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Quantitative assessment of the role of microstructure in corrosion fatigue in a metastable austenitic steel
Scripta METALLURGICA et M A T E R I A L I A
Vol. 28, pp. 8 5 3 - 8 5 6 , 1993 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 r i g h t s r e s e r v e d
QUANTITATIVE ASSESSMENT OF THE ROLE OF MICROSTRUCTURE IN CORROSION FATIGUE IN A METASTABLE AUSTENITIC STEEL Ming Gao and Robert P. Wei Department of Mechanical Enginnering and Mechanics LEHIGH UNIVERSITY Bethlehem, Pennsylvania 18015 USA ( R e c e i v e d D e c e m b e r 2, 1992) ( R e v i s e d J a n u a r y 26, 1993) Introduction In a previous study [i], the morphology of fracture surfaces produced by corrosion fatigue in a hlgh-purity Pel8Crl2Ni steel in deaerated 3.5% NaCI solution and in hydrogen at room temperature was examined, Hydrogen embrittlement was identified as the mechanism for corrosion fatigue crack growth in this steel in 3.5% NaCl solutions, and austenite grain and twin boundaries (IG/TB) as preferred paths for cracking. In this paper, the role of microstructure is further addressed quantitatively by using a superposition model for corrosion fatigue [2,3]. The role of microstructure on environmentally assisted crack growth in the metastable austenitic stainless steels has been extensively studied. One of the fundamental questions relates the importance of strain-induced ~'-martensite formation in enhancing crack growth, which remains, by and large, unresolved in the literature [3,4]. Some believe that e'-martensite is the precursor, and its formation is necessary for stress corrosion cracking (SCC) and hydrogen embrittlement (HE). Others argue, with experimental support, against a crucial role for ~'-martensite formation. A recent study of the role of microstructure in corrosion fatigue of annealed and cold-rolled 304 stainless steels suggested that cyclic strain-lnduced a'-martensite did not play a primary role in environmentally enhancing crack growth, because the amounts of ~'-martensite were lower in the more deleterious environments [3]. The results suggested that other microstructural components, such as austenlte grain and twin boundaries, might have been more important. Because the experiments were limited, quantitative assessments of the contributions from each component could not have been made [3]. In this paper, a critical assessment of the role of microstructure in determining the corrosion crack growth response of a high purity FelSCrl2Ni steel in 3.5% NaCI solutions and in hydrogen is made with the aid of a superposition model for crack growth. Experimental Basis Quantitative assessment of the role of microstructure is based on the measurements of crack growth rates, and post-fracture determinations of areal fractions for each fracture mode and volume fraction of strain-induced ~'-martenslte as a function of the test environments. The crack growth rates and fracture modes were determined in the previous study [1], and are summarized in Table I. Corrosion fatigue crack growth in deaerated 3.5% NaCI solutions and in hydrogen occurred by intergranular (IG) cracking along austenite grain boundaries, twin boundary separation (TB) and quasi-cleavage (QC) through austenite or strain-induced martensite matrix [i]. The crack growth rate and areal fraction of IG/TB separation depended on solution pH; both being higher at the lower pH values but much lower at pH-12. The rate and amount at pH-12 were essentially equal to
853 0 9 5 6 - 7 1 6 X / 9 3 $ 6 . 0 0 + .00 Copyright (c) 1993 P e r g a m o n P r e s s
Ltd.
854
CORROSION
those found in hydrogen at lOOkPa. evidence of IG/TB separation.
The crack
FATIGUE
growth
Vol.
rate was
the lowest' in vacuum,
28,
No.
