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Morphology of corrosion fatigue cracks produced in 3.5% NaCl solution and in hydrogen for a high purity metastable austenitic (Fe18Cr12Ni) steel
Scripta METALLURGICA et M A T E R I A L I A
Vol.
26, pp. 1 1 7 5 - 1 1 8 0 , 1992 P r i n t e d in the U . S . A .
P e r g a m o n P r e s s Ltd. All rights reserved
MORPHOLOGY OF CORROSION FATIGUE CRACKS PRODUCED IN 3.5% NaCI SOLUTION AND IN HYDROGEN FOR A HIGH PURITY METASTABLE AUSTENITIC (Fel8Crl2Ni) STEEL
Ming Cao and Robert P. Wei Department of Mechanical Engineering and Mechanics LEHIGH UNIVERSITY Bethlehem, Pennsylvania 18015 USA (Received
February
12,
1992)
Introduction To further clarify the mechanism for corrosion fatigue crack growth in ferrous alloys in aqueous environments, a direct comparison was made of the morphology of fracture surfaces of a high-purlty metastable (Fel8Crl2Ni) stainless steel that had been produced by corrosion fatigue in 3.5% NaCI solutions and in hydrogen at room temperature. Two basic mechanisms are com~nonly cited; namely, active path dissolution and hydrogen embrlttlement (1,2). Both mechanisms depend upon the coupled electrochemical reactions that occur at the crack tip when new surfaces are exposed. For the active path dissolution mechanism, focus is placed upon the anodic process, or metal dissolution, during the current loading cycle. For the hydrogen embrittlement mechanism, focus is on hydrogen that is produced by the current cathodic process, and the resulting embrlttlement in the subsequent loading cycle. Because of the coupled nature of these processes, it has been difficult to unambiguously distinguish between these two mechanisms (I). Clarification of this issue would help in developing new and improved steels, and in formulating failure prevention programs and life prediction methodologies. Recent studies tended to support hydrogen embrittlement as the more appropriate mechanism for crack growth in steels, including austenitic stainless steels, in aqueous solutions (3-10). The evidence, however, is by and large indirect. To further clarify the mechanisms with more direct evidence, a comparison is made in this study of the fracture surfaces produced by corrosion fatigue in a high purity FelSCrl2Ni austenitic steel in deaerated 3.5% NaCl solution (at several pH levels) and in hydrogen (Ii). Fatigue fracture surfaces produced in vacuum were also examined to provide for reference and comparison. Comparisons were made in terms of specific microstructural features that serve to identify the crack growth mechanisms, and of the areal fractions of these features in relation to the measured crack growth rates and environmental conditions. Material and Experimental Work An 84-mm-square billet of high purity FelSCrl2Ni austenitic steel was used for this study. The billet was laboratory vacuum-inductlon melted by CARTECH Corporation, Reading, Pennsylvania, and was hot-pressed from a 190-mm-square taped ingot in two hot working operations. The finished billet was solution-annealed at I065°C for 1.5 h followed by quenching in water. The chemical composition (in wt%) is as follows: 0.005 C; 17.87 Cr; 11.95 Ni; <0.01 Mn; <0.01 Si; <0.01 Mo; <0.I Cu; <0.005 AI; <0.001 Ca+Mg; 0.003 S; 0.003 P; 0.iii O; 0.003 N; and balance Fe. Corrosion fatigue crack growth tests were conducted at room temperature in deaerated 3.5% NaCl solutions (pH - 2.0, 3.0, 6.5 and 12), hydrogen at (I00 kPa), and vacuum (at <40 ~Pa). Single-edge-notched bend specimens (7.6-mm-thick by 24.8-mm-wide by 132-mm-long) were used for tests in the 3.5% NaCI solutions. The solutions were deaerated to an oxygen concentration of less than 0.03 wppm. The specimens were tested in four point bending at a constant AK of 15 M P a ~ , with (R - 0.i) and I0 Hz, and were potentiostatically controlled at an applied potential of -700 mV (SCE). For the tests in hydrogen and in vacuum, compact tension specimens (12.7-mm-thick by 63.5-mm-wide, with a 15.8-mm-long EDM starter notch) were used. These tests were conducted at 8 Hz; also at AK = 15 M P a ~ and R = 0.i. All of the test specimens were in 1175 0036-9748/92 $ 5 . 0 0 + .00 Copyright (c) 1992 P e r g a m o n P r e s s
Ltd.
