On the role of phase transitions in the hydrogen embrittlement of stainless steels

Scripta METALLURGICA

Vol. 14, pp. 1355-1358, 1980 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

ON THE ROLE OF PHASETRANSITIONS IN THE HYDROGENEMBRITTLEMENT OF STAINLESS STEELS N. Narita and H. K. Birnbaum Department of Metallurgy and Mining Engineering University of I l l i n o i s at Urbana-Champaign Urbana, I l l i n o i s 61801

(Received October 6, 1980)

While the embrittlement of stainless steels by hydrogen has been extensively studied, there is l i t t l e agreement on the experimental observations and even less on the mechanisms of fracture. One persistent theme has been the role of martensitic phase transitions in the fracture mechanisms and in this matter l i t t l e agreement has been forthcoming. Under conditions of high hydrogen fugacities such as cathodic charging (1,2) or stress-corrosion environments (1,3,4), transgranular or intergranular low d u c t i l i t y fracture is observed in both "stable" stainless steels such as Type 310 and "unstable" steels such as Type 304. In a less aggressive hydrogen environment, such as one atmosphere of H~ gas, Type 304 steels exhibit transgranular low d u c t i l i t y fracture (1) in low strain rate-high stress tests. Intergranular fracture (5,6) occurs when Cr depletion occurs at grain boundaries. The "stable" Type 310 steels do not exhibit a low d u c t i l i t y fracture under the low hydrogen fugacity atmospheres. Electron d i f f r a c t i o n from the fracture surfaces of Type 304 fractured in H~ gas (1,7) or by stress-corrosion (1) in MgClp at 155 C has shown that the surface consists of a fine-grained bcc ~ martensitic phase which i~ about l pm in thickness. This result is consistent with magnetic measurements (1,7) made at the fracture surfaces. The effect of hydrogen on the y-phase s t a b i l i t y has also been examined, and i t has shown that solute H decreases the Ms temperature (8,9) and increases the M. temperature (7). Transformation of the y phase to an expandedfcc ¥' phase and a hcp cphas~ was observed (7,9-II) for both Types 310 and 304 steels and transformation of the ~ to bcc ~ has been shown in Type 304 (7,9-II) in the presence of H and stress. Thus while the i n s t a b i l i t y of the y phase is well established as is the presence of ~ at the 304 fracture surface, i t is not known whether the phase transformation is a cause or a result of the hydrogen embrittlement. To examine this question, we f~actured Types 304 and 310 stainless steels in the HVEMin a hydrogen gas atmosphere of about lO Pa at room temperature. The specimens were thinned from sheet which was annealed at l125K in argon and quenched. Deformationwas performed at low strain rates in a displacement-controlled stage. Types 310 and 304 steels tested in vacuum and Type 310 tested in gaseous H~ exhibited duct i l e transgranular fracture. Selected area diffraction (SAD) at the crack t i p ~nd along the crack generally showed Debye rings characteristic of a very fine-grained polycrystalline structure (Fi 9, la). This structure was characteristic only of the region near the fracture since SAD patterns taken about l pm from the surface showed single crystal spot patterns (Fig. Ib). The SAD patterns taken from Type 310 fractured vacuum or in Hp showed only fcc y patterns. Type 304 fractured in vacuum showed predominantly fcc y patterns with a small amount of bcc near the fracture surface. In contrast to this behavior Type 304 steels fractured in lO4 Pa of H~ gas exhibited transgranular fracture such as that shown in Fig. 2. Significant deformation o~curred at the crack t i p as shown by the significant thinning which occurred along the crack surfaces. SAD patterns taken from the crack t i p or along the crack surfaces (Fig. 3) showed Debye rings which were indexed as fcc y and bcc ~ reflections. The grain size of both phases is extremely small as indicated by the number of reflections in each Debye ring. Dark f i e l d images (Fig. 2) were formed using the 200 ~ ring intensities and showed that the ~ phase was located along the fracture surface and in front of the crack t i p . The particle

1355 0036-9748/80/121355-04502.00/0 Copyright (c) 1980 Pergamon Press Ltd.