with no
TABLE I Fatigue Crack Growth Rates and Associated Areal Fraction of IG and TB Separation and Volume Fraction of a'-martenslte in Various Environments at 10 Hz* Crack Growth Rate (10 -9 m/cycle)
Areal Fraction
Volume Fraction
(IG + TB)%
(Martenslte)%
Environment (da/dN) e
Vacuum Hydrogen (i00 kPa)
(da/dN)cf
3.2±0.2
0
0
I00
10.7 ± 0.7
7.5 ± 0.7
8.9 ± 1.3
80
3.5% NaCI pH pH pH pH
-
12 6.5 3.0 2.0
10.4 16.5 17.6 18.9
+ + + +
1.0 3.B 1.0 i.I
7.2 13.0 14.4 16.0
*Except for tests in vacuum and hydrogen, **ND: Not determined
+ + + +
1.0 3.8 1.0 i.I
10.6 24.7 26.1 29.2
_+ + + +
0.3 2.2 2.1 1.9
79 59 ND** 58
where f - 8 Hz
The volume fractions of cyclic straln-induced a'-martensite in the neighborhood of the fatigue fracture surfaces were measured using an X-ray diffraction technique; the experimental details are given elsewhere [3]. The results are also listed in Table I. The variation in volume fraction with solution pH, was opposide to that of IG and TB separation; being highest for the specimen tested in vacuum (100%) and lowest (58%) at pH-2. The amount of a'-martensite in the specimens tested in 3.5% NaCI solution at pH-12 was essentially the same as that at I00 kPa in hydrogen (79 versus 80%), which was in between the amounts in vacuum and at pH-2. These results are consistent with the previous findings on 304 stainless steel [3], and suggest that a more "brittle" (or environmentally sensitive) process had intervened in the transformation. It is to be noted that the volume fraction of a'-martensite should not be equated to the areal fraction of QC separation on the fracture surface. The x-ray data may reflect reduced amounts of transformed a'-martenslte, particularly below the IG/TB regions of the fracture surface, and provide lower bound estimates for the contribution from ~'-martensite. Modeling To assess the role of microstructure, an extension of the superposition earlier for corrosion fatigue is made [2,3]. To account for multiple fracture cycle-dependent corrosion fatigue crack growth rate,(da/dN)cf; is considered as growth rate for each (or the i th) fracture mechanism, (da/dN)cf,i, multiplied by tion, #i: (da/dN)cf - Z(da/dN)cf,i~i;
7J~i - 1
model developed mechanisms, the the sum of c r a c k its areal frac-
(I)
where (da/dN)c f - (da/dN) e - (da/dN)r; and (da/dN)e and (da/dN) r are crack growth rates in the deleterious and inert (for example, in vacuum) environments, respectively.
7
Vol.
28, No.
7
CORROSION
FATIGUE
855
For simplicity, the micromechanisms for corrosion fatigue of FelSCrl2Ni alloy in 3.5% NaCI solutions and in hydrogen are combined into two groups; i.e., IG/TB separation and QC cracking. Denoting the former by b and the latter by m, Eq.(1) then becomes: (da/dN)c f - (da/dN)cf,m~ m + (da/dN)cf,b ~
(2)
and ~m ÷ %
- 1
(3)
It should be noted that, by this grouping, the crack growth rates for IG and TB separations are assumed to be equal in Eq. (2). Substituting Eq.(3) into Eq.(2) and rearranging, Eq.(4) is obtained: (da/dN)c f . (da/dN)cf, m + [(da/dN)cf, b - (da/dN)cf,m]@ b
(4)
Equation (4) shows a linear relationship between (da/dN)cf and the areal fraction of IG/TB separation, @b" This relationship is used for assessing the role of microstructure. Assessment And Discussion Figure I shows a plot of the experimental data on (da/dN)cf and @b, at i0 Hz, given in Table I. The data are seen to follow the linear relationship described by Eq.(4) with a correlation coefficient of 0.994. The crack growth rates for each fracture element are then determined. The crack growth rate for QC, (da/dN)cf,m, is estimated from the intercept, while that for IG/TB separation, (da/dN)cf,b, from the slope, and are as follows: (da/dN)cf, m - (3.25±1.2) x 10 -9 m/cycle and (da/dN)cf, b - (4.52±0.6) x 10 -8 m/cycle. These estimates show that the incremental increase in IG/TB crack growth rate by the environment is approximately fifteen times higher than the growth rate in vacuum of (3.25±1.2) x 10 -9 m/cycle, while the incremental increase in QC crack growth rate is equal to the rate in vacuum. These results clearly show that austenite grain and twin boundaries play a more important role straln-induced ~'-martensite in the enhancement of corrosion fatigue crack growth in metastable stainless steels. Even though strain-induced ~'-martenslte is commonly found near the fracture surfaces, its formation does not appear to be essential for hydrogen embrittlement. Because austenlte grain and twin boundaries are more embrittled, cracking along these boundaries may limlt the extent of cyclic straining and the transformation to a'-martensite in the deleterious environments, Table I. Recent studies suggested that QC cracking might be through hydrlde phases at the crack tip, and that a'-martensite is the decomposition product of these phases [5,6]. Further studies are planned to clarify this issue. than
Summary The role of microstructure in hydrogen assisted fatigue crack growth was quantitatively assessed by using a superposition model for corrosion fatigue for a high purity FelgCrI2NI alloy. The results showed that the environmentally assisted crack growth rate through a'-martenslte, (da/dN)e, m, is only twlce as high as that in vacuum (6.45 x 10 -9 versus 3.25 x 10 -9 m/cycle), while the rate through the austenlte grain and twin boundaries is more than one order of magnitude faster (4.84 x 10-g m/cycle). These results strongly suggest that straln-lnduced ~'-martensite formation is not a principal contributor to the environmental enhancement of corrosion fatigue crack growth and is not necessary for hydrogen embrittlement.
