Self-Diffusion Coefficients of Carbon in Fe 3 C at 723 K via the Kinetics of Formation of This Compound
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Self-Diffusion Coefficients of Carbon in F%C at 723 K via the Kinetics of Formation of This Compound B. OZTURK, V. L. FEARING, J. A. RUTH, Jr,, and G. SIMKOVICH Cementite, Fe3C, is one of the primary phases, in addition to ferrite, in plain carbon and mildly alloyed steels, which represent the major tonnage of the world's steel production. Yet there is a dearth of knowledge concerning this compound. In fact, in the latest edition of The Making, Shaping and Treating of Steel it was noted, concerning Fe3C, that ~ - ' ' L i t t l e is known about its (cementite) properties except that it is hard and brittle." The primary reason underlying the lack of knowledge of the properties of Fe3C is that, in the binary system, ironcarbon, this compound is metastable at all temperatures in respect to its products of decomposition, graphite, and iron saturated with graphite. Recently, however, the Russian scientists, Pilipenkov and Veselov, 2 indicated that Fe3C, free of graphite, could be formed utilizing pure methane as a carburizing gas over extremely fine iron powder at temperatures of less than 773 K (500 ~ At temperatures greater than this graphite deposition occurred and/or the growing Fe3C decomposed to graphite and iron saturated with graphite. Preliminary work by Fearing 3 and Ruth 4 indicated that graphite deposition frequently occurred even at temperatures less than 773 K (500 ~ when utilizing only methane as a carburizing gas and, when mixtures of H2 and CH4 were utilized, graphite deposition frequently occurred, but not always, when the activity of carbon was greater than twenty. Further experiments at carbon activities less than twenty B. OZTURK, formerly Graduate Student, Metallurgical Section, The Pennsylvania State University, University Park, PA, is now Research Scientist, Research Laboratory, United States Steel Corporation, Monroeville, PA 15146. V.L. FEARING is Associate Project Metallurgist, Carpenter Technology, P.O. Box 662, Reading, PA 19603. J.A. RUTH, Jr. is Process Metallurgist, Gillette Company, Gillette Park, South Boston, MA 02106. G. SIMKOVICH is Professor of Metallurgy, Steidle Building, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted December 10, 1981. METALLURGICALTRANSACTIONS A
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revealed no carbon deposition, and the results of these preliminary experiments and the deduced diffusion coefficient of carbon in Fe3C are presented here. To our knowledge these are the first values of the diffusion coefficient of carbon in cementite presented in the literature. Experimental. All of the kinetic tests reported in this communication were conducted at 723 K (450 ~ Weight gains, or losses, during a run were obtained utilizing an automatic recording semimicro Ainsworth balance. Fine iron particles were obtained by oxidizing ferric nitrate (three hours at 403 K [130 ~ followed by six to 12 hours at 503 K [230 ~ to Fe203 and then reducing a weighed portion of the obtained Fe203, contained in a pyrex crucible, at 723 K (450 ~ while suspended from the balance. After the red
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