1176
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FATIGUE
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the LT orientation; i.e., with the crack plane perpendicular crack growth in the transverse direction.
Vol.
26,
No.
8
to the long axis of the billet, and
Fractographic examinations were conducted using an ETEC Autoscan scanning electron microscope (SEM), operated at 20 kV. Working distances of 16 to 20 mm were used, and the specimens were tilted 15 degrees about an axis parallel to the direction of crack growth. The areal fractions of grain and twin boundary separations were determined with the aid of a Micro-Plan II digitizing image analysis system. Six SEMmicrographs, at a magnification of 100X, were analyzed for each test condition to ensure adequate sampling. Each mlcrograph was examined at a further magnification of 3.5X to ensure proper identification of the fractographic features prior to analysis. Results and Discussion Representative SEMmicrographs of fatigue fracture surfaces produced in deaerated 3.5% NaCI solution at pH - 6.5 and in hydrogen are shown in Figs 1 to 4. The essential features of the fracture surface morphology (FSM) for the various environments are identical, and indicate that they were produced by the same mlcro-cracklng mechanisms. The low-magnlflcatlon mlcrofractographs (Fig I) show that the FSM is composed of highly reflective flat facets surrounded by regions that are more irregular in appearance. The highly reflective facets represent intergranular (IG) separation and twin boundary (TB) cracking (ii), which are shown at higher magnifications in Figs 2 and 3. The irregular appearing regions that surround IG and TB separation are associated with quasi-cleavage separation (QC), see Fig 4. Some evidence of secondary cracking may be seen also in Fig 2 at A, and at a higher magnification in Fig 5. These features are essentially the same as those observed in 304 stainless steels in aqueous solution and in air (i0). There is one-on-one matching across the mating fracture surfaces, with no evidence of corrosion attack (or metal dissolution). The FSM of fatigue surfaces produced in vacuum was different and was "ductile" in appearance, with no evidence of IG or TB separation. The crystallographic nature of the fracture surfaces, the absence of discernible corrosion attack, and the identity of FSM with that produced in gaseous hydrogen provide unequivocal support for hydrogen embrittlement as the mechanism for crack growth in aqueous environments.
Fig. I. SEM mlcrographs of Fel8Crl2Ni steel tested in 3.5% NaCl solution at pH - 6.5 (left), and hydrogen (right). The dark areas in the mlcrographs are the highly reflective flat facets.
Vol.
26, No.
8
CORROSION
FATIGUE
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Fig. 2. Similarity of IG and QC cracking in 3.5% NaCI solution (left pair) hydrogen at i00 kPa (right pair) for Fel8Crl2Ni alloy.
1177
and
in
Fig. 3. Similarity of twin boundary separation in 3.5% NaCI solution (left) and hydrogen at i00 kPa (right) for Fel8Crl2Ni alloy.
in
178
CORROSION
FATIGUE
CRACKS
Fig. 4. Similiarity of QC separation in 3.5% NaCI solution hydrogen (right pair) for FelSCrl2Ni steel.
Fig. 5.
Vol.
(left
pair)
and
High-magnlflcatlon microfractographs of secondary cracks shown in Fig 2.
26, No.
in
8
Vol.
26, No.
8
CORROSION
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1179
To quantify the microstructure-environment interactions, the areal fractions of IG and TB separation in the different environments were determined, and compared against the corresponding fatigue crack growth rates in Table I. Both the overall crack growth rates, (da/dN)_, and the environmental contributions, (da/dN)_f, are included (12). It can be seen that The areal fraction of IG and TB separation and tee corresponding crack growth rates at pH 12 are comparable to those in hydrogen, but are less than one-half of those at pH 2 and 6.5. The crack growth rate was the lowest in vacuum, with no evidence of IG and TB separation. These results imply that grain and twin boundaries are the preferred paths for hydrogen assisted cracking (or emhrlttlement). They suggest that hydrogen fugacity was highest at the lower pH values, and was comparable at pH 12 to gaseous hydrogen at i00 kPa.
