Microstructure of Low C Steel Isothermally Transformed in the M S to M f Temperature Range
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INTRODUCTION
THE martensitic transformation in low C steel is generally considered to be a time-independent athermal process, which begins at the martensite start (Ms) temperature and proceeds to completion upon further cooling to the martensite finish (Mf) temperature. A time-dependent isothermal transformation in leanalloyed, low C steels, however, was observed when the quenching was interrupted in the Ms to Mf temperature range.[1,2] The nature of the isothermal transformation below the Ms temperature can, in principle, be (1) the continuation of the athermal martensitic transformation, (2) the initiation of the lower bainitic transformation below the Ms temperature, or (3) the isothermal martensitic transformation below the Ms temperature. The continuation of the stress-induced or straininduced martensitic transformation is possible, because the energy barrier for the successive transformation of the nuclei could be provided by the stress or strain generated in the volume of the already transformed martensite. Van Bohemen et al.[3] reported the isothermal transformation below the Ms temperature in an Fe-0.66C0.69Mn alloy. They concluded that the isothermal transformation below the Ms temperature is bainitic in nature, because (1) the kinetics and microstructure of DONGHWI KIM, formerly Graduate Student, with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, 790-784, South Korea, is now Senior Researcher with the Samsung Heavy Industries Co. Ltd., Geoje 656-710, South Korea. SEOK-JAE LEE, Research Assistant Professor, and BRUNO C. DE COOMAN, Professor, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted December 29, 2011. Article published online October 12, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
the transformation product were similar to the bainite formed above the Ms temperature and (2) no isothermal martensitic transformation below the Ms temperature was reported in hypo-eutectoid steel. However, they did not explain the relatively low activation energy of 43.6 kJ/mol of the isothermal transformation. In addition, they did not consider the fact that the kinetic exponent n in the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation is not necessarily a constant. An isothermal ‘‘martensitic’’ transformation below the Ms temperature was reported for alloyed steel.[4–7] The isothermal transformation, however, was generally observed in high Ni-Mn alloyed or hyper-eutectoid steel. In such alloy systems, the isothermal transformation can be observed in the subzero temperature range in the absence of prior athermal martensite. In contrast, the formation of some volume fraction of athermal martensite cannot be avoided in lean-alloyed, low C steel. A detailed microstructural analysis, therefore, is essential to clarify the nature of the isothermal transformation below the Ms temperature in lean-alloyed, l
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