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size of the ~ phase was very small as indicated by dark f i e l d reflections. By forming dark f i e l d images using different parts of the 200 ~ Debye ring i t was established that the region in front of the crack t i p and along the crack was completely ~ phase. They phase just behind the ~ contributed to the y phase Debye rings and was a fine-grained polycrystalline structure. SAD patterns taken at about one ~m from the crack surface showed single crystal f c c y phase structures indicating that the phase transition and formation of the polycrystalline structure was highly localized to the crack t i p and fracture surfaces. These results are consistent with those obtained from the fracture surfaces of bulk specimens of Type 304 steels fractured in hydrogen atmospheres or by stress corrosion (1,7). The s i g n i f i c a n t additional observation of the present experiments is that the y to ~ phase transition occurs in front of the crack and the fracture occurs through the ~ phase. I t is not known whether the ¥ to ~ t r a n s i t i o n results from the destabilizing effect of stress and hydrogen on the y phase, i . e . an increase in Md temperature (7), or from an increase in the amount of local p l a s t i c i t y due to hydrogen such as has been observed for iron (12,13) and nickel (14), i . e . deformation-induced transformation. In practice l i t t l e difference exists between these viewpoints. Both in fact suggest that the y to ~phase transition is a precursor to the hydrogen-induced fracture of the stainless steels and that the resistance to hydrogen embrittlement is improved by increasing the y phase s t a b i l i t y . This is consistent with the resistance of the more stable Type 310 steels to hydrogen embrittlement in Ho atmospheres. The correlation between y s t a b i l i t y and resistance to hydrogen embrittlement i~ complicated by other possible fracture mechanisms which may occur in the higher strength y phase alloys which do show hydrogen embrittlement despite the stable phase. Acknowledgments This research was supported by the NSF contract DMR 77-09808. These experiments were carried out at the HVEMf a c i l i t y of the Argonne National Laboratory and the authors would like to p a r t i c u l a r l y thank Mr. A. Philippides and Mr. E. Ryan for their assistance. References I. 2. 3. 4. 5. 6. 7. 8. 9. lO. II. 12. 13. 14.

R. Liu, N. Narita, C. A l t s t e t t e r , H. K. Birnbaum and E. N. Pugh, Met. Trans. l I A , (1980) in press. H. Okada, Y. Hosoi and S. Abe, Corrosion-NACE 26, 183 (1970). S. S. Birley and D. Tromans, Corrosion-NACE 27, 63 (1971). R. J. Asaro, A. J. West and W. A. T i l l e r , Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, Unieux-Firming, France (1973). C. L. Briant, Scripta Met. 12, 181 (1978). C. L. Briant, Scripta Met. 12, 541 (1978). N. Narita, C. J. A l t s t e t t e r and H. K. Birnbaum, to be published. T. Suzuki and J. C. Shyne, Proceedings of the First JIM International Symposium on New Aspects of Martensitic Transformation, p. 305, Kobe, Japan (1976). P. Maulik and J. Burke, Scripta Met. 9, 17 (1975). M. L. Holyworth and M. R. Louthan, Jr., Corrosion-NACE 24, llO (1968) H. Mathias, Y. Katz and S. Nadiv, Metal Science 12, 129 (1978). H. Matsui, H. Kimura and S. Moriya, Mat. Sci. and Eng. 40, 207, 217, 227 (1979). I. M. Bernstein, Scripta Met. 8, 343 (1974). J. Eastman, T. Matsumoto and H. K. Birnbaum, to be published,

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HYDROGEN

EMBRITTLEMENT

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OF

STAINLESS

STEELS

1357

222 7 311 220

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Fig.la

Type 310 tested in H2 gas

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Type 304 tested in H2

Fig. 1 Selected Area D i f f r a c t i o n Patterns Taken at Crack Tips. S.A.D. patterns taken at the crack t i p s (Fig. la) showed Debye rings while those taken lum from the crack (Fig. Ib) showed a single crystal spot pattern. 310 a 220 21i 2 0 0 'c~

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311 Y 222 y 400 y

Fig. 3 Selected Area D i f f r a c t i o n Pattern ot lype 304 Tested in H. Gas. The d i f f r a c t i o n pattern was taken at th~ crack t i p .

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HYDROGEN EMBRITTLEMENT OF STAINLESS STEELS

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um

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Dark f i e l d images of crack tips i n Type 304 fractured in H2 gas. The images were formed using the i n t e n s i t y from the 200 ~ Debye rings.

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