Hydride formation and decomposition in electrolytically charged metastable austenitic stainless steels
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INTRODUCTION
PHASE transformations in hydrogen-charged austenitic stainless steels have been investigated over the last 2 decades.t~-sI The primary technique used to detect this transformation was X-ray diffraction (XRD). X-ray diffraction has shown that cathodic hydrogen charging can induce the formation of e and o~'martensites in the absence of straining or of subzero cooling. In the early studies, XRD analyses were not performed immediately after hydrogen charging; therefore, some important phenomena resulting from charging may have disappeared in the time period between charging and XRD analysis and hence were not studied5 ~1 Later, more detailed investigations revealed that hydrogen-induced martensitic transformation in austenitic stainless steels followed a much more complex process and was connected with the intermediate formation of hydride phases, r3-61 In stable austenitic stainless steels, such as AISI 310, 3' austenite transformed to e* hydride during cathodic hydrogen charging. This e* hydride decomposed during subsequent aging to form e(H) martensite (i.e., e martensite with hydrogen in solid solution) which subsequently transformed back to y austenite.Igj The processes suggested that without hydrogen, the e phase is unstable relative to the y phase.t91 No o? martensitic phase was observed after aging in hydrogen-charged AISI 310 stainless steel. I4,61On the other hand, in metastable austenitic stainless steel, such as Fe 18Crl 2Ni, 3" austenite was observed to transform to hcp e* hydride during charging, which decomposed during aging to form a' and e martensitesY ~ The presence of e and a' martensites after hydrogen charging and subsequent aging has been
SHUCHUN CHEN, Post doctoral Fellow, and MING GAO, Principal Research Scientist, Zettlemoyer Center for Surface Studies, and ROBERT P. WEI, Professor and Chairman, Department of Mechanical Engineering and Mechanics, are with Lehigh University, Bethlehem, PA 18015. Manuscript submitted October 5, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
confirmed by transmission electron microscopy (TEM). ~jl 141Nakayama and Takano tl51 used high-voltage TEM to examine phase transformations in front of the crack tip in AISI 304 stainless steel after hydrogen charging and tensile testing and found no hydrides. Their absence may be attributed to the long time delay between hydrogen charging and TEM examinations, during which complete hydride decomposition could have occurred. In spite of extensive studies of hydrogen charging of stainless steels, little is known about the kinetics of hydrogen-induced phase transformations. The mechanisms for hydride formation and decomposition also are not fully understood. In this study, the kinetics and mechanisms of hydrogeninduced phase transformation in two metastable austenitic stainless steels were examined. The kinetics of hydride formation and decomposition were determined with the aid of XRD analyses, and the mechanisms and hydride morphology were investigated by using XRD and TEM. An inert gas fusion technique w
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