In situ X-Ray Diffraction Analysis of Carbon Partitioning During Quenching of Low Carbon Steel
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MARTENSITIC transformation in steels has now been investigated for more than 100 years.[1–5] The interest in martensitic transformations is still very high as numerous industrial applications need this transformation to improve wear resistance, mechanical properties, and fatigue properties of parts in daily productions.[6] Moreover, new interest in fundamentals of martensitic transformations has appeared in the last decades with the challenges of computer simulation, where kinetics and other phenomenon should be well described to obtain reliable simulation results.[7–9] Some recent investigations on medium carbon and low alloyed steels have focused on the characterization of aging/tempering processes and on autotempering, which means tempering effects occurring directly during quenching or bainitic treatments.[10–12] From these studies, it was found that retained austenite is mostly present in the form of thin films (of several nanometers in width) between the martensite laths.[10,11] Carbon diffusion occurs from the original martensitic phase either into the surrounding lattice defects (like voids, dislocations, etc.) or into the retained austenite even at cooling rates up to 103 K/s.[10–12] Strong carbon enrichment at the interface between retained austenite and martensite is then to be expected and was found.[11] Hence, the carbon enrichment of the retained austenite induces a stabilization of retained austenite, which depends on the cooling rate.[10]
New heat treatments called ‘‘Quenching and Partitioning’’ using that large potential for carbon to diffuse out of the freshly formed martensite were even developed.[13,14] To enrich the retained austenite and not to precipitate carbides, the alloys used for those treatments usually contain a high amount of silicon (typically from 0.6 to 2 wt pct) as silicon retards the precipitation of carbides.[13] As most investigations of carbon partitioning during quenching of steel samples were performed on the final state of heat-treated samples, reliable information about the kinetics and generally about ongoing processes are missing. In situ X-ray diffraction (XRD) analysis has become a powerful method for materials characterization stimulated by constant advances in instrumentation and data processing. This method allows, contrarily to dilatometry or resistivity measurements, for obtaining timeresolved precise quantitative information about every phase present in the investigated material.[15,16] In the present investigation, the method of in situ XRD analysis was applied to characterize the behavior of a typical case carburizing steel grade (AISI 5120) during rapid cooling. The kinetics of phase transformation and the evolution of carbon contents in the phases were considered. Moreover, models for the description of transformation kinetics were compared and optimized. II.
JE´RE´MY EPP, Research Engineer, and THOMAS HIRSCH, Research Director, are with the Stiftung Institut fuer Werkstofftechnik, 28359 Bremen, Germany. Contact e-mail: [email protected] CAROLINE CURFS, Sen
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