856
CORROSION
FATIGUE
Vol.
28,
No.
This work was supported by the Office of Basic Energy Sciences, Department of Energy, under Grant No. DE-FGO2-88ER45354. References i. 2. 3. 4. 5. 6.
Ming Gao and Robert P. Wei, Scrlpta Metal. et Mater., 26, 1175-1180 (1992). R.P.Wel and M.Gao, Scripta Met., 17, 959-962 (1983). Mlng Gao,Shuchun Chen and Robert P. Wel, Met. Trans. A, 23A, 355-371 (1992). H.E. Hanninen: Int. Met. Rev., X(3), 85-135 (1979). Shuchun Chert, Mlng Gao and Robert P. Wei, Scripta Met. et Mater., 28, 471-476 (1993). Robert P. Wei and Mlng Gao, "Micromechanlsm for Corrosion Fatigue Crack Growth in Metastable Austenitic Stainless Steels", Proceedings of CDI'92 Conference on Corrosion-Deformation Interactions, Fontainebleau, France, October 5-7, 1992.
20
(do/dN) cf = (d°/dN)cf, m +
E 15 I
{(d°/dN)cf,b - (do/dN)cf,m }*b RVAL 0.994
pH .3
0
v
10
H ~ ~ pH1
o Z "0
o
"o
5
v
Error Bar @ 95~ Conf. Level 0
I
I
I
I
I
I
I
5
10
15
20
25
30
35
Areal Fraction of IG/TB (¢b ~) FIG. 1 A plot of corrosion fatigue crack growth rate as a function Of the areal fraction of IG/TB separation in a high purity FelSCrl2Ni steel.
40
7
Vol. 28, pp. 8 5 3 - 8 5 6 , 1993 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 r i g h t s r e s e r v e d
QUANTITATIVE ASSESSMENT OF THE ROLE OF MICROSTRUCTURE IN CORROSION FATIGUE IN A METASTABLE AUSTENITIC STEEL Ming Gao and Robert P. Wei Department of Mechanical Enginnering and Mechanics LEHIGH UNIVERSITY Bethlehem, Pennsylvania 18015 USA ( R e c e i v e d D e c e m b e r 2, 1992) ( R e v i s e d J a n u a r y 26, 1993) Introduction In a previous study [i], the morphology of fracture surfaces produced by corrosion fatigue in a hlgh-purity Pel8Crl2Ni steel in deaerated 3.5% NaCI solution and in hydrogen at room temperature was examined, Hydrogen embrittlement was identified as the mechanism for corrosion fatigue crack growth in this steel in 3.5% NaCl solutions, and austenite grain and twin boundaries (IG/TB) as preferred paths for cracking. In this paper, the role of microstructure is further addressed quantitatively by using a superposition model for corrosion fatigue [2,3]. The role of microstructure on environmentally assisted crack growth in the metastable austenitic stainless steels has been extensively studied. One of the fundamental questions relates the importance of strain-induced ~'-martensite formation in enhancing crack growth, which remains, by and large, unresolved in the literature [3,4]. Some believe that e'-martensite is the precursor, and its formation is necessary for stress corrosion cracking (SCC) and hydrogen embrittlement (HE). Others argue, with experimental support, against a crucial role for ~'-martensite formation. A recent study of the role of microstructure in corrosion fatigue of annealed and cold-rolled 304 stainless steels suggested that cyclic strain-lnduced a'-martensite did not play a primary role in environmentally enhancing crack growth, because the amounts of ~'-martensite were lower in the more deleterious environments [3]. The results suggested that other microstructural components, such as austenlte grain and twin boundaries, might have been more important. Because the experiments were limited, quantitative assessments of the contributions from each component could not have been made [3]. In this paper, a critical assessment of the role of microstructure in determining the corrosion crack growth response of a high purity FelSCrl2Ni steel in 3.5% NaCI solutions and in hydrogen is made with the aid of a superposition model for crack growth. Experimental Basis Quantitative assessment of the role of microstructure is based on the measurements of crack growth rates, and post-fracture determinations of areal fractions for each fracture mode and volume fraction of strain-induced ~'-martenslte as a function of the test environments. The crack growth rates and fracture modes were determined in the previous study [1], and are summarized in Table I. Corrosion fatigue crack growth in deaerated 3.5% NaCI solutions and in hydrogen occurred by intergranular (IG) cracking along austenite grain boundaries, twin boundary separation (TB) and quasi-cleavage (QC) through austenite or strain-induced martensite matrix [i]. The crack growth rate and areal fraction of IG/TB separation depended on solution pH; both being higher at the lower pH values but much lower at pH-12. The rate and amount at pH-12 were essentially equal to
853 0 9 5 6 - 7 1 6 X / 9 3 $ 6 . 0 0 + .00 Copyright (c) 1993 P e r g a m o n P r e s s
Ltd.
