Isothermal Transformation of a CMnSi Steel Below the M S Temperature
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INTRODUCTION
THE kinetics of the martensitic transformation in steels is controlled by the nucleation process because the growth rate of the martensite plate is extremely rapid. The most commonly observed martensitic transformation in steels takes place athermally (i.e., it is a timeindependent process that begins at the martensite-start [MS] temperature and proceeds upon cooling below the MS temperature). The isothermal austenite decomposition below the MS temperature has been described for high carbon, often hyper-eutectoid, or highly Ni-alloyed steels. In these steels, it is possible to observe the slow growth of the isothermal martensite in isolation at relatively low temperatures. The study of the isothermal martensitic transformation in a low-carbon steel is more difficult because prior athermal martensite formation cannot be avoided and the transformation takes place at temperatures in which diffusion and recovery processes still can occur. The isothermal transformation in lowalloyed hypo-eutectoid or low-carbon steels below the MS temperature have been reported by only a few authors, and the nature of the transformation is still mostly undefined. Averbach and Cohen were the first to report the isothermal decomposition of austenite to martensite in hyper-eutectoid tool steels.[1] Pati and Cohen[2] published results on the nucleation and growth of isothermal martensite in Fe-Mn-Ni steels based on work by Kurdjumov and Maksimova.[3] They concluded that the parent austenite transformed into martensite DONGHWI KIM, PhD Candidate, and B.C. DE COOMAN, Professor, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 790-784, South Korea. Contact e-mail: decooman@ postech.ac.kr JOHN G. SPEER, Professor, is with the Advanced Steel Processing and Products Research Centre, Colorado School of Mines, Golden, CO 80401. Manuscript submitted April 22, 2010. Article published online December 2, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
isothermally, despite less potent embryos and a smaller driving force. Radcliffe and Rollason[4] reported the more rapid onset of the bainite reaction, or the ‘‘swingback’’ phenomenon, at temperatures just below the MS temperature and explained that the transformation acceleration was because of the effect of the presence of athermal martensite plates on the nucleation of bainite in adjacent untransformed austenite. Oka and Okamoto[5] investigated the transformation kinetics and microstructure of the isothermal austenite decomposition around the MS temperature in hyper-eutectoid steels. They reported the presence of both thin, plate-like isothermal martensite and leaf-like isothermal martensite with midrib. Borgenstam and Hillert[6] recently determined the activation energy for isothermal formation of martensite after analyzing the results reported in the literature for different high-alloyed steels. They concluded that the low activation energies they obtained were a result of the movement of transformation dislo
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