TABLE I
Areal Fraction of IG and TB Separation and Associated Fatigue Crack Growth Rates in Various Environments
Crack_Growth Rate* (I0 - m/cycle) Environment
Areal Fraction (da/dN) e
Vacuum
(da/dN)c f
0
0
10.7 + 0.7
7.5 + 0.7
8.9 + 1.3
3.5% NaCI pH - 12
10.4 _+ 1.0
7.2 + 1.0
10.6 ± 0.3
pH - 6.5
16.5 + 3.8
13.0 + 3.8
24.7 + 2.2
pH - 2.0
18.9 + I.i
16.0 ± i.I
29.2 + 1.9
Hydrogen (I00 kPa)
3.2 + 0.2
(IG + TB) %
*Crack growth tests were conducted at i0 Hz, except tests in vacuum and hydrogen (f - 8 Hz).
The indicated effect of pH is consistent with that observed in hydrogen permeation experiments (13). For iron-based alloys, the average concentrations of hydrogen in the material for a low pH solution (0.1N HpSO4, at pH - 1.29) were found to be more than one order magnitude higher than those for a high pH solution (0.2N NaOH, at pH - 13.3) at same charging current densities (13). It should be noted the observed effect of pH on crack growth rates cannot be reconciled with the dlssolutlon-based models, because the amount of charge transferred at pH 12 is higher than, or comparable to, those at the lower pH values (2.0 and 6.5) at the same applied potential (14). Summary The results from this study clearly show hydrogen embrlttlement is the mechanism for corrosion fatigue crack growth in Fel8Crl2Ni steel in 3.5% NaCl solutions. The austenlte and twin boundaries appear to he preferred paths for cracking. The amount of intergranular (IG) and twin boundary (TB) separation depended upon solution pH; the amount at pH 12 was essentially equal to that observed in hydrogen at i00 kPa. The areal fractions of IG/TB separation are correlated with the corrosion fatigue crack growth rates, and their dependence on solution pH is interpreted in terms of its influence on hydrogen fugacity. Further studies to understand this influence are underway and will be reported later.
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FATIGUE
CRACKS
Vol.
26, No.
8
References I. 2. 3. 4. 5. 6. 7. 8. 9. i0. 11. 12. 13.
14.
P.R.Ford: in Envlronment-Sensltlve Fracture, ASTM STP 821, S.W. Dean, E.N. Pugh and G.M. Uglansky, eds., ASTM, Philadelphla, PA, 1984, pp. 32-51. R.P.Wei, G.Shim, and T. Tanaka: in Embrlttlement by the Localized Crack Environment, R. P. Gangloff ed., AIME, New York, NY, 1984, pp.243-63. R.P. Wel, in Microstructure and Mechanical Behavlour of Materials, Vol. II, E.M.A.S, Warley, England, 1986, pp. 507-526. A. Alavl, C.D. Miller and R.P. Wel, Corrosion, 1987, Vol. 43, No. 4, pp. 204-207. Gunchoo Shim and R.P. Wel, Mat'1. Scl. & Eng'g., 1987, Vol. 86, pp. 121-135. J.P. Thomas, A. Alavi and R.P. Wei, Scrlpta Metal1., 1986, Vol. 20, pp. 1015-1018. R.P. Wel and A. Alavl, Scrlpta Met., 1988, Vol. 22, pp. 969-974. D. Allison: Ph.D. Thesis, Lehigh University, Bethlehem, PA 1989. S.J. Hudak, Jr.: Ph.D. Thesis, Lehigh University, Bethlehem, PA 1988. Ming Gao, Shuchun Chen, and R.P.Wei: Metall. Trans., to be published. R.P. Wei and M. Gao, unpublished results, Lehigh University, 1992. R.P.Wei and Ming Gao, Scrlpta Met., 1985, Vol. 17, pp. 959-962. L. Nanls and T.K.G. Namboodhirl, in Stress Corrosion Cracking and Hydrogen Embrlttlement of Iron Base Alloys, NACE-5, R.W. Staehle, J. Hochmann, R.D. McCright, and J.E. Slater ed., NACE, Texas, 1973, pp.432-44. R. P. Wel and M. Gao, unpublished results, Lehigh University (1991).
