Study On Surface Modification Of 5CrNiMo Steel By Scanning Electron Beam

5CrNiMo hot work die steel is inexpensive and often used in the manufacture of large and medium-sized molds,with good hardenability and comprehensive performance at higher temperatures,but the conventional heat treatment of 5CrNiMo hot work die steel is seriously lack of thermal stability,low hardness and poor wear resistance,and high temperature wear is its main form of failure.Scanning electron beam surface modification technology is a new type of surface treatment technology developed in recent decades,which can effectively improve the comprehensive performance of the material surface.This paper takes 5CrNiMo hot work mold steel as the research object,based on the principle of heat transfer,and uses finite element simulation software to establish the mathematical model of scanning electron beam surface modification temperature field,and investigate the temperature field distribution law of the surface at different moments during the scanning electron beam surface modification process;set reasonable scanning electron beam process parameters according to the simulation results,and study the effect of scanning electron beam beam current and electron gun movement speed on the surface morphology,microstructure and mechanical properties of 5CrNiMo hot work mold steel.According to the simulation results,we set reasonable scanning electron beam process parameters to study the effects of scanning electron beam flow and electron gun movement speed on the surface morphology,microstructure and mechanical properties of 5CrNiMo hot work die steel.Finally,the effect of different tempering temperatures on the modified layer of the specimens after the scanning electron beam treatment is investigated according to the actual requirements of the mold production.The results show that the scanning electron beam surface modification treatment can make the surface temperature of 5CrNiMo hot work mold steel heated rapidly above the melting point and reach the molten state,and the surface temperature increases in the scanning electron beam closing stage compared with the stable stage due to the slower heat transfer.After the scanning electron beam treatment,the cross-sectional morphology of the specimen is divided into molten zone,heat-affected zone and substrate.The depth of the modified layer showed an increasing pattern with the increase of the beam current and the decrease of the electron gun movement.The melt zone is characterized by columnar and equiaxed crystals,with a large amount of ultra-fine cryptocrystalline martensite inside the grain boundaries,while the heat affected zone is mainly composed of a small amount of martensite,residual austenite,unfused ferrite,and a large amount of fine alloy carbide particles and white grain-mounted precipitates.As the melt zone tissue grains are refined,resulting in a higher microhardness in the melt zone than in the heat affected zone.With the increase of the scanning electron beam beam current,the surface microhardness of the specimen showed a nonlinear increase,and when the beam current was 4m A,the surface microhardness reached the maximum value of 876.5HV,which was 2.43 times higher than the surface microhardness of the substrate,and with the increase of the electron gun moving speed,the surface microhardness showed a nonlinear decrease,and when the electron gun moving speed was 300mm/min,the surface microhardness of the specimen reached 862.7HV.The surface roughness of the specimen showed a change law of decreasing and then increasing with the increase of beam current and electron gun moving speed,and when the beam current was 3m A and the electron gun moving speed was360mm/min,the surface of the specimen was smooth and flat without defects such as pits,and the surface roughness was reduced from the original 2.437?m to 0.886?m.It has the effect of surface polishing.After the scanning electron beam surface modification treatment,the surface wear resistance of 5CrNiMo hot work mold steel was significantly improved,and the surface wear resistance showed a change pattern of increasing and then decreasing with the increase of beam current and electron gun moving speed.It shows that the scanning electron beam surface modification treatment can effectively improve the comprehensive performance of the surface and extend the service life of the mold.According to the actual processing production requirements of the mold,it is found that the tempering temperature has little effect on the microstructure and mechanical properties of the molten zone of the specimen,while the heat-affected zone generates multi-phase organization,and the microhardness of the heat-affected zone gradually decreases with the increase of tempering temperature.