Study On Electron Beam Surface Modification Of 40CrMn Structural Steel

40CrMn structural steel has high strength and good hardenabitity,which widely used in automotive,aircraft and precision manufacturing fields.Scanning electron beam surface modification technology,as a new surface treatment technology,can improve the mechanical properties of the material surface.Therefore it can extend the service life of parts.Based on the heat conduction theory,a three-dimensional transient heat transfer equation was established,and the distribution law of temperature field during scanning electron beam surface modification process of 40 Cr Mn steel was calculated by the COMSOL Multiphysics simulation software.The effect of process parameters on the microstructure,surface morphology and mechanical properties was studied.The effect of overlap rates on the microstructure,surface morphology and mechanical properties was studied.The results showed that: at the beginning of electron beam scanning,the surface temperature of the sample rose to above the austenitizing temperature.When the electron beam scanning was in the stable stage,the surface temperature of the sample was higer than the melting point of 40 Cr Mn steel,and it was in the melting state.When the scanning electron beam was bandaging stage,the surface temperature of the sample exceeded 3000 K.After single-pass scanning electron beam surface modification of 40 Cr Mn steel,the crosssectional surface morphology of the sample contained three districts as follow: melted layer,heated affected zone and substrate.Martensite phase transformation occurred in the modified layer which acquired the composite microstructure of acicular and lath martensite.The crosssection of microhardness increased firstly and decreased with the increase of depth.The surface microhardness increased nonlinearly with increasing beam current.At a beam current of 5 m A,the surface microhardness increased from 290 HV to 692.8 HV,which was in 2.38 times of the substrate hardness.The surface roughness of the sample decreased firstly and increased with the increase of beam current.When the beam current was 4m A,the surface roughness of the sample reached the minimum value.The surface microhardness decreased nonlinearly with the increase of electron gun moving speed.At a moving speed of 120 mm/min,the surface microhardness reached a maximum value of 666.2 HV.The surface roughness decreased with an increase in the moving speed.At a moving speed of300 mm/min,the surface roughness reduced from 1.754 ?m to 0.801 ?m.The wear loss of the samples after scanning electron beam treatment decreased significantly compared with the initial surface.The optimal process parameters of the single-pass scanning electron beam surface treatment was beam current of 4m A and moving speed of 300 mm/min.After surface treatment of 40 Cr Mn steel by multi-pass scanning electron beam at different overlap rates,the microstructure of the melted zone was mainly composed of tempered martensitic and tempered sorbite.The heat affected zone was mainly composed of coarse grain heated affected zone,fine grain heated affected zone and critical grain heated affected zone.When the overlap rate increased,the surface roughness decreased firstly and increased.When the overlap rate was 0%,the surface microhardness of the sample increased from 436.8HV to664.2HV.Meanwhile,with the increase of the overlap rate,the surface microhardness of the area near the overlap area decreased.Moreover,the wear rate of the sample after electron beam treatment was significantly lower than that of the untreated sample.