Effect Of Combined Chemical Heat Treatment On The Structure And Properties Of G13Cr4Mo4Ni4V Steel For Aviation Bearings

Heat treatment plays a key role in the service life and the properties of bearing steels.G13Cr4Mo4Ni4V(abbreviated as G13)steel belongs to the second generation bearing steel and is developed from 8Cr4Mo4V steel by reducing carbon and increasing the nickel element.G13 is a kind of carburizing steel and requires high strength,high toughness and high wear resistance during operation.In this paper,the G13 steel was carburized and nitrided to achieve the purpose of"hard surface and tough core".The carburized+quenched+tempered G13 steel was taken as the original state,and the nitriding treatment under different process conditions were carried out.Then the changes of the composite layer,carburized layer and core structure after nitriding were studied.According to the change of the micro hardness gradient of the sample combined with microstructure observation the thickness of the composite layer and carburized layer were calculated.In addition,the effect of nitriding time on the thickness of the composite layer and carburized layer were explored.Furthermore,the effects of nitriding treatment on the wear resistance,tensile properties,impact toughness and rotational bending fatigue properties of G13 steel were studied.Finally,the effect of nitriding time on the microstructure and mechanical properties of the steel were determined.Results show that,after nitriding,the composite layer and the carburized layer are composed of tempered martensite and carbides,and the core is composed of lath martensite and a small amount of carbides.Compared to the samples after 50-h and 90-h nitriding,there exist holes in the composite layer and a lot of carbides in the core after 140-h nitriding.After carburizing and nitriding,there is a decarburized layer on the surface of the sample,which causes the existence of the low micro harness region.The highest hardness after carburizing is 785 HV_0.5,while after nitriding,the micro hardness of the sample is greatly improved.After nitriding for 50 h,90 h and 140 h,the maximum micro hardness is 960 HV_0.5,906HV_0.5 and 948 HV_0.5,respectively.In addition,the wear resistance of the sample is improved after the nitriding treatment.The wear rate of the sample after the carburizing+quenching+tempering treatment was 1.0%,and it was 0.97%after the nitriding treatment.The tensile properties and impact toughness of the sample after nitriding are reduced.The tensile strength of the sample after carburizing+quenching+tempering is 2328 MPa,while the tensile strength after nitriding for 50 h is 1979 MPa,and as the nitriding time increase,the tensile strength decreases.The impact property of the sample after carburizing+quenching+tempering treatment was 16.9 J and it was 5.5 J after nitriding treatment.After the nitriding treatment,the rotating-bending fatigue strength of the sample is greatly improved.The rotating-bending fatigue strength of the sample after carburizing+quenching+tempering is 840 MPa,and it is higher than 1000 MPa after nitriding,but the fatigue strength decreases with the increase of nitriding time.