Effect Of Ion Implantation Lanthanum/Yttrium On Contact Fatigue Performance Of 20Cr2Ni4A Carburized Layer

The heavy duty gears of armored vehicles made of 20Cr2Ni4 A alloy steel have the problems of low service life and poor reliability due to their special service environment.In this article,the optimized process of rare earth vacuum carburizing and ion implantation strengthening were explored,and the contact fatigue properties of 20Cr2Ni4 A carburizing steel in different strengthening conditions were studied by combining the microstructure and mechanical properties.The microstructure and mechanical properties of the strengthened layer were tested and analyzed by various characterization and testing techniques,and the influence mechanism of rare earth vacuum carburizing and ion implantation composite strengthening treatment on contact fatigue property were discussed.In the process of rare earth carburizing at 920? and carbon potential of 1.2%,the carbon diffusion coefficients were increased by 1.12 times and 1.17 times respectively and the effective hardening depths of carburized layer were increased by 0.08 mm and 0.11 mm respectively through lanthanum and yttrium ion implantation.The microstructure of carburized layer was improved significantly by rare earth ion implantation pretreatment.The diameter of fine dispersed carbide in the carburized layer was less than 0.32 mm.The surface hardness of carburized layer was increased by 3.8% and 6.0% respectively after ion implantation.In the process of rare earth carburizing,the high density dislocation defect and Cottrell atmosphere formed on the steel surface become the fast diffusion channel of carbon atom,which played an important role in improving the carbon diffusion coefficient and the structure of carburized layer.Through the composite strengthening treatment of transition metal by ion implantation,Tiimplantation and Ti+ Crimplantation form a composite strengthening layer in the rare earth carburized layer,in which there are new phases of Tiand Crcarbonitride,including TiC,TiN,TiNC,CrC and CrN.These strengthening phases have the characteristics of high hardness and high strength,which can significantly improve the contact fatigue life of20Cr2Ni4 A carburizing steel.Under the same pressing load,the surface hardness of the composite strengthening layer was improved and the resistance to plastic deformation was enhanced.After ion implantation of Ti,the nano hardness and elastic modulus were increased by 7.1% and 27.6%,respectively.After ion implantation of Ti+Cr,the nano hardness and elastic modulus were increased by 15.1% and 28.7% respectively.The mechanism of ion implantation strengthening includes the formation of new phases with high-strength on the surface and the formation of radiation damage and lattice distortion in the sub surface.After rare earth carburizing and composite strengthening treatment,the life of carburized layer was significantly increased,and the life dispersion was evidently reduced.The final state of fatigue crack initiation,propagation and spalling has not changed.The P-N curves of rare earth carburized layer and composite strengthened layer were established respectively.The failure probability corresponding to different contact fatigue life of strengthened layer under different external loading could be determined directly without additional test.