Chromium Deposition Process Of Crystallization Process Of Ammonium Tungstate And Its Effect On Cemented Carbide

The ultrafine WC -Co cemented carbides has been widely applied in aerospace, wear resistant materials, new energys and telecommunications with the excellent properties of hardness and strength. Adding inhibitors into the tungsten carbide-cobalt composite powder in the process of ultrafine cemented carbide preparation is a common method. While, dispersed degree of inhibitors in the composite powder has great effect on the properties of cemented carbide. At present, inhibitors are usually added by the method of mechanical mixing. Although the method of adding inhibitors can effectively inhibit WC grain growth, the phenomenon of the WC grain abnomal growth often occurs because of the bad mixing uniformity of the inhibitors in the composite powder. In order to prepare uniform Cr-doped WC powder, doping methods such as wet grinding, before WO3 reduction and W carbonization have been studied in detail. However, doping method of chromium in the process of APT crystallization has not yet been reported in the literature. Currently, there are many kinds of inhibitors, among Cr3C2 has been applied in industrial production due to its excellent inhibitory effect. Hence, the process of doping chromium in the process of APT crystallization is studied. This work including doping chromium in the process of APT crystallization, the treatment of crystallization parent solution containing chromium, and preparation of WC -Co cemented carbides, and main achievements are as follows:The process of doping chromium in the process of APT evaporation crystallization was studied. Precipitation behavior of chromium and effects of initial WO3 concentration, 4NH OH concentration, temperature, stirring speed on the precipitation ratio of chromium are studied. The experimental results show that chromium mainly precipitated in the form of Cr2O3 before APT, and it initially precipitated when p H value was around 9, then the precipitation ratio of chromium reached maxium with p H value 8.5, and then precipitationdissolution epuilibrium of chromium was achieved. Results also indicate that chromium precipitation ratio significantly increased with increasing of chromium and ammonium hydroxide concentration, decreasing of stirring speed. Temperature has slight effect on the chromium precipitation ratio. The optimal technical parameters of doping chromium in the process of APT crystallization were initial WO3 concentration of 200 g/L, initial NH4 OH concentration of 4.00 mol/L, temperature of 100℃, stirring speed of 200 rpm.Physico-chemical properties of Cr-doped APT powder were studied. Results show that contents of impurity elements were in line with the standard of GB/T 10016-2007 APT-0 except for chromium; chromium was mainly existed in APT powder in the form of amorphous chromium hydroxide. As growth of APT grains was inhibited by amorphous chromium hydroxide, hence compared with ordinary products, Cr-doped APT had smaller particle size,bulk density and slightly wider particle size distribution. Chromium was well distributed within the APT powder, both macroscopic and microscopic uniformity.The recycle of APT crystallization parent solution containing chromium and precipitation of chromium in the parent solution were studied. Results show that it was feasible to recycle of the parent solution. The removal of chromium in the parent solution by adding a new reagent(magnesium carbonate hydroxide) was developed. Effects of p H, temperature, reaction time on chromium removal were investigated. The optimal technical parameters were p H of 9.0-9.5, temperature of 80℃, reaction time of 90 min.WC -Co cemented carbides was prepared using the Cr-doped APT powder as raw materials by calcining, reduction, carbonization and subsequent sintering. The effect of Cr-doped APT on subsequent products was investigated. The results shows that chromium evenly distributed in WO3 powder, tungsten powder and WC powder; Cr was existed in the form of Cr2O3 and exerted a little effect on calcining; Cr was existed in the form of Cr2O3 and exerted a great effect on reduction; Cr was existed in the form of Cr3C2. Compared with traditional process, the Cr-doped APT prepared in the evaporative crystallization process had bigger uniformity. According to the properties analysis of tungsten alloy, crystallization process of Cr-doped is better than the traditional process and it effectively inhibits WC grain abnormal growth and improve alloy performances.