An On-Heating Dilation Conversional Model for Austenite Formation in Hypoeutectoid Steels
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INTRODUCTION
DILATOMETRIC measurements are often used to monitor the details of solid-state phase transformations. Most steel transformation studies have focused on the decomposition of austenite into a variety of structures. The kinetics of austenite formation is especially important for processes such as induction hardening or intercritical annealing, where relatively large phase fraction changes can occur as a result of small changes in cycle time or temperature. The amount of each phase present during a transformation can be calculated by applying the lever rule to the dilatometric curve. The lever rule predicts that the volume fraction of each phase is directly proportional to the fraction of transformation strain that has occurred, with consideration of the thermal expansion or contraction strain for each phase. Lever rule predictions are useful for processes involving transformations with a single parent phase and a single product phase, but they do not reveal the transformation details for more complicated starting systems, such as ferrite-pearlite steels. The prediction of austenite phase fraction upon heating is particularly complex for hypoeutectoid ferrite-pearlite steels, where pearlite regions transform first to carbon-enriched austenite regions, followed by the transformation of proeutectoid ferrite regions to austenite.[1] The presence of low-carbon ferrite regions increases the time-temperature combination needed for SEOK-JAE LEE, Research Associate, and CHESTER J. VAN TYNE, FIERF Professor, are with the Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] KESTER D. CLARKE, Postdoctoral Research Associate, is with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted December 18, 2009. Article published online June 22, 2010 2224—VOLUME 41A, SEPTEMBER 2010
austenitization and results in a two-stage transformation sequence. The transition from pearlite decomposition to proeutectoid ferrite decomposition is difficult to interpret from dilatometric data. The ability to accurately predict constituent and phase fractions as a function of temperature/time based on dilatometric data is critical to common industrial practices and heat-treatment modeling, both in verification of the models as well as developing the appropriate constitutive equations for phase transformations during heat treatments. Several studies[2–15] have reported predictions of volume fraction of transformed phase(s) by using the dilatometric transformation strain measured by evaluating the volume strain difference between parent phase and transformed phase, based on atomic lattice parameter calculations. These approaches are termed models for analyzing dilatometric behavior or conversional models of transformation strain. These methods give better results for the determination of phase fractions as compared to the lever rule calculations. M
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