Morphologies Of W/WC Powders And Their Effects On The Structures And Properties Of WC-Co Cemented Carbides

Researches on various preparation methods of cemented carbide powders and sintering technologies for high performance WC-Co cemented carbides with ultrafine grain or special microstructures have been received extensive attention. However, the impacts of powder morphologies on properties of WC-Co cemented carbides need to be explored in depth. Therefore, in this paper, the effects of W/WC powders morphologies on microstructures and properties of WC-Co alloy were researched.In this paper, the morphologies of W/WC powders prepared by different tungsten oxide materials were contrastive analysed, an efficient, controllable and lowcost industrialized process route for nano-WC powders was developed, and the ultrafine grain cemented carbides with excellent comprehensive performances were prepared subsequently. The effects of morphologies of medium-size WC powders, which prepared via high temperature reduction-carbonization process, on properties of cemented carbide were studied, and then high performance WC-6Co (%, mass fraction) alloys with medium-size WC grain and uniform microstructures were prepared. By study the effects of W powders with different morphologies on the flattening morphologies of W powders and the plate-like WC grains, the plate-like grain cemented carbide were prepared. The major contents and important results were given as follows.Different valence tungsten oxide powders and W nano-powders were prepared by yellow tungsten (YTO). W nano-powders were sintered under nitrogen gas with different temperature and duration. The morphology changes of different products in phase-transition were observed and analyzed. The growth mechanism of nano-powders was discussed. The results showed that the reduction products maintained a square shape features were similar to the morphologies of WO3powder particles, which showing morphology genetic characteristics from oxide precursors. Affected by volatile and morphologies of different valence tungsten oxide, the particle sizes of W powders were finer with reducing the valence state of tungsten oxides. The W nano-powders would agglomerate and grow up due to the sintering binding of particles if reduction duration was too long or reduction temperature was too high.W and WC nano-powders were prepared from different tungsten oxide powders. Effects of different raw materials on the morphologies, grain aggregation and uniformity of W and WC powders were researched. It showed that the W nano-powders basically maintained the morphology characteristics of tungsten oxide precursor powders, and WC powder also showed the morphology genetic characteristics. The state of aggregation and uniformity of W and WC powders was different due to the morphologies of different tungsten oxide powders. The W and WC powders prepared through yellow tungsten oxide powders (YTO, SYTO) using APT as raw materials, showed serious grain aggregation and more coarse grains. The W and WC powders prepared by violet tungsten oxide (VTO), fine yellow tungsten (MYTO) or fine violet tungsten oxide (AVTO) also showed both grain aggregation and coarse particles to some extent. But the W and WC nanopowders prepared from fine yellow tungsten (AYTO) using AMT as raw materials showed excellent performance with less coarse grain, less aggregation and better uniformity. All the above conclusions suggested that fine tungsten oxide raw powders with single phase composition, uniform particles size, loose and porous morphology structures were advantage to the preparation of high quality W and WC nano-powders.Effects of W nano-powders with different morphologies on properties of its corresponding WC powders and sintering properties of WC-Co alloy were researched. Impacts of new wet ball mixing method and two step carbonization processes on the uniformity and sintering properties of WC nanopowders were discussed. A novel and lowcost industrialized approach for preparing WC-Co ultrafine grain cemented carbide with excellent comprehensive properties was developed. The experimental results showed that the abnormal coarse WC grains were mainly due to the sintering binding of agglomeration W particles during carbonization process. Homogenous W and WC nanopowders with loose and porous morphology structures can be prepared from fine tungsten oxide powders with similar morphologies structure, and then prepared high performance ultrafine grain WC-Co cemented carbides. Both the wet ball milling method and two step carbonization process could improve the dispersivity of W powders and carbon black; restrain the aggregation and sintering combine phenomenon of nano-powders during carbonization process, which were beneficial to prepare high quality WC nano-powders with homogenous size and good chemical stability. The ultrafine WC-7Co(%, mass fraction) sintered composites prepared by wet ball milling method and two step carbonization method had homogenously microstructures and the excellent properties, the hardness and bending strength were achieved93.8HRA and4450MPa, respectively.High temperature medium-size WC powders were prepared through high temperature reduction-carburization process. Effects of morphologies on properties of alloys were researched. The new preparation method for low cobalt medium-size grain WC-Co hard alloys with homogeneous microstructure and excellent comprehensive performance was developed. It showed that the WC powders prepared by high temperature carburization process were carbonized completely, with uniform and integrated grains, high purity and low lattice defects. The WC-6Co (%, mass fraction) alloys had homogenously microstructure, integrated crystalline, and the grain size of WC crystalline was2μm to2.5μm. The properties of these alloys were better than that of YG6alloys, the hardness and bending strength were90.0HRA and3000MPa, respectively.Effects of the morphologies of tungsten powders prepared by different raw materials on the flattening morphologies of W powders and the microstructures and properties of plate-like grain cemented carbides were researched. It showed that the morphologies of W powders prepared by blue tungsten oxides (BTO) were spherical polyhedron. However, the W powders prepared by yellow tungsten oxides (YTO) were polyhedral equiaxial, which were easier access to prepare plate-like tungsten powders by ball milling, and prepare plate-like WC-Co cemented carbides.