The influence of precipitated austenite on hydrogen embrittlement in 5.5Ni steel
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I.
INTRODUCTION
SEVERAL investigators ~-4 have suggested that retained austenite is beneficial to the hydrogen resistance of lath martensitic steels. The mechanism of the improvement is, however, unclear, and there are indications from other work that the austenite may be harmful. 5 To help clarify this issue it was decided to investigate hydrogen embrittlement in an alloy whose microstructure and fracture modes are well understood. The alloy selected was a commercial 5.5Ni steel manufactured by the Nippon Steel Corporation. This alloy was developed for structural use at cryogenic temperature, and is toughened for cryogenic service by a three-step heat treatment (called the QLT treatment) 6'7 that causes the precipitation of a significant amount of a thermally stable austenite along martensite lath boundaries. The morphology, composition, and mechanical behavior of this precipitated austenite have been studied in detail. 6-" The mechanism of hydrogen embrittlement in commercial 5.5Ni steel is also known. ~2When the alloy is tested in the tempered condition in the presence of hydrogen, it fractures by decohesion along the martensite lath boundaries. The plan of the present work was to use heat treatment to introduce a controlled distribution of precipitated austenite along the martensite lath boundaries and determine the influence of this austenite on the degree and mechanism of hydrogen embrittlement. Since the intercritical tempering treatments that introduce austenite in this steel also temper the dislocated lath martensite matrix, two heat treatments were used. The first, designated QT, provides a tempered martensite with very little precipitated austenite. This was the heat treatment studied in Reference 12. The second, the QLT treatment, provides a tempered martensite matrix that is densely decorated with fine interlath islands of austenite.
Y.-H. KIM is Research Scientist with T.J. Watson Research Center, IBM, P.O. Box 218, Yorktown Heights, NY 10598. H.J. KIM is Research Scientist with Welding Research Institute, Hyundai Heavy Ind. Co., Ltd., 1, Cheonha-dong, Ulsan, Korea. J.W. MORRIS, Jr., is Professor of Metallurgy, University of California, Berkeley, 278 Hearst Mining Building, Berkeley, CA 94720. Manuscript submitted November 13, 1984. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The alloy used for this work was a commercial 5.5Ni steel made by the Nippon Steel Corporation. Its chemical composition is, in weight percent, Fe-5.86Ni-l.21Mn-0.69Cr0.20Mo-0.20Si-0.06C-0.01S-0.008P. The alloy was supplied as a 35 mm plate in the QLT condition. The alloy was annealed at 1200 ~ for 2 hours to remove the effects of prior thermomechanical treatments, austenitized at 800 ~ for 1 hour, and quenched in ice water to room temperature. Samples cut from the plate were then given one of two heat treatments. The first was an intercritical temper for 1 hour at 600 ~ to produce the condition designated QT. The second was the conventional QLT treatment: an intercritical anneal for 1 hour at 670 ~ followed by
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