Abstract
Hard metal which is high performance in hardness and toughness is used as tool materials for high precision processing. Property of hard metal is improved to high hardness and high strength by refinement of WC phase. The further property improvement by refinement of WC phase is expected to tool materials such as micro drill, end mill, ultraprecision die.
The technology of obtaining finer WC powder for micro-grained hard metal by the direct carburization process in which WO3 powder is directly reacted with carbon powder and the inhibitors addition technology in which very fine primary crystals are formed into coarse polycrystalline WC grain by addition of grain growth inhibitor are investigated.
For the direct carburization process, very fine nuclei are formed at reduction step during WO2.72→WO2→W, and the fine nuclei is carburized to WC powder of nano sized from ultrafine grain diameter without grain growth. It was found out that hard metal obtained from the WC powder by the direct carburization process have higher hardness and strength. For the inhibitors addition technology, addition of grain growth inhibitors such as VC, TaC, Cr3C2, NbC, Mo2C at carburization process of coarse tungsten powder was examined. The very fine primary WC crystal are formed by adding Cr into coarse polycrystalline grain at carburizing process. Since Co diffuse into grain boundaries of the polycrystalline WC grain during sintering of hard metal, property of the hard metal was dependent on size of the primary crystal of WC powder. The Cr added WC powder can be contributed to stable production of micro-grined hard metal due to higher apparent density and lower oxygen content of WC powder.
