Abstract
In the present research, X-ray and chemical analyses were carried out on carbides isolated electrolytically from several chromium, molybdenum, tungsten and vanadium steels tempered at 400∼730°C. Moreover, the volume change of these steels on tempering was examined by means of a differential dilatometer, and the correlation of the carbide reactions and the volume change occurring in the fourth stage of tempering was studied. It was found that the formation of VC, Mo2C, W2C and M6C at the expense of cementite causes a distinct expansion of the steel, but the conversion of cementite into M7C3 and M23C6 is not accompanied by a volume change of the steel. The cause is considered to lie in the difference in crystal structure of each carbide. That is, the structures of VC, Mo2C,W2C and M6C are so-called interstitial ones, in which the carbon atoms fit into the octahedral holes, and the interatomic distances between metal to metal atoms are larger than those in the carbides having complex structures as cementite, M7C3 and M23C6. This reasoning is possibly available to explain why the secondary hardening phenomenon occurs in vanadium, molybdenum and tungsten steels, but not in chromium steels, during the fourth stage of tempering.
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