A method for extracting phase change kinetics from dilatation for multistep transformations: Austenitization of a low ca
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INTRODUCTION
THE formation of austenite is an important aspect of many metallurgical processing and fabricating schemes for steels. For example, hot working, heat treating, and welding all require or result in heating into the austenite plus ferrite or austenite phase fields. At the present time, there is widespread interest in modeling these processes as an aid in optimization and control of postprocess microstructure and properties. For these models to be applicable, they must describe the phase transformation kinetics associated with both the on-heating and on-cooling transformations, and these descriptions must be experimentally validated. In general, the formation of austenite in steels has received less attention than the decomposition of austenite, although there have been a number of experimental[1–6] and numerical studies[7–12] of the process. These studies have yielded significant insight into the transformation from both mechanistic and computational perspectives, but there are some limitations and difficulties in applying this insight to large scale process modeling. As discussed by Gavard et al.[13] and Akbay et al.,[9] the formation of austenite differs from its decomposition in two principal ways. First, in the case of diffusion-limited oncooling transformations, the driving force for the reaction increases with increasing undercooling below the equilibrium transformation temperature, while diffusion rates deR.C. DYKHUIZEN, Principal Member of the Technical Staff, Thermal Sciences Department, and C.V. ROBINO, Principal Member of the Technical Staff, and G.A. KNOROVSKY, Senior Member of the Technical Staff, Materials Joining Department, are with Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted April 16, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
crease with increasing undercooling. This balance between driving force and diffusion rates results in the classical Ccurve kinetic behavior, in which the overall transformation rate experiences a maximum at intermediate undercoolings. In contrast, for the on-heating transformation, both the driving force and diffusion rates increase with temperature above the equilibrium transformation temperature, so that the rate of transformation continuously increases with temperature. Second, for the on-cooling reactions from homogeneous austenite, the kinetics can be fully described in terms of the composition and austenite grain size. Such a simplification is not possible for the formation of austenite, however, as a wide variety of starting microstructures are possible. Thus, the complexity of austenite formation implies that formulation of a general model for nonequilibrium conditions is likely to be exceedingly difficult. Therefore, at the current time, it appears that separate models of austenite formation will be required for different initial microstructures. For the case of ferrite/pearlite initial microstructures, the formation of austenite is known to consist of two essentially distinct steps, which are associated with the
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