Si and Ni as Alloying Elements to Vary Carbon Equivalent of Austenitic Ductile Cast Iron- Microstructure and Mechanical Properties-2

7/26/2019 Si and Ni as Alloying Elements to Vary Carbon Equivalent of Austenitic Ductile Cast Iron- Microstructure and Mecha

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Materials Science and Engineering A 504 (2009) 8189

Contents lists available atScienceDirect

Materials Science and Engineering A

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m s e a

C, Si and Ni as alloying elements to vary carbon equivalent of austenitic

ductile cast iron: Microstructure and mechanical properties

Nabil Fatahalla a,, Aly AbuElEzz b, Moenes Semeida b

a Mechanical Department, Faculty of E ngineering, Al Azhar University, Cairo, Egyptb National Institute for Standards, Force and Materials Metrology Department (FMMD), El Haram, Giza, Egypt

a r t i c l e i n f o

Article history:

Received 16 September 2008

Received in revised form 14 October 2008

Accepted 15 October 2008

Keywords:

Austenitic ductile iron

Carbon equivalent

Microstructure

Mechanical properties

Alloying elements

a b s t r a c t

Successful casting of three groups of austenitic ductile irons was achieved covering a carbon equivalent

(CE) range from 3.51% to 5.04%. The three groups implied the change of C, Si or Ni contents to control

the CE%. In case of using Ni element to vary CE%, austenitic ductile iron could be obtained starting from

13.5% up to 34.7% Ni. Generally, the microstructure consisted of graphite nodules embedded in austenitic

matrix. Nodule characteristics were affected by the variation of CE%. Nodularity was almost 100% for all

tested specimens. Slight decrease in hardness andtensile strength (u) was observed with increasing the

CE%. 0.2% proof stress (0.2) showed almost a constant value with increasing CE%. Tensile elongation was

mainly increased with increasing CE% with different degrees owing to the alloying element (C, Si or Ni).

2008 Elsevier B.V. All rights reserved.

1. Introduction

Austenitic ductile cast irons are series of cast irons that con-

tain nickel from 18 up to 36 mass%, having been treated with

magnesium to bring about the formation of nodular graphite

[1]. It contains sufficient nickel to produce an austenitic matrix

structure similar to that of austenitic stainless steel. These irons

have tensile strength ranging from 3870 to 5620 MPa, elongation

from 4% up to 40% and Brinell hardness ranging from 1110 to

1710MPa [16]. These high nickel alloyed ductile cast irons are

made in a number of different compositions to produce the desired

properties [13,713]. While conventional foundry practices are

used for the production of Ni-resist ductile iron castings, specialprecautions, not normally used, must be taken into consideration.

Treating and gating practices, and pouring temperature must be

modified considerably from thoseused in conventional ductile iron

production. For this reason, design engineers and Ni-resist ductile

cast iron producers should reviewproposed casting designs if min-

imum cost and maximum product reliability are to be obtained

[1]. Numerous data have been published about the production,

microstructure and mechanical properties of austempered duc-

tile cast iron (ADI) [17,1419] and conventional ductile iron

[17,2031]. Few information[13,713]do exist for the produc-

Corresponding author. Tel.: +22 24010200; fax: +2 160854008.

E-mail address: [email protected](N. Fatahalla).

tion and properties of austenitic ductile iron in a narrow range of

CE%. To fill this gap, the present investigation focused on studying

the effect of CE% in a wide range, for austenitic ductile cast iron,

on microstructure and mechanical properties. C, Si and Ni were,

each solely, used as alloying elements to vary the CE% in the range

3.515.04.

