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-151-
The Degree of Sensitization and Environmentally Induced Cracking
of AISI 347 Stainless Steel
Tzu-Sheng Chen and Wen-Ta Tsai
AISI 347
NaCl
650 347 EPR
Electrochemical potentiokinetic reactivation, EPR
25
pH= 2 3.5wt% NaCl 90 pH= 2 1wt% NaCl 650
5 15 347
347
304
347
AbstractThe effect of heat treatment on the microstructural change and the evaluation for various
degree of sensitization of 347 SS were investigated. The sensitivity to environmentally-assisted
cracking behavior was also studied using slow strain rate testing technique. The experimental
results indicated that heat treatment at 650 did not cause the occurrence of sensitization. The
reactivation peaks appeared in EPR (Electrochemical potentiokinetic reactivation) curves were not
derived from the corrosion in the Cr depleted grain boundaries, but due to pitting corrosion. Slow
strain rate testing results revealed that in 3.5 wt% NaCl solution of pH=2 at 25 or in 1 wt%
NaCl solution of pH=2 at 90 , the tensile strength and percentage of elongation of 347 SS heat
treated at 650 for 5 or 15h were almost the same as that of solution annealed one. Except at the
edge of the fracture surface where stress corrosion cracking was observed, 347 SS was more
resistant to environmentally-assisted cracking than 304 SS.
Key Word: 347 stainless steel, EPR (Electorchemical Potentodynamic Reactivation),
Sensitization, Environmentally Induced Cracking, Stress corrosion cracking
*
Department of Materials Science and Engineering National Cheng Kung University
151 165 90 9
Journal of Chinese Corrosion Engineering, Vol.15 No.3, PP.151~165(2001)
AISI 347
NbC
C h r o m i u m
Depleted Zone [1]
Sensitization Interganular
Corrosion, IGC Intergranular
Stress Corrosion Cracking, IGSCC
347
Cr23
C6
[2-4]
Oxalic Acid Etch Test [5]
Eletrochemical Potentiokinetic Reaction, EPR [6,7]
347
AISI 347
347
304 316
AISI 347
NaCl
AISI 347
304
1100
/30 SA
650 5 15
SS05
SS15
304 347 1.5 cm
1.5 cm 0.5 cm
HCl HNO3
H2O = 1 1 1
5-10
OM EDS
EPMA 347
304 347
10wt%
H2C
2O
42H
2O
1A/cm2 90
50
OM
Electrochemical Potentiodynamic Reactivation, EPR
EPR
25 0.5M H2SO
4
+ 0.01M KSCN 60
200 mV 120
200 mV 1.67mV/s
Ecorr
EPR
-152-
90 9
347 304
22 mm
2.5 mm 150 mm
SSRT 1 10-6s-1
1200
NaCl pH2
75 90
304 347
1 2 304 347
650
347
3
EDS
EPMA 4
NbC
SEM 347 NbC
2 m 8 m
[8]
5 6 304 347
5 (a) 304
Step
5 (b) 5 (c) 304 650 5
15
Ditch Huey Test
304
347
347 650 5 15
304 347
7 304 EPR
304 5 15
8 347 EPR
347
650 5 15
EPR 9
600 EPR [9-
10]
10 304 347
UTS percentage of elongation, El%
percentage of area reduction, RA%
-153-
AISI 347
dimple
11 12 304 347
25 3.5wt% NaCl pH=2
304 347 3.5wt% NaCl pH=2
304 650 /15
13
14 304 347
SEM
304
304
IGSCC [11-13]
347
14-(a)
347
[14-16] 347
IGSCC [16-17] 347
[14, 15, 18]
347 15
347
NbC
NbC
NbC
16 304 75 1wt% NaCl pH=2
304 650
347 90
1wt%NaCl
17 650 /5h
304 18
(a) (b) (c) 304
18 (d) (e) (f) 347
90 1wt% NaCl pH=2
650 /5h 347
SS, SS05
347
90 NaCl
304
19 304 75 1wt% NaCl
pH=2
SEM
304 SS, SS05 304 SS, SS15
20 (a)
347 90 1wt% NaCl
20 (b)
304
[19, 20]
[11, 19,
20] 347
304
-154-
90 9
1. 650 347
EPR
2. 347 25 90 NaCl
SEM 347
3. 650 347 304
NSC 89-TPC-7- 006-012
1. R. L. Cowan and C. S. Tedmon, Vol. 3, Plenum
Press, New York, p.293, (1973)
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2, p. 78, (1986)
4. H. Tsuge and H. Naggno, Transactions of Iron and
Steeel Institute of japan, vol. 24, p. B-235, (1984)
5. ASTM A262-93a Practice A Oxalic Acid Etch Test.
6. W. L. Clarke, V. M. Romero, J. C. Panko,
Corrosion/77, Preprint No.180, NACE, Honston,
Texas, (1977)
7. W. L. Clarke, R. L. Cowan, W. L. Walker, ASTM
STP656, R. F. Sticgerw ald, Ed. Philadelphia, p.99,
(1978)
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Characterization, Vol.24, p.199, (1990)
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Chemistry and Physics, Vol. 70, P. 208, (2001)
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Nuclear Materials, Vol. 295/2-3, p. 233, (2001)
