Phase transformation of austenitic stainless steels as a result of cathodic hydrogen charging
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INTRODUCTION
AUSTENITIC stainless steels display varying stability with respect to transformation to ~ ' (bcc) and e (hcp) martensites, and the stability of the austenite can have significant effects on the mechanical properties. The stability is usually characterized by an Ms temperature for a ' martensite, which may be derived from the alloy composition according to published formulae, ~'2 and/or an Md temperature, which is usually defined as the temperature above which martensite cannot be induced by deformation. Various formulae for calculating Md for a ' martensite have also been published, TM in which temperatures relating pct transformation to pct deformation have been defined. Although there is general agreement that less stable alloys are more susceptible to hydrogen embrittlement, there has been some controversy about the role of martensite formation. The formation of a ' martensite has been cited as the cause of reduction of tensile ductility of austenitic stainless steels in hydrogen, 5-~~and to be necessary for slow crack growth either in gaseous hydrogen or during cathodic charging."~2 The amount and distribution of martensite may be of importance. It has recently been pointed out 12 that a small amount of a ' martensite formed at a crack tip can lead to enhanced diffusion of hydrogen into the material ahead of the crack tip and result in embrittlement, whereas large volume fractions of martensite may lead to rapid diffusion of hydrogen away from the crack tip, and thus prevent a local build-up of hydrogen concentration. An alternative interpretation of the variation in susceptibility to hydrogen embrittlement of the different austenitic steels is based on a model involving transport of hydrogen by mobile dislocations to embrittlement-susceptible sites such as inclusions and grain boundaries. ~3 This model considers the slip mode to be the critical factor in embrittlement, and there is ample evidence for a correlation of ductility loss in hydrogen with variation of stacking fault energy (7SFE).14The model has been used by several authors A.P. BENTLEY, formerly with the Department of Metallurgy and Materials Science, University of Cambridge, is with the National Institute for Materials Research, CSIR, P.O. Box 395, Pretoria 0001, South Africa. G. C. SMITH is with the Department of Metallurgy and Materials Science, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom. Manuscript submitted October 17, 1985.
METALLURGICALTRANSACTIONS A
to account for ductility losses in hydrogen. 14'1s'16The formation of martensite is considered to be unimportant, and it has been concluded that "strain-induced martensite is neither necessary nor sufficient for hydrogen damage in the alloys studied. ''17 Since unstable alloys have lower stacking fault energies, and are therefore more susceptible to embrittlement irrespective of any transformation, it is difficult to provide definitive evidence for one or the other mechanism. Both approaches, however, emphasize the importance of austenite stab
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