On the influence of carbide formation upon the growth kinetics of proeutectoid ferrite in Fe-C-X alloys
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IN an accompanying paper, Bradley and Aaronson j (BA) report measurements of the thickening and lengthening kinetics of grain boundary ferrite allotriomorphs as a function of isothermal reaction temperature in single Fe-C:Si, Fe-C-Mn, and Fe-C-Cr alloys and in two Fe-C-Ni alloys. The ability of three models: a) local equilibrium with bulk partition of alloying element, 2-~~ b) local equilibrium with localized pileup but no bulk partition of alloying element, ~~ and c) paraequilibrium, 14-~8to account for these data is quantitatively examined. Although it is concluded that the paraequilibrium model best explains the data, significant discrepancies remain between calculated and measured growth kinetics even for this model. While good agreement is obtained for the low Ni, Fe-C-Ni alloy, growth faster than calculated is found for the Fe-C-Si and the high Ni, Fe-C-Ni alloys and slower than calculated for the Fe-C-Mn and Fe-C-Cr alloys. Although several explanations are considered by BA, an obvious one, clearly in need of experimental testing, is that of carbide precipitation at austenite:ferrite boundaries. This can occur either as interphase b o u n d a r y carbide precipi" tation on planar or on (apparently) curved austenite:ferrite boundaries, or as pearlite-like fibrous carbide precipitation, t9-2~Such precipitation might slow down G. J. SHIFLET, formerly Republic Steel Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 and Visiting Graduate Student, CarnegieMellon University, Pittsburgh, PA 15213, is now Assistant Professor, University of Virginia, Department of Materials Science, Charlottesville, VA 22901. H. I. AARONSON, formerly Professor, Department of Metallurgical Engineering, Michigan Technological University, is now R. F. Mehl Professor, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. J. R. BRADLEY, formerly Graduate Student, Department of Metallurgical Engineering, Michigan Technological University, is now with the Physics Department, General Motors Research Laboratories, Warren, MI 48090. Manuscript submitted February 23, 1981. METALLURGICAL TRANSACTIONS A
the growth of ferrite by pinning the austenite: ferrite boundaries. 2~ Alternatively, these carbides could increase the growth kinetics of mobile areas of the austenite:ferrite boundaries by steepening the carbon concentration gradient in austenite driving their growth. Accordingly, the present investigation was undertaken to study this question in the BA alloys within the temperature-time envelopes employed for the growth kinetics studies. The observations reported here also supplement the literature on carbide precipitation per se in Fe-C-X alloys. With the exception of Cr, the alloying elements used are not strong carbide-formers and their effects upon the formation of carbides in association with proeutectoid ferrite have not been as intensively studied.
EXPERIMENTAL PROCEDURE The composition of the alloys used in this investigation
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