11. G.Gragnolino and D.D. Macdonald, Corrosion, Vol.
38, p. 406, (1982)
12. R. M. Latanison and R. W. Staehle, Fundamental
Aspects of Stress Corrosion Cracking, NACE,
Houston, Tx, P. 214, (1969)
13. G. J. Theus and Staehle, Stress Cracking and
Hydrogen Embrittlement of Iron Base Alloys,
NACE, Houston, Tx, p. 845, (1978)
14. A. J. Brophy, Materials Performance, Vol. 13, No.
5, p. 9, (1974)
15. C. H. Samans, Corrosion, Vol. 20, p. 256t, (1969)
16. C. H. Samans, K. Kinoshita, and I. Matsushima,
Corrosion, Vol. 33, p. 271, (1977)
17. M. Kowaka, T. Kado, and K. Ota, Boshoku Gijutsu,
Vol. 25, p.19, (1976)
18. I. Matsushima, Boshoku Gijutsu, Vol. 25, p.141,
(1973)
19. J. E. Truman, Corrosion Science, Vol. 17, p. 737,
(1977)
20. S. W. Dean, Stress Corrosion-New Approaches,
ASTM-STP-610, p. 308, (1976)
-155-
AISI 347
-156-
90 9
304 347
Table 2. Heat treatments and slow strain rate testing
parameters for 304 and 347 SSs.
304
347
Solution Annealing (SA)
SA + 650 /5h (SS05)
SA + 650 /15h (SS15)
Solution Annealing (SA)
SA + 650 /5h (SS05)
SA + 650 /15h (SS15)
SSRT
1. air (25 )
2. 3.5 wt% NaCl, pH=2, (25 )
3. 1 wt% NaCl, pH=2, (75 )
1. air (25 )
2. 3.5 wt% NaCl, pH=2, (25 )
3. 1 wt% NaCl, pH=2, (90 )
304 347
Table 3. Slow stain rate testing results in air for 304 SS
and 347 SS with different heat treatments.
* EL% = Percentage of elongation
RA% = Percentage of area reduction
304 SS Air 347 SS Air
SA SS05 SS15 SA SS05 SS15
UTS(MPa) 537 562 559 585 595 595
EL(%) 64 60 56 54 56 53
RA(%) 75 71 79 67 68 67
304 347 25
3.5 wt%NaCl pH=2
Table 4. Slow stain rate testing results for 304 SS and
347 SS, with different heat treatments, in 3.5
wt% NaCl solution (pH=2) at 25 .
304 SS 347 SS
25 , 3.5wt% NaCl, pH=2 25 , 3.5wt% NaCl, pH=2
SA SS05 SS15 SA SS05 SS15
UTS(MPa) 528 543 541 565 566 567
EL(%) 53 51 44 50 49 51
RA(%) 64 56 40 62 64 60
RAs/RA
Air0.85 0.79 0.51 0.92 0.94 0.90
304 347 75
90 1wt% NaCl pH 2
Table 5. Slow stain rate testing results for 304 SS and
347 SS, with different heat treatments, in 3.5
wt% NaCl solution (pH=2) at 75 and 90 ,
respectively.
304 SS 347 SS
75 , 1wt% NaCl, pH=2 90 , 1wt% NaCl, pH=2
SA SS05 SS15 SA SS05 SS15
UTS(MPa) 475 463 447 478 404 461
EL(%) 43 28 25 35 37 39
RA(%) 47 14 15 38 38 38
RAs/RA
Air0.63 0.20 0.19 0.57 0.56 0.57
304 347 wt%
Table 1. Chemical Compositions (wt%) of 347 SS and
304 SS used.
Fe Cr Ni C Si Mn P S Cu N Nb
304 Bal. 18.3 9.22 0.03 0.51 1.88 0.029 0.008 2.08 0.0118 -----
347 Bal. 18.2 10.6 0.028 0.42 1.17 0.012 0.0004 ---- 0.14 0.64
-157-
AISI 347
1 304 (a) 304 SS,
SA (1100 /30 min) (b) 304 SS, SS05
(SA+650 /5h) (c)304 SS, SS15
(SA+650 /15h)
Figure 1. Metallographs of 304 SS with different heat
treatments.:(a) 304 SS, SA (1100 /30 min) ,
(b) 304 SS, SS05 (SA+650 /5h), and
(c)304 SS, SS15 (SA+650 /15h)
50 m
(a)
50 m
(b)
50 m
(c)
2 347 (a)
SA (1100 /30 min) (b) SS05 (SA+650
/5h) (c) SS15 (SA+650 /15h)
Figure 2. Metallographs of 347 SS with different heat
treatments. (a) SA (1100 /30 min),(b) SS05
(SA+650 /5h), and (c) SS15 (SA+650
/15h)
25 m
(a)
25 m
(b)
25 m
(c)
-158-
90 9
3 347 NbC SEM
EDS
Figure 3. SEM micograph and EDS analysis of NbC in
347 SS.
4 347 NbC SEM
EPMA
Figure 4. SEM micograph and EPMA analysis of NbC
in 347 SS.