2. Experimental procedure

Three heats (A, B and C)were prepared in a 90kg high frequency

(1000 Hz) induction furnace. Charges were low sulphur, low man-

ganese, and low phosphor pig iron (Sorel metal) and steel scrap (cf.Table 1).Necessary amounts ofSi, C and Niwere added toyielda Si-

content 1.635.31mass%, C-content 2.13.5 mass%, and Ni-content

4.9934.70 mass%. Melts were superheated to 17731823 K. Mag-

nesium treatment and inoculation were performed using the

Sandwich Technique[1]for producing ductile cast iron. The fer-

rosilicon alloy containing 10% Mg was used in the spheroidising

treatment. The heats were inoculated with 0.5mass% of the charge

with FeSi alloy (65% Si).The grain size of inoculants ranged from 1.5

to 3 mm. Pure Ni was melted with raw materials to get austenitic

ductile iron in the as cast condition.Table 2lists the actual chem-

ical composition of all heats involved in this study. The melt was

poured at a temperature ranging from 1620 to 1640K into two dif-

ferentmoulds toproduce specimensfor both chemical analysisand

tests. A half-inch Y-block sand mould was used (cf.Fig. 1). Carbon

0921-5093/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2008.10.019


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82 N. Fatahalla et al. / Materials Science and Engineering A 504 (2009) 8189

Table 1

Chemical composition of the raw materials used to produce austenitic ductile cast

iron in the present study.

Raw materials Composition%

C Si Mn S P Ni Fe

Sorel metal 4.0 0.1 0.1 0.02 0.03 0.0 Balance

Steel scrap 0.16 0.15 0.6 0.02 0.03 0.0 Balance

Ferrosilicon 0.0 65.0 0.0 0.0 0.0 0.0 Balance

Carboriser 100.0 0.0 0.0 0.0 0.0 0.0 0.0

Nickel 0.0 0.0 0.0 0.0 0.0 99 Balance

Table 2

Chemical composition of all heats of austenitic ductile cast iron produced in the

present study.

Group symbol Heat no. Composition

C Si Ni Mn Mg

A A1 2.11 2.12 19.77 1.40 0.043

A2 2.31 2.07 19.44 1.40 0.041A3 2.53 2.11 19.41 1.40 0.045

A4 2.71 2.08 19.70 1.40 0.050

A5 2.95 2.12 19.54 1.40 0.045

A6 3.16 2.14 19.41 1.40 0.053

A7 3.29 2.08 19.52 1.40 0.048

A8 3.42 2.16 20.02 1.40 0.059

B B1 2.50 1 .63 21.54 1.34 0.047

B2 2.53 2.17 21.59 1.33 0.040

B3 2.52 2.76 21.90 1.32 0.042

B4 2.56 3.32 21.67 1.33 0.049

B5 2.54 3.89 21.86 1.34 0.051

B6 2.51 4.41 21.87 1.34 0.049

B7 2.53 4.92 21.65 1.33 0.038

B8 2.50 5.31 21.58 1.33 0.036

C C1 2.90 1.86 4.99 1.77 0.045

C2 2.85 1.82 9.09 1.72 0.069

C3 2.79 1.84 13.50 1.48 0.061

C4 2.80 1.85 16.10 1.56 0.065

C5 2.83 1.75 19.80 1.71 0.051

C6 2.78 1.79 23.90 1.60 0.063

C7 2.77 1.85 30.40 1.59 0.067

C8 2.91 1.83 34.70 1.39 0.062

Table 3

Effect of CE% on nodule-characteristics of all groups of austenitic ductile cast iron

produced in the present study.

Group

symbol

Heat no. CE% Nodule count

nodule (mm2)

Nodule size

(m)

Nodularity

(%)

A A1 3.51 80 15 80A2 3.69 125 28 100

A3 3.91 125 25 100

A4 4.1 125 25 100

A5 4.34 70 20 100

A6 4.55 220 25 100

A7 4.67 220 25 100

A8 5.04 220 25 100

B B1 3.86 130 28 100

B2 4 160 25 100

B3 4.16 200 25 100

B4 4.28 200 25 100

B5 4.38 225 22 100

B6 4.46 225 22 100

B7 4.59 225 22 100

B8 4.64 250 20 100

C C1 3.7 125 28 100C2 3.79 200 10 100

C3 3.9 250 15 100

C4 4 180 15 100

C5 4.15 200 15 100

C6 4.26 250 15 100

C7 4.49 250 15 100

C8 4.8 250 15 100

equivalent was calculated according to the following formula[1]:

C.E. = C%+ 0.33Si% + 0.047Ni% (0.0055Ni% Si%)

Standard microstructure examination procedures for cast irons

were used[3]. Vickers hardness test was performed at room tem-

perature of 298 K using Otto Wolpert Werk tester. Squared baseddiamond indenter (Angle 136), with 125kg load and 15s dura-

tion was applied. Tensile tests were carried out according to ASTM

(A370-2002). Specimens were machined to 5 mm gauge diameter

and30 mm gaugelength. Tests were conductedin Instron universal

testing machine connected to computer to draw the stressstrain

curves and recording the tensile strength (u), 0.2 proof stress

Fig. 1. Schematic of a half-inch Y-block. Dimensions in mm.


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N. Fatahalla et al. / Materials Science and Engineering A 504 (2009) 8189 83

Fig. 2. As-polished microstructure of austenitic ductile cast iron with different %CE ranging from 3.51 to 5.04. Group A: different C% ranging from 2.11 to 3.42.

and elongation. Tensile tests were performed at room temperature

(298K) at a strain rate of 6105 s1 up to fracture.

3. Results and discussion

3.1. Production of austenitic ductile cast iron having different CE%

Successful trials have been achieved, in the present investiga-

tion, to obtain austenitic ductile cast iron having different CE%

ranging from 3.51 to5.04.Threegroups (A, B andC) were produced;

each one of which containedeight heats of different CE%. The mainvariable was CE% as controlled by; (i) C in group A, (ii) Si in group

B and (iii) Ni in group C (cf.Table 3).

3.2. Microstructure of austenitic ductile cast iron

Figs.24 show the as-polishedmicrostructure of austenitic duc-

tile cast iron forall groups; A, B, andC, respectively. Generally, these

photos show dark graphite nodules embedded in a single bright

matrix (austenite). Low nodule count (80 nodule/mm2) is observed

inFig. 2a. This result stems from the low C-content (2.11%) of this

specific heat. Thereafter, the nodule count (125 nodule/mm2) was

almostconstant for CE%ranging from 3.69% to 4.1% (cf. Fig. 2(bd)).

Nodule count reached its lowest value (70 nodule/mm2) at a CE%

close to the eutectic composition (cf.Fig.2e).This result may be dueto formation of secondary graphite at this composition. Secondary

graphite is the miniature graphite particles observed around the


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Fig. 3. As-polished microstructure of austenitic ductile cast iron with different %CE ranging from 3.86 to 4.64. Group B with different Si% ranging from 2.50 to 5.31.

graphite nodules. Thereafter, an increase in nodule count (220

nodule/mm2) has been observed for alloys of CE% ranging from

4.55 to 5.04 (cf.Fig. 2fh). This result may refer to the relatively

high C-content of alloys in this range (2.953.42%C).

Fig. 3 shows the increase in nodule count due to an increase

in CE%, which may refer to the increase of Si-content [15,7].

Moreover, Fig. 4 shows, also, the increase in nodule count with

increasing CE% which, on the other hand, may refer to increasing

Ni-content [15,7]. Table 3 summarises theeffectof CE%on nodule-

characteristics of all groups of austenitic ductile cast iron producedin the present study. Nodule count ranged from 70 to 220for group

A, and from 130 to 250 for group B and finally from 125 to 250

nodule/mm2 for group C. These results are believed to depend on

the variation of CE%and thealloying element in each case [15]. On

the other hand, nodule size was around 25m for groups A and

B and it was around 15m for group C. Result for nodule size of

group C may refer to the effect of Ni-content and different chem-

ical composition in these heats[15]. Nodularity was almost 100%

for all heats with one exception for A1 (80%). This is believed to

refer to both, the low C-content and low CE%[15].Table 3shows

that the nodule count of groups B and C are generally higher

than those of group A. These results stem from the higher Si-content in B than in A. The increase in Si-content avoids the

formation of carbides and allows increasing the amount of free


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Fig. 4. As-polished microstructure of austenitic ductile cast iron with different %CE ranging from 3.70 to 4.80. Group C with different Ni% ranging from 4.99 to 34.70.

carbon[15]. On the other hand, the nodule size in group C is

smaller than that in A(cf.Table 3); consequently,the nodule count

for the former is higher than that for the latter.