854
CORROSION
those found in hydrogen at lOOkPa. evidence of IG/TB separation.
The crack
FATIGUE
growth
Vol.
rate was
the lowest' in vacuum,
28,
No.
with no
TABLE I Fatigue Crack Growth Rates and Associated Areal Fraction of IG and TB Separation and Volume Fraction of a'-martenslte in Various Environments at 10 Hz* Crack Growth Rate (10 -9 m/cycle)
Areal Fraction
Volume Fraction
(IG + TB)%
(Martenslte)%
Environment (da/dN) e
Vacuum Hydrogen (i00 kPa)
(da/dN)cf
3.2±0.2
0
0
I00
10.7 ± 0.7
7.5 ± 0.7
8.9 ± 1.3
80
3.5% NaCI pH pH pH pH
-
12 6.5 3.0 2.0
10.4 16.5 17.6 18.9
+ + + +
1.0 3.B 1.0 i.I
7.2 13.0 14.4 16.0
*Except for tests in vacuum and hydrogen, **ND: Not determined
+ + + +
1.0 3.8 1.0 i.I
10.6 24.7 26.1 29.2
_+ + + +
0.3 2.2 2.1 1.9
79 59 ND** 58
where f - 8 Hz
The volume fractions of cyclic straln-induced a'-martensite in the neighborhood of the fatigue fracture surfaces were measured using an X-ray diffraction technique; the experimental details are given elsewhere [3]. The results are also listed in Table I. The variation in volume fraction with solution pH, was opposide to that of IG and TB separation; being highest for the specimen tested in vacuum (100%) and lowest (58%) at pH-2. The amount of a'-martensite in the specimens tested in 3.5% NaCI solution at pH-12 was essentially the same as that at I00 kPa in hydrogen (79 versus 80%), which was in between the amounts in vacuum and at pH-2. These results are consistent with the previous findings on 304 stainless steel [3], and suggest that a more "brittle" (or environmentally sensitive) process had intervened in the transformation. It is to be noted that the volume fraction of a'-martensite should not be equated to the areal fraction of QC separation on the fracture surface. The x-ray data may reflect reduced amounts of transformed a'-martenslte, particularly below the IG/TB regions of the fracture surface, and provide lower bound estimates for the contribution from ~'-martensite. Modeling To assess the role of microstructure, an extension of the superposition earlier for corrosion fatigue is made [2,3]. To account for multiple fracture cycle-dependent corrosion fatigue crack growth rate,(da/dN)cf; is considered as growth rate for each (or the i th) fracture mechanism, (da/dN)cf,i, multiplied by tion, #i: (da/dN)cf - Z(da/dN)cf,i~i;
7J~i - 1
model developed mechanisms, the the sum of c r a c k its areal frac-
(I)
where (da/dN)c f - (da/dN) e - (da/dN)r; and (da/dN)e and (da/dN) r are crack growth rates in the deleterious and inert (for example, in vacuum) environments, respectively.
7
Vol.
28, No.
