Study On Synthesis Of Nano WC-Cr₂（C,N）-Co Composite Powder And Preparation Mechanism Of Cemented Carbide

WC based cemented carbide is widely used in tunneling,rail transit and other fields because of its high hardness,high toughness,wear and corrosion resistance,but the domestic cemented carbide still has problems such as insufficient strength and toughness,leading to the urgent need of high-end cemented carbide in our country mainly depends on imports.In order to further improve the mechanical properties of the alloy,this study proposes to prepare high-strength and toughness ultra-fine crystal cemented carbide by in-situ synthesis of nano-WC-Cr₂（C,N）-Co composite powder combined with discharge plasma rapid sintering technology.Differential thermal analysis（TG-DSC）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and thermodynamic analysis were used.The synthesis mechanism of nano-WC-Cr₂（C,N）-Co composite powder,the optimization of process parameters and the microstructure and mechanical properties of cemented carbide were studied.（1）Through thermodynamic calculation,the reaction process of composite powder was studied,and combined with carbothermal reduction nitrification method,the phase and morphology evolution law of composite powder synthesis process was analyzed.The results show that the high-valence oxide of W is first reduced to the low-valence state.With the increase of temperature,the low-valence oxide is completely reduced,and the carbonation reaction occurs to form WC.The phase evolution of WC-Cr₂（C,N）-Co formation follows(amorphous phase,WO_2.72)→(CoWO₄,WO_2.72,WO₂,CrWO₃)→（WO₂,CoWO₄,Co₆W₆C,CrWO₃）→（W₂C,Co₃W₃C,WC）→（WC,Co,Cr₂（C,N））With the increase of the heat treatment temperature,a large number of pores appeared on the surface of the spheres formed by the aggregation of precursors,and finally evolved into a spherical morphology formed by the aggregation of a large number of powder particles.（2）By optimizing the key process parameters of composite powder synthesis,the optimum process conditions were studied.The results show that with the increase of reaction temperature,the phase composition is basically unchanged,but the powder size gradually increases.With the extension of reaction time,the W₂C phase and the lack of carbon phase（Co₃W₃C,Co₆W₆C）gradually disappeared,and the WC grains grew significantly after 60 min of holding.The Co₃W₃C phase in the product decreased gradually with increasing carbon content.When the carbon content exceeded 5.89 wt.%,the carbon deficient phase（Co₃W₃C）disappeared completely.When the reaction temperature was 1050°C,the reaction time was 60min,and the carbon content was5.89 wt.%,the average particle size of the powder was～67 nm,and the free carbon,oxygen and nitrogen contents were 0.19 wt.%,0.22 wt.%and 0.14 wt.%,respectively.In the composite powder,Co was tightly bound to WC,Cr was uniformly distributed on the surface of WC,and the relationship between Cr and WC was（10-10）WC//（022）Cr₂（C,N）orientation.（3）With the help of SPS sintering technology,the sintering densification behavior of the composite powder was analyzed,the microstructure and mechanical properties of the cemented carbide were studied.The results showed that the composite powders achieved final densification through three main stages:1)bridging between powder particles to form sintering neck;2)a large number of atoms migrate,the pores are significantly reduced,and the compaction speed is accelerated;3)further remove a small amount of closed pores.With the increase of sintering temperature,the grain size of cemented carbide gradually increases,the Vickers hardness increases to 1895 HV30,and the fracture toughness increases first and then decreases.The maximum fracture toughness at 1150℃is 12.24 MPa·m^1/2.With the extension of the holding time,the grain size of the cemented carbide gradually increased,the Vickers hardness increased to 1942 HV₃₀,and the fracture toughness gradually decreased to 10.37 MPa·m^1/2,and increased slightly at 1250℃.The results show that the best ultrafine carbide prepared at 1200℃for 5 min has an average grain size of～0.48μm,a Vickers hardness of 1842HV₃₀ and a fracture toughness of 11.41 MPa·m^1/2...