Isothermal martensite formation in an AISI 52100 ball bearing steel
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INTRODUCTION
A V E R B A C H , Cohen, and Fletcher ~ first revealed isothermal martensite formation at near room temperature in quenched and quench-tempered 1C-1.5Cr steel. Isothermal austenite decomposition proceeds rapidly during the first 1.5 hours after quenching and continues at a diminishing rate for several months. 2 Prosvirin and Entin 3 recognized the formation of isothermal martensite in an as-quenched 1C-I.5Cr steel at a temperature range from 80 to 150 ~ Chang and Hsu 4 investigated the isothermal transformation of austenite within the temperature range between M~ and room temperature for 5 hours after quenching from 1100 ~ to room temperature in a ball bearing steel by use of electrical resistance measurements. They concluded that the isothermal transformation of retained austenite is martensitic and takes place mainly by the growth of already existing martensite. Hsu et al. s revealed directly from metallographic investigation that beside the growth of the preexisting athermal martensite, the formation of isothermal martensite may occur through nucleation and growth processes in retained austenite. Recent work on the isothermal martensitic transformation in Fe-Ni-C alloys reveals that the overall kinetics of the transformation at temperatures below Ms (Mb) also exhibits C curve behavior, 6 similar to that found in Fe-Ni-Mn alloy, v8 The present work attempts to study the formation of isothermal martensite in a quenched AISI 52100 steel in much detail mainly concerning the kinetics and mechanism of the isothermal reaction.
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EXPERIMENTAL
Commercial AISI 52100 ball bearing steel with composition 1.02C, 1.56Cr, 0.27Si, 0.12Ni, 0.007S, 0.009P, T .Y. HSU (Xu Zuyao), Professor, and CHEN WEIYE, formerly Graduate Student, now Research Asststant, are with the Department of Materials ScLence and Engineering, Shanghai Jlao Tong University, Shanghai, China. CHEN YEXIN, formerly Graduate Student, Department of Materials Science and Engineering, is with the Anhui Institute of Technology, Hefei, Anhm Province, China. Manuscript submitted September 29,1986. METALLURGICALTRANSACTIONS A
0.008Cu is used. Specimens are heat treated through austenitization at 1150 ~ in an argon atmosphere, followed by quenching to oil at a temperature of 55 ~ cooling to room temperature, and then isothermal holding at 97 to 115 ~ The structure of isothermal martensite on a polished specimen is shown through a hot stage of optical microscopy with phase contrast and TEM. Specimens for X-ray diffraction were electropolished with a solution containing 100 g CrO3, 540 ml glacial CH3COOH, and 28 ml H20. A graphite monochromator is used for measuring the amount of retained austenite. Differential method with a back reflection camera is applied to determine lattice constants of martensite and retained austenite. Changes in lattice constants in the martensite in the course of isothermal holding are determined by a RIGAKU X-ray diffractometer with a hot stage and a differential sample attachment, taking a quenched specimen as reference.
