 3 9 ) , a A l O , , = 0 . 1 ( 2 3 2 )
and an _ s 0.8 - 0 . 9 ( 2 1 6 ' 2 3 2 )
CaO
h C a h s s 1.35 X 10 8 A-V (1)
h A l h O = 9.6 X 10 6 A-V (2)
hCa hO S 3.29 X 10 1 1 A-V (3)
h Q = 2.35 X 10" 3 h s A-V (4)