Study On Characterization And Fatigue Behavior Of Surface Metamorphic Layer Of 18CrNiMo7-6 Alloy Steel

18CrNiMo7-6 alloy steel,with high strength and wear resistance after carburizing and quenching,is widely used in the manufacturing of machine parts.After carburizing heat treatment,a certain thickness of modified layer named surface modified layer will be produce near the surface of parts.The microstructure and fatigue performance of the material,which have a great effect on the sensitivity of the surface stress concentration and the service life of the component,will be greatly changed after the introduction of the surface modified layer.Therefore,it is of great significance to study the microstructure and fatigue performance of the surface modified layer.In this thesis,the characterization and fatigue behavior of the surface modified layer of 18 Cr Ni Mo7-6 alloy steel after carburizing and quenching were studied.The main research contents are as follows:(1)The residual stress,corrosion resistance,microstructure of the surface modification layer was characterized.The residual stress in different modification layer depth was measured,the impedance spectrum and polarization curve related to corrosion resistance in different metamorphic layer depth was tested,the microstructure of the metamorphic layer with different depth was observed and the content of different microstructure was calculated.Then the effect of residual stress and microstructure on corrosion resistance of carburised 18 Cr Ni Mo7-6 steel was discussed.(2)The fatigue properties of each surface modification layer were tested,and the S-N curves were obtained.As the depth increases,the slope of the S-N curve becomes slower and slower.The fatigue fracture of the modification layer with different depth was analyzed,the fracture mode changed from brittle to brittle-ductile,and the dimple size becomes larger and larger with the increase of the modification layer depth.The fracture under different stress at the same metamorphic layer depth was also analyzed,and it was found that the smaller the stress was,the larger the fatigue propagation area was.(3)The fatigue micro-crack propagation of 18 Cr Ni Mo7-6 carburized alloy steel matrix was studied.The quasi-in situ observation of the micro-cracks during the fatigue process obtained the micro-crack propagation path and micro-crack growth rate;the fatigue micro-crack propagation characteristics were analyzed,and it was concluded that the micro-crack growth would stop expanding or rotate to meet certain grain boundaries angle.