WC-Co cemented carbides are a tool material with good performance. The ultra-fine WC-Co cemented carbides have both high strength and hardness. It is a hot topic to study the preparation of ultrafine WC-Co cemented carbides. Spark Plasma Sintering (SPS) is a new technology for this material preparation. Because of its advantages and prospects, the technology of sintering WC-Co cemented carbides has been studied. A new process that can in-situ synthesize WC-6Co cemented carbides. It uses (WO2.9+Co3O4+C) powders by direct reduction and carbonization in a short time at a low cost.
Test 1: In-situ synthesizing the normal-sized WC-6Co cemented carbides
The DSC-TG tests of the (WO2.9+Co3O4+C) powders indicate that experiments and DSC can determine the reaction temperature. Furthermore, it studied the effects of carbon content and synthesis process parameters on the material’s phase, density, microstructure, and performance. The results show that when in-situ synthesizing the normal-sized (φ20mm) WC-6Co cemented carbides, the optimal carbon content is 14.3wt%. The optimal processing parameters are a reaction temperature of 980-990°. And a sintering temperature of 1250-1270°C, 3-8 minutes holding time, and a pressure of 60MPa. It calls to obtain pure phases of WC and Co. The relative density reaches 99％, and the average grain size is 0.8 μm. The Vickers hardness is over 1900kg/mm2; the HRA hardness is 93.5; the fracture toughness (Kic) is 11.2 MN·m-3/2.
Test 2: In-situ synthesizing the large-sized WC-6Co cemented carbide
This new process has prepared the large size (φ45mm) of WC-6Co cemented carbides for the first time. It tested the bending strength. The adjusted carbon content is 14.5wt％, and the adjusted processing parameters are a reaction temperature of 1000°C. The sintering temperature of 1200-1230°C, the holding time of 3 minutes, and the pressure of 60MPa. However, the bending strength is less than 1000MPa.
The reasons are the inhomogeneous microstructures and the existence of a large number of pores. The second SPS sintering temperature is 1300°C to improve the mechanical properties, and the bending strength reaches 1740MPa. It obtained better mechanical properties when using the second process of SPS and adding 6-hour vacuum annealing. The results showed that the relative density is 98.4％, and the average grain size is 1.0 μm. The Vickers hardness is 1850kg/mm2, and the HRA hardness is 91; the bending strength increases to 2036MPa.
Test results by SPS (Spark Plasma Sintering)
To analyze and discuss the mechanism of (WO2.9+Co3O4+C) in-situ synthesis. The results showed that there are two stages of carbonization and sintering densification. Increasing the temperature, carbon can rapidly reduce (WO2.9+Co3O4) to WC-Co by challenging courses. It explained why the sintering temperature of WC-Co was decreased to below 1340°C by SPS.
Based on the study, it is known that the normal-sized WC-6Co cemented carbides can be prepared using the SPS (Spark Plasma Sintering) in-situ synthesis and has better mechanical properties than that from the traditional method. But for the large-sized WC-6Co samples, good mechanical properties can only be obtained by subsequent treatment after the SPS process.
