Electron microscopy study of the aging and first stage of tempering of high-carbon Fe-C martensite
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INTRODUCTION
IF the tempering of martensite is now understood, aging phenomena occurring at temperatures lower than those at which tempering occurs and known under the generic name of aging still constitute a very active field of interest, as is testified by several recent contributions, t~-Tl All of these studies converge on the general idea that there exists a continuous redistribution of the carbon interstitials which starts at a temperature as low as about 200 K and which evolves into the in situ precipitation of the e- or r/-carbide observed during the first stage of tempering. This study accompanies that of the kinetics of aging and of the first stage of tempering of high-carbon martensite investigated by M6ssbauer spectroscopyt71 (Figure 1). Several electron microscopy observations have been made, among which those of Nagakura and coworkers ts-~2~or Taylor and co-workers E4,5,61are presently the most exhaustive. The experimental information reported in our study does confirm most of their previous findings. However, the present work will clarify several specific salient points which are not yet well settled. The kinetic study quoted above E7] allows for the determination of the most suitable heat treatment for attaining a given stage of aging or tempering. Moreover, by using only high-carbon binary alloys, one discards any confusion which might exist in ternary Fe-Ni-C alloys where nickel could affect the aging and tempering behavior, t4,5,61Besides the fact that it obviously enhances and thus facilitates the observations, high carbon content prevents the complications that could arise due to high M, (martensite start) temperature inherent with low-carbon alloys.
O.N.C. UWAKWEH is a former Graduate Student of the University of Nancy and Laboratoire CNRS Maurice Letort. J.-M.R. GI~NIN, Professor and Department Head, and J.-F. SILVAIN, Research Scientist, are with the Department of Materials Science, University of Nancy, ESSTIN and the Laboratoire CNRS Maurice Letort, Parc R. Bentz, 54500 Vandoeuvre, France. Manuscript submitted March 14, 1990. METALLURGICAL TRANSACTIONS A
EXPERIMENTAL
Sample preparation carefully follows the procedure already described for the kinetics study, tTl Several of the samples are actually those already used for the M6ssbauer investigation, and the others are prepared exactly the same way. Therefore, one can insure that the microscopy observations correlate with the phenomena revealed from M6ssbauer spectroscopy, which describes the statistical distribution of the environments of the iron nuclei. Thus, samples are made of 80-/xm-thick ARMCO* iron foils *ARMCO is a trademark of Armco, Inc., Middletown, OH.
carburized at 1140 ~ in a 1/5CH4-H2 gas mixture, for obtaining about 1.95 wt pct C, and quenched at room temperature, where the samples are 100 pet retained austenite. The Ms temperature of this type of alloy is below 235 K. Then, after a second quench to 78 K is carried out, up to 60 pct of the austenite transforms to the martensitic phase a', as evidenced from M6ssba
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