-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)

2. J. J. Smith and R. A. Farr, International Materials

Reviews, Vol.38, p. 25 (1993)

3. R. M. Boothby, Materials Science Technology, Vol.

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)

8. W. T. Tsai, C. M. Laiw, and J.T. Lee, Material

Characterization, Vol.24, p.199, (1990)

9. T. F. Wu, T. Z. Chey and W. T. Tsai, Material

Chemistry and Physics, Vol. 70, P. 208, (2001)

10. T. F. Wu, T. Z. Chey and W. T. Tsai, Journal of

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)