The composition of precipitated austenite in 5.5ni steel
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THE ferritic Fe-Ni steels which have been developed for structural use at cryogenic temperatures are typically given a heat treatment which finishes with an intercritical temper at a relatively low temperature in the two-phase (a + y) region, t-6 During the intercritical temper a fine admixture of austenite phase is precipitated within the prior martensite matrix. If the tempering temperature is sufficiently low and the tempering time sufficiently long, a substantial fraction of the precipitated austenite is retained after cooling to room temperature and remains present on subsequent cooling to service temperatures as low as liquid nitrogen temperature (77 K). The final intercritical temper is used to lower the ductile-brittle transition temperature of Fe-Ni steels so that good cryogenic structural properties are obtained. Prior research 1-~~suggests that the retained austenite plays a direct role in establishing good low temperature toughness. The toughening mechanism is apparently a combination of the gettering action of the austenite for deleterious species such as carbon 6 and the grainrefining action of the austenite in disrupting the crystallographic alignment of martensite packets? The effectiveness of the austenite in the latter respect depends on the fineness of its distribution through the parent matrix and also depends on its thermal stability; s the available evidence8 suggests that the austenite must be present in the alloy at the time the load is applied if it is to contribute to cryogenic toughness. While several metallurgical factors may contribute to the thermal stability of precipitated austenite, including its state of coherence and internal strain, the dominant influence should be its composition. Since the austenite is small in size and finely dispersed through the martensite matrix, however, its composition is difficult J. I. KIM is Assistant Research Engineer; Department of Materials Science & Mineral Engineering, University of California, Berkeley, CA 94720. J. W. MORRIS, JR. is Professor, Department of Materials Science & Mineral Engineering, University of California, Berkeley, CA 94720 and also Faculty Senior Scientist, Molecular & Materials Research Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720. Manuscript submitted October 30, 1980.
to determine. The only published estimate of the auste~ite composition in an intercritically tempered cryogenic steel known to us is that by Segawa et al, II who used chemical etching techniques to extract retained austenite from a commercial grade 9Ni steel in the quench and tempered condition. These results indicate an enrichment of the austenite in Ni and Mn which is both consistent with the equilibrium phase diagram and with the observed retention of austenite after cooling to liquid nitrogen temperature. Given this background, the present work was undertaken to gain additional information on the composition of precipitated austenite in intercritically tempered cryogenic steels and to study the sensitivity of the composition to details of th
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