Preparation Of Ultrafine Tungsten Carbide Powder And Ultrafine-gained Cemented Carbides Based On Carbothermic Reduction

Cemented carbides,consisting of a hard phase(such as WC,TaC,TiC,etc)and a binder(such as Co,Ni,Fe,etc),are among the important cemented materials owing to their outstanding combination of high hardness,excellent strength,high toughness,good wear resistance and chemical stability.As the rapid development of society,WC-based cemented carbides are widely used in various fields.Different application environments require more excellent performance of cemented carbides.It is recognized that mechanical properties of WC-Co cemented carbides could be improved significantly by decreasing the grain size of WC to the ultrafine scale or even nano scale.The ultrafine WC powder is the key raw materials for the preparation of ultrafine-grained WC-Co cemented carbides.Thus,the preparation of ultrafine WC powder and ultrafine-grain cemented carbides has already become a research focus.Currently,the widely method for the industrial production of WC powder and WC-Co cemented carbides is a three-step process.Step 1:hydrogen reduction of tungsten oxides to prepare W powder;Step 2:preparation of WC powder by carbonization of W powder;Step 3:cemented carbides prepared by sintering of WC and Co powder.The chemical vapor transports(CVT)mechanism caused by the formation of gaseous hydrate in the process of hydrogen reduction of tungsten oxide accelerates the growth of W particles,which restricts the preparation of ultrafine WC powder and ultrafine cemented carbides.Therefore,in order to realize the lowcost and high-efficiency preparation of ultrafine WC powder,this paper abandons the process of preparing tungsten powder by hydrogen reduction method,and develops the new preparation processes of "carbothermic reduction followed by solid-phase carburization" and "carbothermic reduction followed by gas-phase carburization" based on carbothermal reduction method.Furthermore,the new short process of "carbothermic reduction and further direct carbonization+sintering" and the in-situ enhancement of WC-Co cemented carbides were further developed by combining the carburizing and sintering processes to realize the preparation of ultrafine grained cemented carbides.In this paper,the effects of thermodynamics and reaction mechanism of material preparation process,molar ratio of C/WO3,reaction temperature and the holding time on the phase transition,morphology evolution,grain size and properties of products were investigated systematically.The research results are described as follows:(1)Based on the carbothermal reduction method,the "carbothermal reduction followed by solid-phase carburization" process was developed to prepare ultrafine WC powder.Removing all oxygen atoms through the first stage of carbothermal reduction WO3 and controlling the particle size of the product;Then,according to the discrepancy between the carbon content of reduction product and theoretical value of WC,the second stage solid-state carburization was completed after precise carbon supplement to control the purity of the product,so as to successfully prepare ultrafine WC powder.It was found that the C/WO3 ratio of raw materials and the reduction temperature had an important influence on the particle size of the product.With the increase of C/WO3 molar ratio and the decrease of reduction temperature,the particle size of the product decreased gradually.By adjusting the C/WO3 ratios of raw materials and the reduction temperature,the WC powder with particle size of 178-825 nm could be obtained.(2)Focusing on the key point of "inhibit the growth of WC particles in the carburization stage",the new method of "carbothermic reduction followed by gasphase carbonization" was developed to prepare ultrafine WC powder.Using CH4H2 mixed gases for low-temperature gas-phase carburizing effectively solved the problem of the growth of WC particles in the carburization stage.The reduction products could be completely carburized by adjusting the CH4 ratio in the mixed gas,and avoiding the occurrence of the carbon deposition reaction of CH4,which was beneficial to control the purity of the final product.Aiming at the problem of carbon black remaining in the reduction product with high carbon ratio,a new strategy was proposed that the reduction production with high C/WO3 ratios was further roasted before gas-phase carbonization to remove the free carbon.When the C/WO3 ratio was in the range of 2.1-2.6,it was noted that the free carbon content of the reduction product was low,and the CH4-H2 mixed gases could be directly used for gas-phase carburizing.When the carbon ratio was in the range of 2.7-3.5,due to the residual unreacted carbon black in the reduction product,the prereduction product was roasted at 1100℃ for 4h to convert the free carbon into the bonding carbon in W2C or WC,and then complete gas-phase carburization.Finally,ultrafine WC powder with particle size of 77-192 nm was successfully prepared.(3)According to the characteristics of the above "carbothermic reduction followed by solid-phase carburization" and "carbothermic reduction followed by gas-phase carburization" processes,the reduction temperature in the carbothermal reduction stage is further reduced and the holding time is prolonged to prepare the reduction products with finer particle size.Different low-temperature carburizing processes are selected according to the residual carbon content of the reduction products.When a low C/WO3 ratio(2.1-2.6)was adopted,the residual carbon content of the reduction product was at a very low level,and in this case,the products were carburized by 10%CH4-90%H2 mixed gases at low temperature in the next gas-phase carburization stage.When a high C/WO3 ratio(2.7-3.8)was adopted,a number of unreacted carbon black would remain in the reduction product.In this case,the desired carbon black was mixed with reduction product to generate WC in the next solid-phase carburization stage.Finally,the WC powder with a BET value of 3.88-4.25 m2/g and a particle size of 90.39-98.81 nm was prepared.(4)Based on the research foundation of "carbothermic reduction followed by solid-phase carbonization" process,a new short-flow process of”carbothermic reduction and further direct carbonization+sintering" was developed to prepare fine-grained WC-Co cemented carbides.The mixture of WO3 and Co2O3 was reduced by carbon black at 1150℃ for 2h;then,the mixture of carbothermic reduction product and an appropriate content of carbon black was compacted,and the green compact was first carbonized at 1200℃ for 2h and then sintered at 1450℃ for 4 h to prepare cemented carbides with hardness of 1972 HV and fracture toughness of 12.3 MPa·m1/2.This process could prepare WC-Co cemented carbides with different Co content according to the different target products.(5)On the basis of the short process,using ammonium paratungstate,cobalt oxalate,ammonium metavanadate and carbon black as raw materials,the ultrafinegrained WC-Co cemented carbides was in-situ enhanced by the short process,and the ultrafine-grained cemented carbides with mean WC grain size of 0.46 μm,homogenous distribution of W,Co and V elements and excellent performances were successfully prepared.The study found that the addition of V element and the high C/WO3 ratio were beneficial to refine the particle size of the reduction product,and the in-situ formation of VC in the final product significantly enhanced the properties of WC-Co cemented carbide.It was revealed that when the addition amount of VC was less than 2%,V element existed in WC/Co phase boundaries in the form of V4WC5 thin layer.When the VC content was higher than 2%,the V4WC5 with larger particles were generated except the V4WC5 thin-layer.