An experimental and theoretical study of cementite dissolution in an Fe-Cr-C alloy
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I.
INTRODUCTION
THE dissolution
of cementite in chromium-containing alloys has been extensively investigated by Molinder t~l in the temperature range of 750 ~ to 900 ~ and by Nilsson tzj in the range of 770 ~ to 910 ~ Both Molinder and Nilsson applied optical microscopy and X-ray analysis. Later, Hillert et al. t31 analyzed Nilsson's experimental data in detail by means of a mathematical model based on local equilibrium at the moving phase interface and diffusion-controlled growth. They concluded that the dissolution rate is controlled by the rate of diffusion of carbon in the early stages and the alloy element in the later stages. Adopting a similar approach, Gullberg t4] studied the dissolution of M23C 6 carbide in austenite at 910 ~ and was able to obtain good agreement between experimental and calculated results. On the other hand, Nolfi et al. tS] investigated the dissolution of Fe3C in ferrite at much lower temperatures, namely, 650 ~ and 700 ~ by means of electrical resistivity measurements and concluded that the process is controlled entirely by the interfacial reaction. Recently, Agren t61 developed a computer-operated method for the simulation of diffusional reactions in multicomponent systems, applied it to a number of problems, including the dissolution of carbide precipitates in both plain carbon, I7j manganese-alloyed, tSj and chromiumalloyed t91 steels, and compared the resulting data with available experimental information at that time. However, the experimental information concerned only the average carbon content of the matrix, e.g., austenite or ferrite; there was no detailed experimental information on the distribution of alloy elements in the matrix and carbides during the dissolution process. ZI-KUI LIU, Graduate Student, LARS HOGLUND, Graduate Student, BJ(~RN JONSSON, Research Associate, and JOHN/~GREN, Professor, are with the Division of Physical Metallurgy, Royal Institute of Technology, S-10044 Stockholm, Sweden. Manuscript submitted June 26, 1990. METALLURGICAL TRANSACTIONS A
Over the last decade, there has been an extensive development in the field of high spatial resolution analytical electron microscopy (AEM). tl~ In the present study, AEM is used to investigate the dissolution process of (Fe, Cr)3C in austenite. The material is a synthetic Fe-Cr-C alloy. To avoid the influence of slow austenite formation and possible interfacial effects at low temperatures, the process is examined at a high temperature where ferrite transforms to austenite very rapidly. The dissolution rate will thus be controlled by volume diffusion in austenite and carbide. The theoretical calculation by means of a newly developed computer program, DICTRA, till based on/~gren's method t61 is presented. II.
EXPERIMENTAL PROCEDURE
A . Material
A high-purity Fe-2.06Cr-3.91C (at. pct) alloy was used in the present study. The raw material was a 0.15-mm-thick band which was made from electrolytic iron and chromium and purified graphite and then hot rolled and cold rolled down to the final thickness. The band was cu
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