Photomicrographs for etched specimens of all heats are shown

inFigs. 57. Generally, all photos show graphite nodules embed-

ded in austenitic matrix. Austenitic ductile cast irons must contain

sufficient amount of Ni to produce an austenitic matrix similar

to that of austenitic stainless steel [3]. Literature indicates a CE%

of around 4.3% for standard types of austenitic ductile cast iron

according to ASTM A439. Successful production of austenitic duc-

tile cast iron was achieved in the present study with a wide CE%

range (3.515.04).Fig. 5a shows a matrix containing iron carbide together with

austenite. Gradual decrease in iron carbide-content and corre-

sponding gradual increase in the soft phase of austenite can be

clearly seen through Fig. 5(bh). Thepresence of carbides in Fig. 5(a

and b) is believed to stem from the low C-content (2.112.31%C) in

A1 and A2 heats.

Fig. 6, for group B shows wholly austenitic matrix from the

beginning (cf. Fig. 6a) to the end (cf. Fig. 6h). This refers to the

sufficient amount of C and Si-content of this group[15].

InFig. 7(a and b) pearlite (dark areas) and martensite (bright

areas) can be observed, in small fractions, beside the austenitic

matrix. Appearance of pearlite andmartensite is due to insufficient

amount of Ni-content[15]. Literature showed that the sufficient

amount of Ni-content have been necessary to obtain austeniticmatrix andit was reported to have a minimum value of 18 mass%Ni

[15]. The present study achieved successful production of


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Fig. 5. Effect of variation of %CE ranging from 3.51 to 5.04 on the microstructure of austenitic ductile cast iron etched with 0.5 nital. Group A with different C% ranging from2.11 to 3.42.

austenitic ductile cast iron having wholly austenitic matrix using

only 13.5%Ni. The production cost can, therefore, be reduced using

lesser Ni-content.

3.3. Mechanical properties of austenitic ductile cast iron

3.3.1. Hardness

Fig. 8shows the influence of CE% ranging from 3.51 to 5.04 on

hardness Vickers (HV) forthe three groups. These curvesshow that

HV is slightly decreased or roughly can be considered constant in

the investigated CE% range. It is worthy to mention that heats C1

and C2 are not implied in the austenitic ductile iron category (theirmicrostructures revealed small amounts of pearlite and marten-

site). The high values of HV for these two specific heats refer to the

existence of pearlite andmartensitein their matrices. Although the

present results cover a wider range of CE% compared to literature,

however, it agreed with literature (ASTM A439) in the range of CE%

reported previously.

Table 4lists the mechanical properties of the austenitic ductile

cast iron reported in the literature. It shows that Brinell hardness

(HB) of standard grades falls within 12102020 MPa[1]. However,

the chemical compositions of standardgrades were near the eutec-

tic value of 4.3%. The present study covered the CE% range from

3.51% to 5.04%.

3.3.2. Tensile properties of austenitic ductile cast iron3.3.2.1. Tensilestrength (u). Fig.9 showsthe influence of CE%rang-

ing from 3.51 to 5.04 on u for all heats investigated. The graph


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Fig. 6. Effectof variation of %CErangingfrom 3.86 to 4.64 on themicrostructure of austeniticductile cast iron. Etchedwith nital 0.5. GroupB with differentSi% ranging from

2.50 to 5.31.

Table 4

Summaryof theproperties of austenitic ductile castiron available in a narrow range

of CE% ASTM A439[3].