7
CORROSION
FATIGUE
855
For simplicity, the micromechanisms for corrosion fatigue of FelSCrl2Ni alloy in 3.5% NaCI solutions and in hydrogen are combined into two groups; i.e., IG/TB separation and QC cracking. Denoting the former by b and the latter by m, Eq.(1) then becomes: (da/dN)c f - (da/dN)cf,m~ m + (da/dN)cf,b ~
(2)
and ~m ÷ %
- 1
(3)
It should be noted that, by this grouping, the crack growth rates for IG and TB separations are assumed to be equal in Eq. (2). Substituting Eq.(3) into Eq.(2) and rearranging, Eq.(4) is obtained: (da/dN)c f . (da/dN)cf, m + [(da/dN)cf, b - (da/dN)cf,m]@ b
(4)
Equation (4) shows a linear relationship between (da/dN)cf and the areal fraction of IG/TB separation, @b" This relationship is used for assessing the role of microstructure. Assessment And Discussion Figure I shows a plot of the experimental data on (da/dN)cf and @b, at i0 Hz, given in Table I. The data are seen to follow the linear relationship described by Eq.(4) with a correlation coefficient of 0.994. The crack growth rates for each fracture element are then determined. The crack growth rate for QC, (da/dN)cf,m, is estimated from the intercept, while that for IG/TB separation, (da/dN)cf,b, from the slope, and are as follows: (da/dN)cf, m - (3.25±1.2) x 10 -9 m/cycle and (da/dN)cf, b - (4.52±0.6) x 10 -8 m/cycle. These estimates show that the incremental increase in IG/TB crack growth rate by the environment is approximately fifteen times higher than the growth rate in vacuum of (3.25±1.2) x 10 -9 m/cycle, while the incremental increase in QC crack growth rate is equal to the rate in vacuum. These results clearly show that austenite grain and twin boundaries play a more important role straln-induced ~'-martensite in the enhancement of corrosion fatigue crack growth in metastable stainless steels. Even though strain-induced ~'-martenslte is commonly found near the fracture surfaces, its formation does not appear to be essential for hydrogen embrittlement. Because austenlte grain and twin boundaries are more embrittled, cracking along these boundaries may limlt the extent of cyclic straining and the transformation to a'-martensite in the deleterious environments, Table I. Recent studies suggested that QC cracking might be through hydrlde phases at the crack tip, and that a'-martensite is the decomposition product of these phases [5,6]. Further studies are planned to clarify this issue. than
Summary The role of microstructure in hydrogen assisted fatigue crack growth was quantitatively assessed by using a superposition model for corrosion fatigue for a high purity FelgCrI2NI alloy. The results showed that the environmentally assisted crack growth rate through a'-martenslte, (da/dN)e, m, is only twlce as high as that in vacuum (6.45 x 10 -9 versus 3.25 x 10 -9 m/cycle), while the rate through the austenlte grain and twin boundaries is more than one order of magnitude faster (4.84 x 10-g m/cycle). These results strongly suggest that straln-lnduced ~'-martensite formation is not a principal contributor to the environmental enhancement of corrosion fatigue crack growth and is not necessary for hydrogen embrittlement.
856
CORROSION
FATIGUE
Vol.
28,
No.
This work was supported by the Office of Basic Energy Sciences, Department of Energy, under Grant No. DE-FGO2-88ER45354. References i. 2. 3. 4. 5. 6.
Ming Gao and Robert P. Wei, Scrlpta Metal. et Mater., 26, 1175-1180 (1992). R.P.Wel and M.Gao, Scripta Met., 17, 959-962 (1983). Mlng Gao,Shuchun Chen and Robert P. Wel, Met. Trans. A, 23A, 355-371 (1992). H.E. Hanninen: Int. Met. Rev., X(3), 85-135 (1979). Shuchun Chert, Mlng Gao and Robert P. Wei, Scripta Met. et Mater., 28, 471-476 (1993). Robert P. Wei and Mlng Gao, "Micromechanlsm for Corrosion Fatigue Crack Growth in Metastable Austenitic Stainless Steels", Proceedings of CDI'92 Conference on Corrosion-Deformation Interactions, Fontainebleau, France, October 5-7, 1992.
20
(do/dN) cf = (d°/dN)cf, m +
E 15 I
{(d°/dN)cf,b - (do/dN)cf,m }*b RVAL 0.994
pH .3
0
v
10
H ~ ~ pH1
o Z "0
o
"o
5
v
Error Bar @ 95~ Conf. Level 0
I
I
I
I
I
I
I
5
10
15
20
25
30
35
Areal Fraction of IG/TB (¢b ~) FIG. 1 A plot of corrosion fatigue crack growth rate as a function Of the areal fraction of IG/TB separation in a high purity FelSCrl2Ni steel.
40
7