Refining Mechanisms Of Arsenic In The Hydrogen Reduction Process Of Tungsten Oxide And Its Application In Ultrafine-grained WC-Co Cemented Carbide

Due to possessing high hardness and high strength, ultrafine-grained WC-Co cemented carbide is commonly used in many important areas, such as cutting tools, impact tools and wear-resistant parts. The characteristics of WC powders genetic from W powders directly, so producing nano W powders with fine dispersion and uniform particle size is a key technology for the preparation of ultrafine-grained WC-Co cemented carbide. In this paper, nano W powders, ultrafine WC powders and ultrafine-grained WC-Co cemented carbide were prepared by a series processes of calcination, reduction, carbonization and liquid phase sintering combined with the method of the solid-liquid mixed, which adding a certain amount of As element into ammonium para-tungstate. In our research, the phase consittution of all samples were detected by X-ray diffraction. The morphology and grain size of synthesized powders and alloys were characterized by scanning electron microscopy, and using the energy dispersed spectroscopy to analysis the distribution of As elements in samples. The existence form and phase evolution regularity of As elements in tungsten oxide powders, tungsten powders and tungsten carbide powders were analysied by transmission electron microscope. The value of As content in the samples of APT-1%As, WO3-1%As, W-1%As and WC-1%As were tested by Inductively Coupled Plasma(ICP). The particle size or grain size distributions of samples were determined through the method of image statistics. The performance of WC-Co cemented carbide, such as density, hardness and fracture toughness were tested by drainage method, micro-vickers hardness tester and Vickers indentation crack method, respectively. The main conclusions of research are as follows:1. A part of As elements escaped from APT-As precursor composite powders in the form of gaseous in the process of calcination, which causing WO3–1%As aggregate particles possessed more loose sponge structure compared with pure WO3. This loose sponge structure could promote hydrogen entry and the discharge of water vapour, resulting in lower water vapor partial pressure inside the furnace and inhibited W particles growing in vapor deposition during the hydrogen reduction process of tungsten oxide. Other part of As elements combined with O and W elements and formed W2O3(AsO4)2 ternary compound. Eventually, As elements existed in tungsten trioxide powder as the form of As2O3 and W2O3(AsO4)2.2. The particle size of W–As composite powders decreased with the increase of arsenic adding amount. The 0.1 wt% adding quantity was the critical point of arsenic refining tungsten powders and W–As composite powders possessed an average particle size of 80 nm when the adding quantity of As reached 1.0 wt%. On the one hand, the WAs2 intermediate phase played an important role as heterogeneous nucleation core in the process of W nucleation and formed a core-shell structure with WAs2 as core and W as shell, as a result of increasing the number of the nucleation. On the other hand, the WAs2 particles attached at the surface of W particles and hindered W particles growth through grain boundary migration. Moreover, the formation of volatile As2O3 and As reduced the partial pressure of water vapor and inhibited W particles growing in vapor deposition.3. As elements existed in WC powders as the form of WAs2 after carbonization, because of WAs2 was stable. The WC-1%As composite powders were fine dispersion and uniform with the average particle size of 0.22 ?m. During the process of carbonization, the WAs2 particles which as nucleation core of W were bare and distributed between small WC particles, as result of hindering WC particles grew up through grain boundary migration and weaking the agglomeration between WC particles.4. The WAs2 was dissolved in the Co phase and hindered the process of dissolution-precipitation of WC in the Co phase during the process of sintering, which inhibiting WC grains grew up. YG6-1%As alloy was ultrafine-grained WC-Co cemented carbide with the average particle size of 0.47 ?m. The density, hardness and fracture toughness of YG6-1%As alloy were 97.3%?1239.8 Hv?10.86 Mpa·m1/2, respectively, which improved significantly compared with YG6 alloy.