Modified Microstructure,Property And Thermal Stability Of WC-10%Co Cemented Carbide Irradiated By High Current Pulsed Electron Beam

Surface wear is one of the main factors in the failure of cemented carbide materials as machining tools during stressful service,which is a critical problem in the design of better cemented carbide tools.High current pulsed electron beam(HCPEB)technique has shown gratifying development in the field of surface modification of materials in recent years.The irradiation of a short pulsed intensive electron beam flux induces a heat-stress coupling effect,leading to the in situ microstructure changes in the surface region of the target material.In the present work,to enhance the surface tribology and anti-wear properties of cemented carbide machining tools,HCPEB was utilized in the tailoring of surface microstructure of WC-Co cemented carbide.A battery of experiments for WC-10%Co cemented carbide were carried out on HOPE-I type HCPEB apparatus with varied treating parameters.Microhardness and tribological tests were conducted to assess the property of WC-10%Co cemented carbide before and after surface modification.Various material characterization methods such as X-ray diffractometer,3D confocal laser scanning microscope,scanning electron microscope and field emission transmission electron microscope were utilized to analys the evolution of the morphology and microstructure.The influence of the WC grain size on the process and outcome of the HCPEB modification was examined by comparing the microstructure and properties between three types of cemented carbide specimens with different intial WC grain sizes.In the mechanim analysis,methods such as temperature field simulation,high temperature atomic diffusion calculation and calculation on nucleation thermodynamics were incorporated,where the precipitation characteristics of nano graphite and phase competition mechanism occurred during non-equilibrium solidification induced by HCPEB irradiation were revealed.HCPEB irradiation induced rapid surface melting,resulting in the formation of typical features such as craters,microcracks and "hills and valleys".Further characterization revealed a compact modified surface layer(MSL)?1 ?m,where the micro-WC grains transformed into numerous nano-scaled grains(mainly metastable cubic phase WC1-x),along with scattered nano graphite precipitations.With more irradiations,the thickness of the MSL gradually increased,meanwhile,the grain size was further refined and the graphite particles redistributed and concentrated at the bottom of the MSL.The HCPEB-modified surface of WC-10%Co cemented carbide was hardened.After 35 pulses of irradiation,micro hardness of YG10,YG10X,YG10S specimens were increased from their initial state 1735 HV,2167 HV,2240 HV up to 3128 HV,2903 HV,2883 HV,maximum increment was achieved on YG10,?80%.Tribological property was also considerably improved,take YG10 for instance,the friction coefficient and the wear rate were reduce from?0.70 and?3.83×l0-4 mm3/min down to?0.23 and?1.63×10-4 mm3/min,respectively.The nano graphite particles in the MSL exhibited lubricating effect comparing the morphology and element analysis of the wear scars.Under the irradiation of HCPEB,the surface temperature field of cemented carbide changed rapidly,as shown in the numerical simulation,heating and cooling rate were as high as?1010 K/s and?108 K/s,and the temperature could reach up to?3200 K.The diffusivities of W,C,Co atoms vary significantly with such thermal changes,diffusion coefficient grealy decreases with the temperature.According to the elemental distribution characteristics,the modified surface could be divided into the Co-poor region and the Co-rich region considering the precipitation of graphite.The formation of the graphite in the Co-poor region was mainly realized via decomposition of WC during melting,they are smaller;whereas in the Co-rich region,C atoms diffused into the region and precipitated as graphite,since the melting point of Co is much lower,the region stayed liquidus longer,the graphite grew bigger in size.The solidification of the W-C melt in the surface region of cemented carbide is a superfast non-equilibrium process,during which a competiton between stable phase WC and the metastable phase WC1-x took place.Nucleation thermodynamic analysis showed that,the redistribution at the solidification front was limited by the diffusivity of atoms in the undercooled melt,therefore at proper composition(C content below 39 at.%),nucleation of the metastable phase procedes the stable phase.Considering the high temperature from the application of the cemented carbide materials in severe abrasive conditions and the metastable state of the modified microstructure achived by HCPEB irradiation,the thermal stability was studied along with the relationship between surface property and microstructures by the method annealing at varied annealing temperature(AT).As the results showed,the modified microstructure remained stable when the AT was at 500? and below.When the AT was at 600-700?,solid state phase transformation WC1-x?HEX*+HCP*?WC+W2C occurred in the MSL.Further elevating the AT to 900?and above,considerable amount of ternary phases,such as Co6W6C,Co2W4C were formed,along with some Co3C and polymorphous carbon phases.According to the microhardness test,annealing at proper temperature could lead to further hardening.