Grade CE% 0.2(MPa) u(MPa) Elongation % HB ( MPa)

D2 4.44 207 400 8 13902020

D2B 4.44 207 400 7 14802120

D2C 4.37 183 400 20 12101710

D3 4.33 207 379 6 13902020

D3A 4.92 207 379 10 13101930

D4 4.33 NA 414 NA 20202730

D5 4.31 207 379 20 13101850D5B 4.31 207 379 6 13901930

D5S 3.17 207 449 10 13101930

shows that as CE% increases the u decreases slightly for all heats.

uforheats of group A is higher than its value for heats of groups

B and C. This is believed to stem from the higher C-content in

the former. The higher u of heats of group B compared to that

of group C refers to the higher Si-content in the former. Again,

the results of the two heats C1 and C2, although shown on the dia-

gram, however, are not comparable with other points since they

revealed pearlite and martensite in their matrices (not austenitic

ductile iron). Additionally, these two specific heats, C1 and C2, had

low Ni-content (less than 13.5%Ni) (cf.Table 2). The decrease in uis related to the amount of iron carbide (hard phase) and austenite

(soft phase) in the matrices. The u values obtained in the present

study agree with those reported in the literature in the commonrange of CE%[15]. However, the present research covered a wider

range of CE% than that reported in the literature.


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Fig. 7. Effect of variation of %CE ranging from 3.70 to 4.80 on the microstructure of austenitic ductile cast iron etched with 0.5 nital. Group C with different Ni% ranging from

4.99 to 34.70.

3.3.3. 0.2% Proof stress (0.2)

Fig. 10shows the variation of 0.2 with the change in CE% of

austenitic ductile cast iron of all groups investigated. The graph

shows slight decrease in 0.2 for groups A and C and slight

increase in 0.2 for group B. It is believed that the increase in

Si-content of group B resulted in corresponding increase in 0.2.

Fig. 10 also shows that while thetrendof0.2forgroups Aand C

is similar,but the values of0.2 of group C are higher than those

for the former. This result may stem from the higher Ni-content of

C group.This higher0.2may also refer to the secondary graphite

particles generated (cf. Fig. 4(ah)). It is suggested to clarify this

phenomenon through future research. Emphases of the presentresults are given by the literature[15]for the common range of

CE%.Fig.8. Variation of hardnessHV with%CE of austenitic ductile castiron of all groups

(A, B and C). *This graph implies two alloys of pearliticmartensitic DI of group C.


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Fig. 9. Variation of ultimatetensile strength with%CE of austenitic ductile castiron

of all groups (A, B and C). *This graph implies two alloys of pearliticmartensitic DI

of group C.

Fig. 10. Variation of 0.2 proof stress with %CE of austenitic ductile cast iron of all

groups (A, B and C). *This graph implies two alloys of pearliticmartensitic DI of

group C.

Fig. 11. Variation of elongation%with %CEof austenitic ductile castiron of allgroups

(A, B and C). *This graph implies two alloys of pearliticmartensitic DI of group C.

3.3.4. Elongation%

Fig. 11delineates the effect of CE% on elongation of austenitic

ductile cast iron for all heats produced in the present study. The

elongation of all the heats of groups A and B has the trend of

slightincrease with increasing CE%. Elongation of theheatsof group

B is generally higher than that of group A; this maybe due to

higher Si-content which prevents the formation of any carbides

and increases the amount of soft phase (austenite). The increase in

Ni-content in the third group C resulted in a slight decrease in

ductility as can be seen inFig. 11.

4. Conclusions

The goals, of the present investigation, have been successfully

achieved implying:

(1) Successful production of austenitic ductile cast iron cover-

ing a wide range of carbon equivalent (3.515.04%). This was

achieved using; carbon, silicon or nickel as alloying elements.

The present results are generally in consistence with those

reported in the literature in the common CE% range. However,

the present researchfilled thegaps thatdo exist in theliterature.(2) Successful casting procedure produced austenitic ductile iron

for a heat with only 13.5mass%Ni. Therefore, a promising

cheaper production cost will be available less than that

presently used (more than 18 mass%Ni).

(3) The microstructure of the produced austenitic ductile cast iron

consisted of graphite nodules embedded in austenitic matrix.

The nodule characteristics were affected by the change of CE%.

(4) Slight decrease in hardness, tensile strength, and 0.2% proof

stress with increasing CE% was observed. On the other hand,

a slight increase in ductility was observed with increasing the

CE%.

References

[1] American Foundrymens Society, Ductile Iron Handbook, Inc. Des Plaines, Illi-nois, (1993).

