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Using microscale tungsten oxide, cobalt oxide and carbon black as the raw materials, the ultrafine WC-Co composite powder was synthesized by reduction and carburization reactions. Then the composite powder was sintered to prepare the ultrafine-grained cemented carbides bulk by vacuum sintering. The microstructure and phases of the composite powder and the sintered bulk were investigated by scanning electron microscopy (SEM) and X-ray diffraction meter (XRD). The effects of the carbon addition in the raw powders on the phase formation in the composite powder and on the phases and mechanical properties of the sintered bulk were investigated. The results show that the main phases of the ultrafine WC-Co composite powder are WC and Co, there is a little η(Co3W3C) phase coexisting. The raw powders with carbon black content of 16.69% is suitable. The ultrafine-grained WC-Co bulk material with pure phases and a mean grain size of 470 nm was obtained, which has the transverse rupture strength of 2464 MPa. WC-Co bulk material prepared with the raw powders containing carbon black as 16.85% has a little free carbon and the transverse rupture strength is 1946 MPa.
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