The present study investigated the microstructure of cutting tool in order to clarify their wear mechanism of cutting tool made of high speed tool steel in the metal cutting process. A special protective oxide surface, which mainly consist of iron, vanadium and oxygen is formed on the surface of the tool during dry cutting wear test. Iron could be diffused from cutting tool and cutting material, and vanadium which alloyed to improve tool life as MC carbide in high speed steel is from cutting tool. During cutting wear test, an amorphous oxide surface seems to exist in a liquid state. At the cutting temperature on the contact point of tool, the surface as so-called "Belag" is melted as a result of eutectic reaction of iron oxide and vanadium oxide. The surface has a role of fluid lubrication between work material and tool. Therefore, the surface is effective in protecting against tool wear at this cutting speed.
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.
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