Epsilon carbide precipitation during tempering of plain carbon martensite
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I.
INTRODUCTION
T E M P E R I N G processes of steel martensite above ambient temperatures have been classified into four stages, i.e., (a) the 0-th stage, where carbon atoms redistribute to form a planar modulated structure prior to carbide precipitation, 1~-51 (b) the first stage, where the martensite ( a ' ) decomposes into low-carbon martensite (a") containing 0.2 - 0.3 wt pct C and e-carbide particles, 16-13] (c) the second stage, within which the retained austenite transforms into lower bainite, 1~~ and finally, (d) the third stage, the faulted (or two-dimensional) cementite precipitation with complete loss of the tetragonality.]8-1~ Hirotsu and Nagakura ]2Lz2]examined the electron diffraction patterns from a high-carbon martensite tempered at 393 K very closely and found unusual diffuse extrareflections which were different from fundamental e-carbide reflections. In order to explain these reflections consistently, they proposed that the carbide is not close-packed hexagonal and is isomorphous to the orthorhombic C02N. They named this "r/-carbide" instead of e. On this identification, however, there have been some arguments, such as the problems arising from the equivalent variants of the e-carbide/martensite orientation relationship. [23] The morphologies of the e-carbide precipitate are also of interest, but comprehensive explanation for the various shapes obtained so far has not been given. For example, Murphy and Whiteman 111]showed that e-carbide particles are long needles parallel to the (121)~.11(1io0)~, but the rodlike morphology elongated in (100)~.11(1120), directions E9,~2,241and that of a disklike shape on {100}o planes ~8,25]have also been observed. The aims of the present study, therefore, are to reexamine the diffraction patterns from e-carbide to confu'm YASUYA OHMORI, Professor, is with the Department of Materials Science and Engineering, Ehime University, Matsuyama 790, Japan. IMAO TAMURA, Professor Emeritus, Kyoto University, is Manager of the Research Institute for Applied Sciences, Kyoto 606, Japan. Manuscript submitted October 25, 1991. METALLURGICAL TRANSACTIONS A
the crystal structure and to clarify the morphological change of e-carbide during tempering.
II.
EXPERIMENTAL PROCEDURE
The materials used are an Fe-0.69 wt pct C binary alloy and a plain carbon steel with the composition of 1.03C-0.36Si-0.68Mn in weight percent. They were received in the shape of 20-mm-diameter bars and were machined into 1.5-mm-thick disks. They were austenitized at 1373 K for 10 minutes in a dynamic argon atmosphere and quenched into iced brine to form martensite. Tempering was carried out by a constant heating rate of 5 Ks -1 in a dynamic helium atmosphere, and the specimens were quenched into iced water upon reaching the given tempering temperatures, the highest tempering temperature being 523 K. Thin foils for transmission electron microscopy were prepared from these specimens by a conventional twin-jet polishing method in CH3COOH-HC104 electrolyte and were examined by both a 200-kV and high-resolu
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