Chi-carbide in tempered high carbon martensite
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INTRODUCTION
T H E tempering of high carbon martensitic Fe-C alloys and steels leads eventually to a microstructure consisting of ferrite and cementite. Lement, et al ~'2 characterized the development of this microstructure as the third stage of tempering, and indicated that the initial portion of the reaction consisted of the solution of a transition carbide and the subsequent precipitation of cementite (0-carbide) within the martensite plates and along the martensite plate boundaries. However, some investigators 3~ have presented evidence that the carbide which forms at the beginning of the third stage of tempering is H~igg or x-carbide rather than cementite. Chi-carbide, as identified with X-ray powder patterns by Jack and Wild, 7 is monoclinic with lattice parameters a = 1.1562 nm, b = 0.4573 nm, and c = 0.5060 nm and has the stoichiometry M5C2 where M may represent any combination of Fe and Mn atoms. Duggin et al 8 have also determined the structure of X-carbide, but by neutron diffraction techniques. Transmission electron microscopy (TEM) shows that X-carbide tends to nucleate and grow on transformation twins of the parent martensite. 3,4'5 The electron diffraction evidence3'4'5 for x-carbide formation during tempering is based on the matching of the interplanar spacings and angles obtained from carbide distributions in tempered steels to those obtained from synthetically produced x-carbide. 7'8 Such selected area diffraction evidence frequently does not uniquely identify x-carbide because cementite has a very similar structure and many of C-B. MA, Associate Scientist, is with Institute of Nuclear Energy Research Lung-Tan, Taiwan 325, Republic of China. T. ANDO, formerly Research Assistant at the Colorado School of Mines, is now Senior Researcher, Tokyo Kohan Company, Ltd., Chiyoda-ku, Tokyo 100, Japan. D.L. WILLIAMSON, Associate Professor of Physics, Department of Physics, and G. KRAUSS, AMAX Foundation Professor, Department of Metallurgical Engineering, are both with Colorado School of Mines, Golden, CO 80401. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.
METALLURGICALTRANSACTIONS A
the interplanar spacings of X and 0 are identical.7'9 Although there is agreement that the carbide structure that forms at the beginning of the third stage of tempering is different from that of cementite, some investigators~~ attribute this structure to the constraints of the parent tempered martensite on the first-formed cementite rather than to the formation of X-carbideM6ssbauer effect spectroscopy (MES) is a technique which has often been applied to the phase characterization of Fe-C alloys, particularly for retained austenite determinations. ~2'13'14The MES parameters of X and 0 carbides are well known primarily from studies of synthesized and extracted carbides. Results of these studie
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