Vol.
26, pp. 1 1 7 5 - 1 1 8 0 , 1992 P r i n t e d in the U . S . A .
P e r g a m o n P r e s s Ltd. All rights reserved
MORPHOLOGY OF CORROSION FATIGUE CRACKS PRODUCED IN 3.5% NaCI SOLUTION AND IN HYDROGEN FOR A HIGH PURITY METASTABLE AUSTENITIC (Fel8Crl2Ni) STEEL
Ming Cao and Robert P. Wei Department of Mechanical Engineering and Mechanics LEHIGH UNIVERSITY Bethlehem, Pennsylvania 18015 USA (Received
February
12,
1992)
Introduction To further clarify the mechanism for corrosion fatigue crack growth in ferrous alloys in aqueous environments, a direct comparison was made of the morphology of fracture surfaces of a high-purlty metastable (Fel8Crl2Ni) stainless steel that had been produced by corrosion fatigue in 3.5% NaCI solutions and in hydrogen at room temperature. Two basic mechanisms are com~nonly cited; namely, active path dissolution and hydrogen embrlttlement (1,2). Both mechanisms depend upon the coupled electrochemical reactions that occur at the crack tip when new surfaces are exposed. For the active path dissolution mechanism, focus is placed upon the anodic process, or metal dissolution, during the current loading cycle. For the hydrogen embrittlement mechanism, focus is on hydrogen that is produced by the current cathodic process, and the resulting embrlttlement in the subsequent loading cycle. Because of the coupled nature of these processes, it has been difficult to unambiguously distinguish between these two mechanisms (I). Clarification of this issue would help in developing new and improved steels, and in formulating failure prevention programs and life prediction methodologies. Recent studies tended to support hydrogen embrittlement as the more appropriate mechanism for crack growth in steels, including austenitic stainless steels, in aqueous solutions (3-10). The evidence, however, is by and large indirect. To further clarify the mechanisms with more direct evidence, a comparison is made in this study of the fracture surfaces produced by corrosion fatigue in a high purity FelSCrl2Ni austenitic steel in deaerated 3.5% NaCl solution (at several pH levels) and in hydrogen (Ii). Fatigue fracture surfaces produced in vacuum were also examined to provide for reference and comparison. Comparisons were made in terms of specific microstructural features that serve to identify the crack growth mechanisms, and of the areal fractions of these features in relation to the measured crack growth rates and environmental conditions. Material and Experimental Work An 84-mm-square billet of high purity FelSCrl2Ni austenitic steel was used for this study. The billet was laboratory vacuum-inductlon melted by CARTECH Corporation, Reading, Pennsylvania, and was hot-pressed from a 190-mm-square taped ingot in two hot working operations. The finished billet was solution-annealed at I065°C for 1.5 h followed by quenching in water. The chemical composition (in wt%) is as follows: 0.005 C; 17.87 Cr; 11.95 Ni; <0.01 Mn; <0.01 Si; <0.01 Mo; <0.I Cu; <0.005 AI; <0.001 Ca+Mg; 0.003 S; 0.003 P; 0.iii O; 0.003 N; and balance Fe. Corrosion fatigue crack growth tests were conducted at room temperature in deaerated 3.5% NaCl solutions (pH - 2.0, 3.0, 6.5 and 12), hydrogen at (I00 kPa), and vacuum (at <40 ~Pa). Single-edge-notched bend specimens (7.6-mm-thick by 24.8-mm-wide by 132-mm-long) were used for tests in the 3.5% NaCI solutions. The solutions were deaerated to an oxygen concentration of less than 0.03 wppm. The specimens were tested in four point bending at a constant AK of 15 M P a ~ , with (R - 0.i) and I0 Hz, and were potentiostatically controlled at an applied potential of -700 mV (SCE). For the tests in hydrogen and in vacuum, compact tension specimens (12.7-mm-thick by 63.5-mm-wide, with a 15.8-mm-long EDM starter notch) were used. These tests were conducted at 8 Hz; also at AK = 15 M P a ~ and R = 0.i. All of the test specimens were in 1175 0036-9748/92 $ 5 . 0 0 + .00 Copyright (c) 1992 P e r g a m o n P r e s s
Ltd.
