A Study of Diffusion- and Interface-Controlled Migration of the Austenite/Ferrite Front during Austenitization of a Case
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austenitization of medium to low-alloy carbon steels is an important part of many industrial heat treating procedures. It has not, however, received the more rigorous treatment that has been accorded to austenite decomposition into its several products (e.g., References 1 through 6). This lack of detailed analysis can be explained through a number of prohibitive reasons, rooted in the nature of the ferrite-to-austenite transformation. The austenitization may proceed too rapidly in many cases for classical investigative methods, which may involve soaking a sample for various times and then evaluating a partially transformed microstructure through sectioning, polishing, and optical or electron microscopy. Even if this were not the case, the austenite simply cannot be retained for characterization in many circumstances, as it decomposes upon cooling. Due to the significant difficulty and expenditure of resources that might be dedicated to studying it, industrial practices mostly ignore the details of this process and simply use the fact that it does occur (i.e., solution annealing) when designing heat treating proceERIC D. SCHMIDT, Graduate Student, and SEETHARAMAN SRIDHAR, Professor, are with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA. Contact e-mail: [email protected] E. BUDDY DAMM, Principal Materials Engineer, is with The Timken Corporation, Canton, OH 44706-0930, USA. Manuscript submitted August 4, 2006. 244—VOLUME 38A, FEBRUARY 2007
dures. As a result, one could argue that there has not been a strong driving force for research and analysis of the topic. It is, however, the opinion of the present authors that a fundamental understanding of the formation of austenite in alloy steels, in terms of kinetics, morphology, and mechanism, can significantly benefit thermal processing technology by improving the performance of the heat-treating and quenching process of alloy steels, reducing the number of steps and especially increasing the predictability of the final microstructure. Furthermore, the procedures used in the analysis contained here can certainly be applied in a study of other phase transformation phenomena. There have been sporadic studies of austenite formation in iron and low-alloy steels during the last 40 years. These include experimental studies on the influence of starting microstructures on the transformation kinetics,[7–10] dilatometry,[9,11,12] and modeling of the transformation during continuous heating at varying scales of complexity.[13–16] As a result of these studies, there is some understanding of the austenite nucleation and growth process under limited conditions. For pure iron, austenite is assumed to nucleate at ferrite grain boundaries and the interface migration rate is controlled by the interface reaction (i.e., the rate at which atoms jump across and are incorporated into the crystal on the opposite side of the interface).[7] When carbon is added to the iron, the growing austenite precipitates have a much higher concentratio
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