Effects of Tempering Time and Repeated Tempering on Carbides in High Speed Steels.

This report deals with a study on the effects of tempering time and repeated tempering at 575°C for carbides in low-W high speed steels.The carbides were isolated electrolytically from specimens of several heat-treated high speed steels. Chemical composition and crystalline structure of the isolated carbides were determined by chemical analysis and X-ray diffraction, and the shape of the cabides were observed by electron microscopy. The results were obtaind as follows:(1) The amount of precipitated carbides was increased gradually with progress of tempering time, but the amount of precipitated carbides during tempering at 575°C×100h were less than at 650°C×1h. The carbides in the steel on a short-time tempering were composed of M6C and MC types, while M23C6 carbide appeared in the steel of tempering at more than 50 hours.(2) On the repeated tempering at 575°C, the amount of carbides was increased, especially on twice-repeated tempering suddenly. The carbides in the steel on repeated tempering at 575°C were precipitated as much as those in the steel on continuous tempering of 10-50 hours at the same temperature. Presumably it should promote the precipitation of carbides during repeated tempering.