[2] MetalsHandbook on Propertiesand Selection: Iron andSteel,AmericanSocietyfor Metals, Metals Park, Ohio USA, vol. 1, 2005.

[3] Metals Handbook on Metallography and Microstructure, American Society forMetals, Metals Park, Ohio USA, vol. 9, 2004.

[4] H.T. Angus, Cast Iron: Physicaland Engineering Properties, 2nd ed., British CastIron Research Association, London, 1978.

[5] N. Fatahalla, H. Hakim, A. AboElEzz, M. Mohamed, Zeitschrift fr Metallkunde89 (1998) 507513.

[6] H. Bayati, R. Elliott, Materials Science and Technology 11 (1995) 284293.[7] S. Karsay, Ductile Iron: Production Practices, 2nd printing, American Foundry-

mens Society for Metals, 1979.[8] D.W. Zeng, C.S. Xie, K.C. Yung, Materials Science and Engineering A333 (2002)

223231.[9] H. Qin, Y. Zhang, Y. Yougshen, Materials Science and Engineering A393 (2005)

310314.[10] I. Karaman, H. Sehitoglu, H.J. Maier, Y.I. Chumlyakov, Acta Materialia 49 (2001)

471476.[11] Y.A. Alzafin, A.H.I. Mourad, M. AbouZour, O.A. Abuzeid, Engineering Failure

Analysis 14 (2007) 12941300.[12] Dsseldorf,AusteniticCast Ironfor HeavyDuty Applications Technical Report,

Press Release, July 23, 2003.[13] F. Mampaey, AFS Transactions 02024 (2002) 120.[14] E. Olivera, D. Rajnovic, S. Zec, L. Sidjanin, M.T. Jovanovic, Materials Characteri-

zation 57 (2006) 211217.[15] U. Seker, H. Hasirci, Journal of Materials Processing Technology 173 (2006)

260268.[16] S.K.Putatunda, S.Kesani,R.Tackett,G. Lawes, MaterialsScienceand Engineering

A435 (2006) 112122.[17] Y. Sahin, M. Erdogan, V. Kilicli, Materials Science and Engineering A444 (2007)

3138.[18] B.Stokes, N.Gao, A.S.Reed,MaterialsScienceand EngineeringA445446(2007)

374385.[19] A. Refaey, N. Fatahalla, Journal of Materials Science 38 (2003) 351362.[20] K. Aslantas, S. Tasgetiren, Y. Yalcin, Engineering Failure Analysis 11 (2004)

935941.[21] K. Kocatepe, M. Cerah, M. Erdogan, Materials and Design 28 (2005) 172181.[22] G. Tokas, M. Tayanc, A. Tokas, Materials Characterization 57 (2006) 290299.[23] A.N. Damir, A. ElKhatib, G. Nassef, International Journal of Fatigue 29 (2006)

499507.[24] K. Kocatepe, M. Cerah, M. Erdogan, Journal of Materials Processing Technology

28 ( 2007) 172181.[25] U. Seker, I. Ciftci, H. Hasirci, Materials and Design 24 (2003) 4751.[26] A. AbdAlAal, K.M. Ibrahim, Z. AbdElHamid, Wear 260 (200 6) 10701075.[27] N.Fatahalla,H.A. Hakim, A. AboElezz, M.Mohamed, Journalof Materials Science

31 (1996) 4 9334937.[28] R.C. Dommarco, M.E. Sousa, J. A. Sikora, Wear 257 (2004) 11851192.[29] N. Fatahalla, T. Gomaa, S. Bahi, M. Negm, Zeitschrift fr Metallkunde 89 (1998)

554561.[30] A. Nofal, N. Fatahalla, M. Morad, Foundry International, Technical Report,

September, 1998, pp. 3136.[31] N. Fatahalla, S. Bahi, O. Hussein, Journal of Materials Science 31 (1996)

57655772.

Documents

Si and Ni as Alloying Elements to Vary Carbon Equivalent of Austenitic Ductile Cast Iron- Microstructure and Mechanical Properties-2