Characterization of the Carbon and Retained Austenite Distributions in Martensitic Medium Carbon, High Silicon Steel
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THE segregation of carbon to lattice defects, austenite, grain and phase boundaries, carbon clusters, and carbide during both quenching and tempering has a significant effect on the properties of martensitic steels. Silicon is known to retard the precipitation of cementite during tempering, but the effect of silicon on other carbon segregation processes is not known in great detail. Models to simulate steel transformations and processing rely on understanding these segregation DONALD H. SHERMAN, Senior Engineer, and STEVEN M. CROSS, Engineer, are with the Advanced Materials Technology, Technology & Solutions Division, Caterpillar Inc., Peoria, IL 616561875. Contact e-mail: [email protected] SANGHO KIM, formerly Postdoctoral Researcher, Division of Engineering, Brown University, Providence, RI 02912, is Researcher, Plate and Rod Group, POSCO, Pohang 790-785, South Korea. FERNANDE GRANDJEAN, Professor of Physics, is with the Department of Physics, University of Liege, B-4000, Sart-Tilman, Belgium. GARY J. LONG, Professor of Chemistry, is with the Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409-0010. MICHAEL K. MILLER, Senior Research Staff Member, is with the Microscopy and Microanalysis Sciences Group, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6136, USA. Manuscript submitted October 2, 2006. Article published online July 6, 2007. 1698—VOLUME 38A, AUGUST 2007
processes.[1–4] This article reports the characterization of the carbon and retained austenite distributions as functions of cooling rate and tempering temperature in the martensitic microstructure of a representative medium carbon, high silicon steel (MCHSS). Particular focus is on the redistribution of carbon during both cooling and low-temperature tempering. The carbon redistribution results are compared to results published in the literature. In general, martensitic transformations are diffusionless and displacive with respect to substitutional elements.[5–9] However, for medium carbon, low alloy steel with martensite start temperatures (Ms) between 200 C and 480 C, significant carbon segregation (autotempering) occurs during cooling at rates up to 103 K/s.[10] The Ms and quench rate determine the extent of autotempering; alloys with the lowest Ms and the highest cooling rates have the least autotempering.[11] Several carbon redistribution processes (autotempering) are both thermodynamically and kinetically possible during cooling.[12,13,14] These processes include the segregation of carbon atoms to microstructural features[9,15–19] (i.e., voids, dislocations, phase boundaries, or retained austenite), spinodal decomposition,[20–30] and carbide precipitation.[31–37] In addition, carbon atoms can segregate to form coherent clusters that are METALLURGICAL AND MATERIALS TRANSACTIONS A
randomly dispersed throughout a carbon-depleted martensitic matrix.[20] Carbon enrichment in the austenite by diffusion of carbon from the martensite eventually suppresses the martensite finish temperature
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