Effects Of Composite Surface Modification On Surface Integrity And High Cycle Fatigue Properties Of M50NiL Steel

With the rapid development of the modern industry,a higher requirement for the life of the aviation engine.Single surface modification means is unable to meet higher performance requirements,there have been two or more process composite surface modification methods,and it has developed rapidly.M50NiL steel is a new type of carbon steel developed for a new generation of aerospace engine,has been widely used in bearings and gears.At present,the research on M50NiL steel mainly focuses on carburizing treatment,and the performance research mainly focuses on contact fatigue performance and friction and wear performance,while the research on composite surface modification and high cycle fatigue performance of M50NiL steel is less.For this reason,this article takes M50NiL steel as the research object,firstly carburizes M50NiL steel,and then carries out surface deformation strengthening treatment（ultrasonic surface rolling process,abrasive water jet shot peening and abrasive water jet shot peening+ultrasonic surface rolling process）and plasma nitriding treatment respectively.The influence of different treatments on the surface integrity of the sample was explored.The tension and compression fatigue limit and fatigue strengthening mechanism of the three state samples are studied,which provides a theoretical basis for the research and application of the composite surface modification treatment of M50NiL steel.The surface integrity and fatigue limit of carburized M50NiL steel were studied.When the carburizing temperature is 950℃and the carburizing time is 90 min,the surface hardness of the sample is 695 HV_0.1,the depth of the Carburized layer is 1200μm,the surface residual stress is-103 MPa,and the average surface roughness Ra and Rz are 0.13μm and 0.81μm,respectively.The fatigue limit of carburized M50NiL steel is 412 MPa,the fatigue crack initiation is on the surface of the sample,and the fatigue failure mechanism is“surface defect induced fatigue failure”.The surface failure is caused by high hardness of carburized layer,stress concentration caused by surface defects and low residual compressive stress on the surface.The fatigue properties of carburizing+ultrasonic rolling high-energy composite modified samples were studied.after high-energy composite surface treatment,the surface residual stress and hardness were increased,the surface residual stress was increased to-1320MPa,the surface hardness was increased to 750 HV_0.1,and the surface roughness was greatly reduced,Ra and Rz decreased by 46.1%and 40.7%,respectively.After high energy composite surface treatment,the fatigue limit of the sample is 676 MPa,and the fatigue limit is 64.1%higher than that of the carburized sample.The initiation location of the fatigue crack is transferred from the surface to the matrix area of the specimen,the source of the fatigue crack is the inhomogeneous structure in the matrix,and the fatigue failure mechanism is“fatigue failure induced by the inhomogeneous structure of the matrix”.The improvement of fatigue properties of the samples after high energy composite modification is the comprehensive effect of high surface residual compressive stress,surface work hardening and low surface roughness.The fatigue properties of carburized and nitrided specimens were studied.After duplex hardening treatment,the depth of nitrided layer is 90μm,the surface hardness is 1121 HV_0.1,and the surface residual compressive stress is-998 MPa.After duplex hardening treatment,the fatigue limit is 777 MPa,and the fatigue limit is 88.6%higher than that of the carburized sample.The initiation location of the fatigue crack is transferred from the surface to the matrix area of the specimen,the source of the fatigue crack is the inhomogeneous structure in the matrix,and the fatigue failure mechanism is“fatigue failure induced by the inhomogeneous structure of the matrix”.The improvement of fatigue limit by duplex hardening treatment is the comprehensive effect of high residual compressive stress and high surface hardness.The fatigue limit of the duplex hardening sample is about 15%higher than that of the high energy composite modified specimen.In addition,under the maximum stressσ_a=800 MPa,the tensile-compression fatigue life of the high-energy composite modified specimen is 30 times longer than that of the carburized specimen,and the tensile-compression fatigue life of the duplex hardening specimen is 104 times longer than that of the carburized specimen.The tensile-compression fatigue life of duplex hardening specimens is 2.4 times longer than that of the high-energy composite modified specimens.Compared with the high-energy composite modified sample,the duplex hardening sample has a higher fatigue crack threshold,a longer and more numerous of secondary fatigue cracks,a more tortuous fatigue crack propagation path and a higher crack growth resistance.In addition,the fatigue crack growth rate of duplex hardening samples is lower.The residual stress relaxation test shows that the duplex hardening sample has higher residual stress stability than the high-energy composite modified sample,and the high residual stress stability is the reason for the higher fatigue properties of duplex hardening specimen.