Microstructure Evolution And Strengthening Mechanism Of 42CrMo And 20CrMnTi Steel After Composite Strengthening By LSP

In order to improve the wear resistance and corrosion resistance of materials,laser shock peening of square spot was adopted to strengthen the surface of 20 CrMnTi steel and 42 CrMo steel.The layered style morphology,microstructure in the steel after LSP were observed by 3D microscope,SEM and TEM;the surface roughness,micro hardness and residual stress after LSP were measured;The corrosion resistance of 20 CrMnTi and the wear resistance of 42 CrMo were tested after LSP,and achieved the following resultsThe 20 CrMnTi steel was strengthened by pulsed energy of 22 J,26J and 30 J.The results show that so many changes are appeared after laser shock peening: the plastic deformation appeared on the material surface;surface roughness gets lower than original sample;hardness increased from 262.52 HV to 301.84HV;the distance of surface hardened layer is about 85 ?m;the residual stress value increased from-81.5MPa to over-520MPa;Laser shock peening can effectively improve the wear resistance of materials.Material corrosion resistance has great influence on the laser pulse energy of laser shock peening: low laser pulse energy can increase the density of the strengthened surface layer and passive film of materials,so that Cl-is difficult to penetrate the surface layer and corrode the substrate.The transverse residual compressive stress makes the passive film more stable,and improves the corrosion resistance of materials.But the too large laser power density will cause the generation of pit defects in the material surface and the ablation of Al foil.Thus the surface passive film on the material is destroyed and the Cl-are accumulated at the pit defects,and degrade the performance of corrosion resistance.The 42 CrMo steel was strengthened by pulsed energy of 32 J,34J and 36 J.The result show that: with the treatment of laser shock strengthening,the hardness of the material increases from 385.83 HV to 445.62 HV,the distance of hardening layer is about 75 ?m,and the residual stress value increases from-113.5MPa to-482.7Mpa.LSP can strengthen the wear resistance of materials,probably causes are: the reducing roughness of materials after laser shock peening;the generation of small twin sheet after LSP;the multiplication of a large number of dislocation and dislocation movement after LSP;the dispersion strengthening action with broken pearlite lamellae and dispersed carbides after LSP;the generation of residual compressive stress field after LSP.Laser shock processing results in the generation of dislocation cells,dislocation bands,dislocation walls,sub-grain and twin crystals,and the fracture of strip carbides.This is due to the high speed plastic deformation by laser shock peening.The dislocation motion,lead to the generation of forest dislocation and dislocation tangles,further lead to the dislocation bands,dislocation walls,sub grains and twins.When the dislocation movement encountered strip carbides,dislocation will be gathered here.The accumulation of much dislocation formed around long carbides caused the large distortion stress field,resulting in carbide deformation and fracture,and even lead to the short distance displacement of the carbide.The carbide particles can act as a dispersion strengthening agent on the matrix.