1176
CORROSION
FATIGUE
CRACKS
the LT orientation; i.e., with the crack plane perpendicular crack growth in the transverse direction.
Vol.
26,
No.
8
to the long axis of the billet, and
Fractographic examinations were conducted using an ETEC Autoscan scanning electron microscope (SEM), operated at 20 kV. Working distances of 16 to 20 mm were used, and the specimens were tilted 15 degrees about an axis parallel to the direction of crack growth. The areal fractions of grain and twin boundary separations were determined with the aid of a Micro-Plan II digitizing image analysis system. Six SEMmicrographs, at a magnification of 100X, were analyzed for each test condition to ensure adequate sampling. Each mlcrograph was examined at a further magnification of 3.5X to ensure proper identification of the fractographic features prior to analysis. Results and Discussion Representative SEMmicrographs of fatigue fracture surfaces produced in deaerated 3.5% NaCI solution at pH - 6.5 and in hydrogen are shown in Figs 1 to 4. The essential features of the fracture surface morphology (FSM) for the various environments are identical, and indicate that they were produced by the same mlcro-cracklng mechanisms. The low-magnlflcatlon mlcrofractographs (Fig I) show that the FSM is composed of highly reflective flat facets surrounded by regions that are more irregular in appearance. The highly reflective facets represent intergranular (IG) separation and twin boundary (TB) cracking (ii), which are shown at higher magnifications in Figs 2 and 3. The irregular appearing regions that surround IG and TB separation are associated with quasi-cleavage separation (QC), see Fig 4. Some evidence of secondary cracking may be seen also in Fig 2 at A, and at a higher magnification in Fig 5. These features are essentially the same as those observed in 304 stainless steels in aqueous solution and in air (i0). There is one-on-one matching across the mating fracture surfaces, with no evidence of corrosion attack (or metal dissolution). The FSM of fatigue surfaces produced in vacuum was different and was "ductile" in appearance, with no evidence of IG or TB separation. The crystallographic nature of the fracture surfaces, the absence of discernible corrosion attack, and the identity of FSM with that produced in gaseous hydrogen provide unequivocal support for hydrogen embrittlement as the mechanism for crack growth in aqueous environments.
Fig. I. SEM mlcrographs of Fel8Crl2Ni steel tested in 3.5% NaCl solution at pH - 6.5 (left), and hydrogen (right). The dark areas in the mlcrographs are the highly reflective flat facets.
Vol.
26, No.
8
CORROSION
FATIGUE
CRACKS
Fig. 2. Similarity of IG and QC cracking in 3.5% NaCI solution (left pair) hydrogen at i00 kPa (right pair) for Fel8Crl2Ni alloy.
1177
and
in
Fig. 3. Similarity of twin boundary separation in 3.5% NaCI solution (left) and hydrogen at i00 kPa (right) for Fel8Crl2Ni alloy.
in
178
CORROSION
FATIGUE
CRACKS
Fig. 4. Similiarity of QC separation in 3.5% NaCI solution hydrogen (right pair) for FelSCrl2Ni steel.
Fig. 5.
Vol.
(left
pair)
and
High-magnlflcatlon microfractographs of secondary cracks shown in Fig 2.
26, No.
in
8
Vol.
26, No.
8
CORROSION
FATIGUE
CRACKS
1179
To quantify the microstructure-environment interactions, the areal fractions of IG and TB separation in the different environments were determined, and compared against the corresponding fatigue crack growth rates in Table I. Both the overall crack growth rates, (da/dN)_, and the environmental contributions, (da/dN)_f, are included (12). It can be seen that The areal fraction of IG and TB separation and tee corresponding crack growth rates at pH 12 are comparable to those in hydrogen, but are less than one-half of those at pH 2 and 6.5. The crack growth rate was the lowest in vacuum, with no evidence of IG and TB separation. These results imply that grain and twin boundaries are the preferred paths for hydrogen assisted cracking (or emhrlttlement). They suggest that hydrogen fugacity was highest at the lower pH values, and was comparable at pH 12 to gaseous hydrogen at i00 kPa.
