Preparation of extra coarse-grained WC-Co cemented carbides by doping sodium in the ammonium tungstate solution during evaporation crystallization process

Abstract

Coarse tungsten powders are usually used as raw material for preparing coarse grained WC-Co cemented carbides. Sodium can effectively promote the growth of tungsten powder during the reduction of tungsten oxide with hydrogen. Conventional addition of sodium salt into tungsten oxide powders by mechanical milling has the disadvantage of uneven distribution of sodium, which leads to abnormal growth of tungsten powders and affects the performance of WC-Co cemented carbides. In this study, uniform sodium doped ammonium paratungstate (APT) was prepared by adding sodium carbonate into ammonium tungstate solution during the evaporation and crystallization process based on the formation of ammonium-sodium paratungstate. Experimental results showed that sodium was uniformly distributed in the APT and promoted the growth of APT crystal. The sodium-doped APT was then used to produce coarse tungsten powders. Compared with tungsten powders obtained from tungsten trioxide with addition of sodium by mechanical milling, the coarse tungsten powders had larger average particle size and more uniform particle size distribution. The coarse tungsten powders were subsequently prepared into tungsten carbide powders and WC-Co cemented carbides by carburization and pressure sintering, respectively. Results showed that the obtained WC-Co cemented carbides had larger average WC grain size (6.8–7.2 μm), higher fracture toughness and transverse ruptures strength, and more uniform microstructure than that of WC-Co cemented carbides obtained by addition of sodium with mechanical milling. Moreover, sodium thoroughly volatilized during hydrogen reduction and carburization process, so that the purity of WC-Co cemented carbides was not reduced. Therefore, this study provided a new route for preparation of extra coarse-grained WC-Co cemented carbides.