The stability of precipitated austenite and the toughness of 9Ni steel
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INTRODUCTION
I N 1942 9Ni steel was developed by the International Nickel Company as a ferritic material for cryogenic service.L'2 After tempering for about one hour at 600 ~ 9Ni steel exhibits a beneficial suppression of its ductile-to-brittle transition temperature (DBTT) by more than 100 ~ This tempering temperature is within the austenite plus ferrite two-phase region of the equilibrium phase diagram. After tempering, a few percent of austenite (T-phase) is found between the martensite (a '-phase) laths and along the prior austenite grain boundaries. 3-6 The formation of austenite is the most prominent microstructural change observed after tempering. Furthermore, the presence of austenite is the only clear difference between the microstructure of 9Ni steel and the microstructure of 6Ni steel, for which there is no improvement in DBTT after a simple tempering. It is therefore widely accepted that precipitated austenite can be beneficial to toughness at cryogenic temperatures. 7-1~ It is also recognized that the mere presence of austenite is insufficient to ensure good cryogenic toughness. C. W. Marschall, et a l . 7 performed a systematic study of the effects of different tempering treatments on the Charpy toughness of 9Ni steel at 77 K and 290 K. They correlated these toughness data to the amount of austenite present at 290 K, both before and after the material was immersed in liquid nitrogen. In this way they found it necessary for the austenite to be thermally stable against martensitic transformation if good cryogenic toughness is to be obtained. Tempering for much longer than 10 hours at 600 ~ or tempering at higher temperatures, was found to be deleterious to both austenite stability and to cryogenic toughness. Similar systematics have been reported by others. 1~-14
B. FULTZ is Assistant Professor of Materials Science, Keck Laboratory, California Institute of Technology, Pasadena, CA 91125. J. I. KIM, Project Leader, and Y. H. KIM, Researcher, are with T. J. Watson Research Laboratory, IBM, Yorktown Heights, NY 10598. H.J. KIM is Manager with Welding Research Institute, Ulsan, Korea. G. O. FIOR is Researcher with TEGAL, P.O. Box 10, Novato, CA 94948. J. W. MORRIS, Jr. is Professor of Metallurgy, University of California, Berkeley and Faculty Senior Scientist, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. Manuscript submitted August 31, 1984. METALLURGICAL TRANSACTIONS A
Relationships between the stability of austenite and cryogenic fracture toughness have been an important, albeit a controversial topic of r e s e a r c h . 3-5'7-23 Early ideas that the soft austenite phase served to b.znt a propagating crack, as well as ideas that any fresh untempered martensite near the crack tip will promote brittle fracture, have been ruled out by observations that all austenite transforms to martensite in the plastic zone ahead of the crack tip. 3'll'ls'19 It has also been suggested that the austenite serves as an "interstitial scavenger" and promotes a cleaner and more ductile mart
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