Mechanism Of W Nano-powders Prepared By Carbon-hydrogen Co-reduction And Application In Preparing WC Nano-powders And Ultrafine Grained WC-Co Cemented Carbides

Ultrafine/nano-grained WC-Co cemented carbides have been the development direction of cemented carbides due to the “double-high” properties,namely both high hardness and high strength.To produce W and WC nano-powders with excellent performance is the key factor to manufacture ultrafine/nano-grained WC-Co cemented carbides.However,the rapid and abnormal growth of W particles may happen during hydrogen reduction process of tungsten oxides because of the volatilization-deposition effect.In this thesis,the carbon-hydrogen co-reduction method was used to prepare W nano-powders,while two-stage carbonization method and carbon-hydrogen co-reduction direct carbonization method were used to produce WC nano-powders respectively.Ultrafine-grained WC-Co cemented carbides were prepared by low pressure sintering using the prepared WC nano-powders as raw materials.This thesis conducts a systematical study on the production process,performance and mechanism of W nano-powders,WC nano-powders and its sintered alloys.Its major contents are as follows:1.The phase transformation and morphology evolution during the process of the carbon-hydrogen co-reduction were studied,and the co-reduction process parameters were investigated.The result shows that the reduction process follows stepwise reduction rules.In this process,amorphous precursor turns into such low-price intermediate oxides as WO_2.9,WO_2.72 and WO₂ in sequence and is finally reduced to ?-W.As crystal structure transfers and carbon and vapor reaction is consumed,the reduction production became loose and porous.The average particle size of W powders reduces as the carbon addition ratio of precursor increases.When the C/W molar ratio is higher than 2.6,residual carbon content would increase greatly.As reduction temperature increases,carbon in the precursor would participate in the reduction and be consumed while the growth tendency of W powder be weakened.Therefore,the average particle size and residual carbon content of the reduced W powders would decrease.Precursor with a carbon addition ratio of 2.6 would be reduced to spherical W powder with an average particle size of 56 nm after reduction at 760 oC for 60 min.2.The morphology,particle size and microstructure of W powders prepared by different reduction methods were studied.The preparation mechanism of the carbon-hydrogen co-reduction was proposed after analyzing the grain growth curves of W powders under different reduction temperatures.It turns out that the growth rate of W grains would slow down as the reduction temperature increases.When the reduction temperature is above 760 oC,vapor generated in the process reacts with carbon and generated CO and H₂,which signally reduces the p[H₂O]/p[H₂] in the system and restrains the production of volatile aqua-complex WO₂?OH?₂.The dominant growth mechanism of W grain also changes from the original volatilization-deposition to the atomic diffusion.Reduction methods have an important influence on the particle size and morphology of W powders.It is found that the W particles obtained by the carbon-hydrogen co-reduction are fine and uniform spherical particle with loose structure and excellent dispersity.However,the W particles prepared by ordinary hydrogen reduction are coarse,grown entirely,and had the polyhedral morphology of the W intrinsic crystal.The reason for that is,the W grain growth mechanism in the carbon-hydrogen co-reduction process primarily is solid phase topochemical reaction,while the growth mechanism in the ordinary hydrogen reduction process is the volatilization-deposition.3.Using W nano-powders obtained by the carbon-hydrogen co-reduction and carbon black as the raw materials,the WC nano-powders were synthesized by the two-stage carbonization.The effect of two-stage carbonization process parameters?carbonization temperatures and soaking times?to the WC phase,morphology and particle size was studied.It is found that the particle size of WC powder depends on the carbonization and growth rate of W particles.High carbonization rate and low growth rate would reduce particle size of WC.Pre-carbonization process under low temperature would form thin WC layers on the surface of W particles,changing the contact status of particles from W/W contact to WC/WC contact.It could hinder the coarsening of WC powders arise from the sintering combination growth of W nano-powders in the early carbonization stage.The optimal process of the pre-carbonization and second-stage carbonization are 900 oC for 60 and 1100 oC for 90 min,respectively.After the two-stage carbonization with 900 oC+1150 oC,the W nano-powders?mean particle size 56 nm?are carbonized into WC nano-powders?mean particle size 106 nm?.4.WC nano-powders were synthesized by carbon-hydrogen co-reduction direct carbonization method.The effect of the carbon addition ratio to the carbon content of WC powders was studied.The effect of the reduction and carbonization temperature to the morphology and particle size of WC powders was investigated.The result shows that the combined carbon content of WC powders would increase gradually as the carbon addition ratio of precursor increases.When the C/W molar ratio is 3.6,combined carbon content would reach the theoretical value of 6.12%,while free carbon content is 0.06%.When the C/W molar ratio is higher than 3.6,the free carbon content would increase rapidly.The structure heredity exists in the transformation process from W to WC,and the growth coefficient is between 1.4 and 1.6.The average particle size of WC would decrease as the reduction temperature elevates.Increasing carbonization temperature would accelerate the crystal boundary migration and the average particle size of WC powders would increase as carbonization temperature elevates.The optimal process of the carbon-hydrogen co-reduction direct carbonization is the C/W molar ratio of 3.6,the reduction temperature of 760⁸00 oC and the carbonization temperature of 1100¹200 oC.Under the optimal process,the fine and homogeneous spherical WC powder with the average particle size of 87.3 nm is obtained.5.Ultrafine-grained WC-Co cemented carbides were prepared by low pressure sintering using the prepared WC nano-powders as raw material.The effect of the sintering process parameters to the microstructural and mechanical properties of WC-Co cemented carbides was studied.It is found that as the sintering temperature increases and the soaking time extends,the density and the mean grain size of the sintered alloys would increase.While the hardness and bending strength of samples would improve as the density of sintered alloys increases.If the sintering temperature is excessively high or the soaking time is excessively long,grain size of the sintered alloys would grow up abnormally,leading to the declination of density and mechanical performance.The optimal sintering parameter of WC-6Co alloys is the sintering temperature of 1360 oC with 60 min duration.Under the optimal process,the mean grain size of the sintered sample was 305 nm,the hardness and bending strength are 94.6 HRA and 4450 MPa,respectively.