TABLE I
Areal Fraction of IG and TB Separation and Associated Fatigue Crack Growth Rates in Various Environments
Crack_Growth Rate* (I0 - m/cycle) Environment
Areal Fraction (da/dN) e
Vacuum
(da/dN)c f
0
0
10.7 + 0.7
7.5 + 0.7
8.9 + 1.3
3.5% NaCI pH - 12
10.4 _+ 1.0
7.2 + 1.0
10.6 ± 0.3
pH - 6.5
16.5 + 3.8
13.0 + 3.8
24.7 + 2.2
pH - 2.0
18.9 + I.i
16.0 ± i.I
29.2 + 1.9
Hydrogen (I00 kPa)
3.2 + 0.2
(IG + TB) %
*Crack growth tests were conducted at i0 Hz, except tests in vacuum and hydrogen (f - 8 Hz).
The indicated effect of pH is consistent with that observed in hydrogen permeation experiments (13). For iron-based alloys, the average concentrations of hydrogen in the material for a low pH solution (0.1N HpSO4, at pH - 1.29) were found to be more than one order magnitude higher than those for a high pH solution (0.2N NaOH, at pH - 13.3) at same charging current densities (13). It should be noted the observed effect of pH on crack growth rates cannot be reconciled with the dlssolutlon-based models, because the amount of charge transferred at pH 12 is higher than, or comparable to, those at the lower pH values (2.0 and 6.5) at the same applied potential (14). Summary The results from this study clearly show hydrogen embrlttlement is the mechanism for corrosion fatigue crack growth in Fel8Crl2Ni steel in 3.5% NaCl solutions. The austenlte and twin boundaries appear to he preferred paths for cracking. The amount of intergranular (IG) and twin boundary (TB) separation depended upon solution pH; the amount at pH 12 was essentially equal to that observed in hydrogen at i00 kPa. The areal fractions of IG/TB separation are correlated with the corrosion fatigue crack growth rates, and their dependence on solution pH is interpreted in terms of its influence on hydrogen fugacity. Further studies to understand this influence are underway and will be reported later.
1180
CORROSION
FATIGUE
CRACKS
Vol.
26, No.
8
References I. 2. 3. 4. 5. 6. 7. 8. 9. i0. 11. 12. 13.
14.
P.R.Ford: in Envlronment-Sensltlve Fracture, ASTM STP 821, S.W. Dean, E.N. Pugh and G.M. Uglansky, eds., ASTM, Philadelphla, PA, 1984, pp. 32-51. R.P.Wei, G.Shim, and T. Tanaka: in Embrlttlement by the Localized Crack Environment, R. P. Gangloff ed., AIME, New York, NY, 1984, pp.243-63. R.P. Wel, in Microstructure and Mechanical Behavlour of Materials, Vol. II, E.M.A.S, Warley, England, 1986, pp. 507-526. A. Alavl, C.D. Miller and R.P. Wel, Corrosion, 1987, Vol. 43, No. 4, pp. 204-207. Gunchoo Shim and R.P. Wel, Mat'1. Scl. & Eng'g., 1987, Vol. 86, pp. 121-135. J.P. Thomas, A. Alavi and R.P. Wei, Scrlpta Metal1., 1986, Vol. 20, pp. 1015-1018. R.P. Wel and A. Alavl, Scrlpta Met., 1988, Vol. 22, pp. 969-974. D. Allison: Ph.D. Thesis, Lehigh University, Bethlehem, PA 1989. S.J. Hudak, Jr.: Ph.D. Thesis, Lehigh University, Bethlehem, PA 1988. Ming Gao, Shuchun Chen, and R.P.Wei: Metall. Trans., to be published. R.P. Wei and M. Gao, unpublished results, Lehigh University, 1992. R.P.Wei and Ming Gao, Scrlpta Met., 1985, Vol. 17, pp. 959-962. L. Nanls and T.K.G. Namboodhirl, in Stress Corrosion Cracking and Hydrogen Embrlttlement of Iron Base Alloys, NACE-5, R.W. Staehle, J. Hochmann, R.D. McCright, and J.E. Slater ed., NACE, Texas, 1973, pp.432-44. R. P. Wel and M. Gao, unpublished results, Lehigh University (1991).