Preparation Of Nanocrystalline Gradient Cemented Carbides And Its Microstructure And Properties

With the rapid development of modern industry,the impact of hardmetal drilling tools in service and the violent collision between the surface and the rock become more and more obvious.Highly efficient,durable and economical cemented carbide has become a tool for mining tools.And research workers are pursuing the goal.However,the conventional mining cemented carbide tools have the contradiction between hardness and toughness:the higher the hardness,the lower the toughness,which becomes the bottleneck when the traditional cemented carbide materials are used.However,the gradient cemented carbide and nano crystalline cemented carbide provide a new way to solve this contradiction.Since the nanocrystalline cemented carbide can have both high strength and high hardness at the same time,the surface-poor Co-gradient cemented carbide outer layer has high hardness and good wear resistance inside.The inner layer of the alloy has good toughness in the nominal Co content,structures makes its strong hardness,wear resistance and toughness superior to ordinary cemented carbide and a substantial increase in service life.If you can combine nanocrystalline cemented carbide and the surface-poor Co-gradient cemented carbide,you can have both the advantages.In order to suppress the growth of WC grains,the preparation of nanocrystalline cemented carbide has high requirements for the mixed powder and sintering process.However,the traditional surface-poor Co-gradient cemented carbide is prepared by carburizing process and the vacuum degree of equipment is very high,there is a certain security risk.Therefore,this paper proposes a method of controlling the change of carbon content inside and outside by adjusting the composition of the initial raw materials,thereby obtaining the gradient distribution of the Co phase and maintaining the WC grains in the range of the nanocrystals to prepare the surface-poor Co nanocrystalline gradient cemented carbide.In this paper,a two-step sintering method is used,that is,after the pre-sintering to obtain a dense nano crystalline alloy,gradient sintering is performed to form a gradient layer with a certain thickness.Firstly,the effect of mixing process on the particle size of mixed powder and WC powder on the properties of mixed powder and sintered alloy was studied.Secondly,the effects of different pre-sintering methods and pre-sintering temperature on the grain size,microstructure and properties of the alloy were studied.The effects of the addition of V(C,N)or VN on the micro structure and properties of the alloy during pre-sintering and gradient sintering were discussed.Based on the results,the effects of gradient sintering on the layer thickness,grain size,alloy microstructure and propertie of the law.The results show:(1)Compared with the high-energy agitating ball mill,the rolling ball mill,when mixed powder distribution more uniform.With the increase of the ratio of material to powder,the resulting mixed powder is more and more uniform and less agglomerated.With the increase of WC powder particle size,the agglomeration of the powder decreases,but the grain size increases after sintering.(2)SPS pre-sintering was more abnormal than the low-pressure pre-sintering,with fewer grains and more uniform grain size distribution.With the increase of SPS temperature,the average grain size of WC increases,the density increases,the fracture toughness decreases gradually,and the hardness increases firstly and then decreases.(3)By controlling the composition of the raw materials,when the V compound is V(C,N)or VN,the surface-poor Co nanocrystalline gradient structure cemented carbide can be obtained.With the increase of the content of V compound,the thickness of the obtained gradient layer gradually increases,the grain size decreases gradually,and the hardness and fracture toughness increase and then decrease.Compared with VN,when adding V(C,N),the prepared nanocrystalline gradient cemented carbide has a thicker gradient layer,fmer WC grains and a higher hardness and density.(4)With the increase of the gradient sintering temperature or the holding time,the thickness of the alloy gradient layer increases,the average grain size of WC increases,the hardness and fracture toughness increase at first and then decrease,the density of the alloy decreases first and then increases.(5)When adding 4 wt.%V(C,N),ball material ratio is 15:1,SPS pre-sintering and temperautur 1150? as an sintering process,1400? holding 60 min as a gradient sintering process,the surface-poor Co nanocrystalline gradient structure cemented carbide prepared the best overall performance.After sintering,the grain size of the alloy is 84 nm,the hardness was HV30 1803 and the relative density was 93.45%.After graded sintered,gradient layer thickness is 33 ?m and grain size is 117 nm.The obtained hardness was HV30 2279 and the fracture toughness was 9.75 Mpa·m1/2.The relative density was 96.7%.