Effect of Alloy Elements on Eutectic Carbide Morphology for High-speed Tool Steel

Abstract

Effect of alloying elements on the composition distribution, morphology and volume fraction of eutectic carbides were investigated for high-speed tool steels with uniform hardness from 68 to 70HRC after hot working and quenching and tempering. The stability of eutectic carbides at high temperature was also evaluated. M₂C, M₆C and MC eutectic carbide are observed in as-cast samples similar to the general high-speed tool steels. M₂C type carbide increases in volume with increasing Si and V contents, and M₆C and MC carbides appear with increasing W and V contents. The amount of M₂C eutectic carbide varies with the composition in liquid phase just prior to eutectic solidification. The morphology of the eutectic carbide changes from fine fiber or lamellar type to coarse lamellar or feather type, and the interlayer spacing of eutectic carbide tends to increase with increasing the area fraction of M₂C eutectic carbide. Moreover, after heat treatment at 1140℃ for 16 hours, some M₂C carbides remain stabled but MC and M₆C carbide appears.