The Effect Of Carburizing And Shot Peening On The Microstructure And Surface Properties Of 17CrNiMo6 Steel

In this paper,the surface layer microstructure and properties of 17CrNiMo6 steel are studied after the carburizing,shot peening(SP)and their combination.17CrNiMo6 steel is mainly used to manufacture gears under heavy load,which are widely used in industry.In general,these industrial components have higher requirements on the surface quality in production.Hence,the carburizing is used to improve the surface properties of 17CrNiMo6 steel.The carburizing can ensure that the surface layer of 17CrNiMo6 steel has higher hardness,and the parent metal keeps well toughness.This paper adopts Jmatpro and Thermo-calc software to simulate material thermodynamics as well as properties calculation,analysis the surface layer microstructure and properties.Comparing the simulation results with the experimental results,it can provide a reference for the optimization of carburizing process.The results show that the thickness of the carburized layer is about 3.8mm.The carbon concentration of the carburized layer reduces with the increase of layer depth.The carbon concentration gradient of the surface area with 0~2.2mm change faster,which change slower in the layer depth with 2.2mm~3.8mm,and gradually transition to the parent metal components.The max hardness of the carburized layer is 743.1HV,appeared in 78μm from the surface.The hardness decreases with the increase of the layer depth,and the thickness of effective hardened layer is about 3mm~3.2mm.The surface layer microstructure of17CrNiMo6 steel is martensite,residual austenite and carbide.The content of residual austenite and cementite decreases along with the increase of the layer depth.The residual stress of carburized layer is compressive residual stress(CRS).The surface CRS is-21 MPa.And the max CRS is-217 MPa,appeared in 204μm from the surface.The SP treatment is carried out on the carburizing specimen surface with the intensity from 0.2mmA to 0.6mmA,and not carburizing specimen surface with 0.3mmA~0.6mmA,respectively.The analysis results indicate that SP treatment can further improve the specimen surface microstructure and properties.The crater can be observed obviously on the not carburizing specimen surface after SP treatment.The greater SP intensity induces the higher surface roughness.The surface roughness of specimen with SP intensity of 0.6mmA is Ra/4.35μm,which increase 41.2% than 0.3mmA.The surface hardness is improved with theincrease of SP intensity.The hardness of specimen with SP intensity of 0.6mmA is 624.22 HV,which is improved 55.2% than the parent metal.The residual austenite of carburizing specimen surface layer disappeared after SP treatment,owing to phase transformation of carburized layer.And the surface grain refinement is more obvious with the increase of SP intensity.SP treatment can further improve the hardness of carburizing specimen.Moreover the hardness is much higher with the increase of SP intensity.The hardness of specimen with SP intensity of 0.6mmA is977.90 HV,which is improved 12.0% than 0.3mmA and 110.3% than the parent metal.But the surface roughness of specimen with SP intensity of 0.6mmA is Ra/1.51μm,which increase 91.1% than 0.3mmA.This paper studies the residual stress of carburizing specimen surface layer after SP,and the residual stress is CRS in all specimens surface.Because of the ball impact the surface of the parts,the elastic and plastic deformation is produced in near surface layer.The elastic deformation zone inhibited the plastic deformation zone to expand.Hence,the specimen can obtain a CRS field in near surface layer.The max CRS value is not in the top surface,but in the subsurface.The position of the max CRS value is among 50μm~80μm.The CRS value increase with the increase of SP intensity.The max CRS value of 0.5mmA is-1221 MPa,which is greater than 0.6mmA(-1198MPa).Therefore,SP intensity of 0.5mmA has the best strengthening effect for the carburizing specimen of 17CrNiMo6 steel.