Research On Surface Strengthening Treatment Of 40Cr Steel With Scanning Electron Beam

40Cr steel is currently the most widely used alloy quenched and tempered steel in my country.Because of its excellent hardenability and machinability,it is often used in shaft parts,connecting rod bolts and important gears.However,in the practical application,the workpiece is kept in a bad working environment for a long time,and the surface is worn and corroded,causing the workpiece to fail.The traditional heat treatment process is difficult to deal with these failure forms.Scanning electron beam is a new surface modification technology of energy beam,which has the advantages of high energy efficiency,low deformation and high repeatability.In order to improve the mechanical properties of 40 Cr steel surface and prolong its service life,the scanning electron beam surface intensification treatment of 40 Cr steel is carried out.In this paper,40 Cr steel is used as the research object,a finite element model for surface strengthening of 40 Cr steel by scanning electron beam is established based on threedimensional transient heat transfer partial differential equation and Gaussian heat source model.The distribution of temperature field at different times in the scanning process and cooling process is explored to determine the reasonable technological parameters of the surface strengthening treatment test of scanning electron beam.The effects of electron beam processing parameters(electron beam current and electron gun velocity)and overlap rate on the microstructure,surface morphology and mechanical properties of 40 Cr steel are studied.The results show that the surface temperature of the sample is higher than the melting point of 40 Cr steel in the process of scanning electron beam treatment.The surface temperature of the sample will rise sharply because the heat cannot be diffused in the scanning electron beam.After surface strengthening by scanning electron beam,the crosssectional morphology of 40 Cr steel is composed of three regions: molten layer,heat-affected zone and matrix.The temperature of the modified layer(the melting layer and heat-affected zone)is higher than the transformation temperature,martensite transformation occurs,and the microstructure is mainly composed of martensite.The microstructure of martensite in the heat affected zone is smaller than that in the molten layer.With the increase of the beam current,the martensite structure of the molten layer becomes coarser,while with the increase of the electron gun moving speed,the martensite structure of the molten layer becomes smaller.The microhardness of the cross section of the sample increases first and then decreases with the increase of depth,and the maximum hardness appears in the heat-affected zone.The surface microhardness of 40 Cr steel increases non-linearly with the increase of beam current.When the beam current is 6m A,the surface microhardness reaches 756.5HV.As the scanning speed increases,the surface microhardness decreases nonlinearly.As the beam current increases,the surface roughness of the sample decreases first and then increases;as the moving speed of the electron gun increases,it first decreases and then increases.When the beam current is 4m A and the moving speed of the electron gun is300mm/min,the surface roughness decreases from 2.306?m to 1.354?m.After the scanning electron beam treatment,the surface wear resistance of the sample is significantly improved.As the beam current increases,the wear resistance of the surface of 40 Cr steel first decreases and then increases.As the moving speed of the electron gun increases,it first decreases and then increases.When the beam is 4m A and the electron gun moving speed is 240mm/min,the wear resistance is the best.After the surface strengthening of 40 Cr steel with multichannel scanning electron beam,the overlapping region with repeated heating will appear,and the second scanning heating will have a tempering effect on the first part of the regional microstructure,tempering martensite,tempering sorbite,tempering trositic and other microstructures.The surface roughness of the sample decreases first and then increases with the increase of the overlap rate.The average microhardness is 627.4HV when the overlap rate is 0%.When the lap rate is 25%,50% and 75%,the microhardness varies in the range of 431?658HV,and the microhardness of quenching zone is higher than that of overlap zone and intersection zone.The wear decreases first and then increases with the increase of overlap rate.When the overlap rate is 25%,the wear resistance is the best.Scanning electron beam surface hardening treatment can significantly increase the surface microhardness,reduce the surface roughness and enhance the surface wear resistance of 40 Cr steel.Scanning electron beam surface strengthening technology is used to extend the working life and extend the working range of the workpiece in practical production.