Effect of Alloying Element Partition in Pearlite on the Growth of Austenite in High-Carbon Low Alloy Steel
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INTRODUCTION
THE kinetics of austenitization from pearlite in high-carbon low alloy steel depends strongly on steel composition, i.e., the species of alloying element.[1–4] Hillert[5] proposed two modes of austenite formation and cementite dissolution in alloy steel. At a low superheating above the eutectoid temperature, the long-range redistribution of alloy element is required, while above a certain critical temperature, the redistribution of alloy element is no longer necessary, and the growth of austenite occurs under carbon diffusion control. The diffusion of alloy element, if it occurs, is confined to a very narrow region ahead of the interface, which in effect maintains local equilibrium of alloy element at the interface. A similar transition occurs in the growth of proeutectoid ferrite from austenite; the long-range redistribution of alloy element between ferrite and austenite takes place at a small undercooling below Ae3 which is called partition local equilibrium (PLE) mode.[6,7] Below a certain temperature, alloy element partition is not required, and the growth is controlled by carbon diffusion, which is called Z. N. YANG, Graduate Student, C. ZHANG, Associate Professor, and Z.G. YANG, Professor, are with the School of Materials Science and Engineering, Key Laboratory for Advanced Materials of Ministry of Education, Tsinghua University, Beijing 100084, P.R. China. Y. XIA, Graduate Student, is with the School of Materials Science and Engineering, Key Laboratory for Advanced Materials of Ministry of Education, Tsinghua University, and also Current Researcher at Wuchang Shipbuilding Industry Corp. Ltd., Wuhan 430060, P.R. China. M. ENOMOTO, Emeritus Professor, is with Ibaraki University, Hitachi 316-8511, Japan. Contact e-mail: masato.enomoto. [email protected] Manuscript submitted February 12, 2015. Article published online December 28, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
no-partition local equilibrium (NPLE) mode. Because the transformation temperature is often lower in ferrite transformation, the ferrite growth is thought to occur also under paraequilibrium in which the diffusion of alloy element is entirely suppressed.[8] Xia et al.[9] calculated the transition temperature between the two modes of austenite growth, which they denoted partition-to-no-partition transition temperature (PNTT), in Fe-0.6 pctC-M alloys (M=Mn, Si, Cr, and Ni) and ascertained that PNTT and the growth rate varied widely with the species of alloy element. They assumed in the calculation that the pearlite formed at 923 K (650 C) and alloy element partitioned to full equilibrium between pearlitic ferrite and cementite prior to austenitization. According to the literature, however, the extent of alloy element partitioning between ferrite and cementite is considerably dependent on pearlite transformation temperature, here designated Tptr.[10–19] Equilibrium partitioning can be achieved only at a very small undercooling. Moreover, the extent of partitioning increases during post-transformation aging which usually is acco
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