The chemical composition of precipitated austenite in 9Ni steel
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I.
INTRODUCTION
COMMERCIAL 9Ni steel is prepared for service at cryogenic temperatures by an intercritical tempering which forms small fcc austenite particles within the bcc martensite matrix. The cryogenic toughness of 9Ni steel requires not only the presence of austenite, but also a certain degree of stability of the austenite particles against the fcc--, bcc martensitic transformation. Correlations between the stability of austenite particles and the cryogenic toughness of 9Ni steel are well-established.~'2'3 The austenite particles, however, are expected to be thermodynamically unstable at lower temperatures, so their stability against the fcc --~ bcc martensitic transformation is a question of fundamental as well as of practical interest. Equilibrium phase diagrams suggest that the solutes Ni, Mn, Cr, or C should segregate to austenite particles during the intercritical tempering of 9Ni steel. These elements stabilize the austenite phase, and are expected to be at least partially responsible for the austenite stability at low temperatures. Unfortunately, it is difficult to measure the solute concentration of austenite particles because they are so small, having dimensions of only a few hundred/~ in the very early stages of intercritical tempering. 4 Modem experimental techniques are required for these composition measurements, and here we report data obtained by analytical scanning transmission electron microscopy (STEM) and by transmission M/Sssbauer spectrometry (TMS). A dramatic reduction in the austenite stability occurs as intercritical tempering proceeds, ~ and this change in austenite stability with tempering time poses a well-defined problem for laboratory investigation. We sought to determine the extent to which this change in austenite stability during tempering is attributable to composition changes of the austenite particles. In addition, an analysis of both the amount of austenite and its solute composition provides basic information about how the austenite particles are formed during intercritical tempering. B. FULTZ is Assistant Professor with Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, CA 91125. J.I. KIM and Y. H. KIM, Research Scientists, are with T. J. Watson Research Laboratory, IBM, Yorktown Heights, NY 10598. J.W. MORRIS, Jr. is Professor, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted July 25, 1985. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL
PROCEDURES
AND DATA ANALYSIS Commercial 9Ni steel plates were kindly supplied by the Nippon Kokan Company, and were prepared as described previously. ~The chemical composition of this material was (in wt pct): 9. INi, 0.50Mn, 0.20Si, 0.17Cr, 0.08C, 0.004P, 0.004S with a balance of Fe. Intercritical tempering was performed at 590 ~ for times ranging from 0.8 to 627 hours. Specimens for study by STEM and TMS were prepared from wafers cut from bulk material with an abrasive saw under flood cooling. These wafers were thinned in
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