4 m 2 m
Energy (KeV)
-159-
AISI 347
5 304
(a) SA (b) SS05 (c) SS15
Figure 5. Surfure Morphologies of 304 SS with
different heat treatments after Oxalic Acid
Etch Test: (a) SA, (b) SS05, and (c) SS15.
50 m
(a)
50 m
(b)
50 m
(c)
6 347
(a) SA (b) SS05 (c) SS15
Figure 6. Surfure Morphologies of 347 SS with
different heat treatments after Oxalic Acid
Etch Test: (a) SA, (b) SS05, and(c) SS15.
25 m
(a)
25 m
(b)
25 m
(c)
-160-
90 9
8 347 EPR (a) SA
(b) SS05 (c) SS15
Figure 8. Metallographs of 347 SS after EPMA test:
(a) SA, (b) SS05, and (c) SS15.
25 m
(a)
25 m
(b)
25 m
(c)
7 304 EPR (a) SA
(b) SS05 (c) SS15
Figure 7. Metallographs of 304 SS after EPMA test:
(a) SA, (b) SS05, and (c) SS15.
25 m
(a)
25 m
(b)
25 m
(c)
-161-
AISI 347
9 347 0.5 M H2SO
4+
0.01 M KSCN EPR
Figure 9. Electrochemical Potentiodynamic
Reactivation curves of 347 SS, with
different heat treatment, in 0.5 M H2SO
4+
0.01 M KSCN electrolyte.
10 304 347
Figure 10. Stress-Elongation curves of 304 SS and 347
SS with different heat treatment, slow strain
rate tests in air.0.01 M KSCN electrolyte.
12 347 25 3.5
wt% NaCl pH=2
Figure 12. Stress-Elongation curves of 347 SS with
different heat treatment tested in 3.5 wt%
NaCl solution (pH=2) at 25 .
11 304 25 3.5 wt% NaCl pH=2
Figure 11. Stress-Elongation curves of 304 SS tested in
3.5 wt% NaCl solution (pH=2) at 25 .
-162-
90 9
13 304 25
3.5wt% NaCl pH=2
SEM
(a) SA (b) SS05 (c) SS15
Figure 13. SEM fractographs of 304 SS with different
heat treatment after slow strain rate test in
3.5 wt% NaCl solution (pH=2) at 25 : (a)
SA, (b) SS05, and (c) SS15.
(a)
(b)
(c)
14 650 /15h 347 25
3.5wt% NaCl pH=2
SEM (a) (b)
Figure 14. SEM fractographs of 347 SS with heat
treatment (650 /15hr) after slow strain rate
test in 3.5 wt% NaCl solution (pH=2) at 25
: (a)center of the fracture surface, (b)edge
of the fracture surface.
(a)
(b)
-163-
AISI 347
15 650 /15h 347 25
3.5 wt% NaCl pH=2
SEM (a) (b)
Figure 15. SEM micrographs showing the edge
surfaces of 347 SS heat treated at 650 for
15h, after slow strain rate test in 3.5 wt%
NaCl solution (pH=2) at 25 : (a) expanded
surface pits, (b ) pit induced cracks.
(a)
(b)
17 347 90 1wt% NaCl pH=2
Figure 17. Stress-Elongation curves of 347 SS after
slow strain rate test in 1 wt% NaCl solution
(pH=2) at 90 .
16 304 75 1wt% NaCl pH=2
Figure 16. Stress-Elongation curves of 304 SS after
slow strain rate test in 1 wt% NaCl solution
(pH=2) at 75 .
-164-
90 9
18 304 347 75 90
1wt% NaCl pH=2
(a) 304
SS, SA (b) 304 SS, SS05 (c) 304 SS,
SS15 (d) 347 SS, SA (e) 347 SS, SS05
(f) 3 47 SS, SS15
(a)
(b)
(c)
Figure 18. SEM micrographs showing the side views
of 304 SS and 347 SS after slow strain rate
tests in 1 wt% NaCl solution (pH=2) at 75
and 90 , respectively: (a)304 SS, SA;
(b)304 SS, SS05; (c)304 SS, SS15; (d)347
SS, SA; (e)347 SS, SS05; and (f)347 SS,
SS15.
(d)
(e)
(f)
-165-
AISI 347
19 304 75 1wt% NaCl pH=2
SEM (a) SA (b) SS05
(c) SS15
Figure 19. SEM fractographs of 304 SS after slow
strain rate test in 1 wt% NaCl solution
(pH=2) at 75 : (a) SA, (b) SS05, and (c)
SS15.
(a)
(b)
20 650 /15h 347 90 1wt%
NaCl pH=2
SEM (a) dimple
(b)
Figure 20. SEM fractographs of 347 SS heat treated at
650 for 15h, after slow strain rate test in 1
wt% NaCl solution (pH=2) at 90 : (a)
dimple, (b) brittle fracture at edge.
(a)
(b)
(c)