| The microstructure and property of 32Cr3MoVE nitrided bearing steel produced by vacuum induction was investigated under different heat treatment process in this paper.The hardness distribution and microstructure characteristic of nitrided layer of32Cr3MoVE nitrided bearing steel were studied and analyzed.The room temperature rolling contact fatigue test and room temperature rotating bending fatigue test were carried out.The research results are as follows:The heat treatment process for 32Cr3MoVE bearing steel is 930°C×1h?OQ?+550°C×2 h?AC?.The microstructure after quenching and tempering treatment is tempered sorbite,the tensile strength is 1 525 MPa,and the yield strength is 1 283 MPa as well as impact absorption energy at room temperature is 76 J,and the strengthening phase in the steel is mainly the long strips of M3C and M2C precipitated between the tempered sorbite strips.The non-metallic inclusions in steel are mainly MgO-Al2O3,MgO-Al2O3-CaO composite inclusions,and Al2O3-MnS composite inclusions.The average diameters were all less than 5?m,and the area of large-sized non-metallic inclusions were less than 15?m2.The amount of MgO-Al2O3 inclusions accounts for about 52.1%of the total number of non-metallic inclusions in the measured area.The surface hardness of the nitride layer of 32Cr3MoVE bearing steel reaches 1000HV or more and the effective nitride layer depth is 400?m.The nitride layer is composed of compound layer and diffusion layer,and the compound layer is about 16?m thick and composed by?'phase?Fe4N?and CrN.The residual compressive stress of the nitrided layer reached a peak at 300?m,which was 610 MPa.The venous tissue appears in the diffusion layer.The room temperature rotating bending fatigue limit of 32Cr3MoVE bearing steel is793 MPa,and the stress distribution is within the three-level stress gradient.The failure mode is mainly caused by surface cracking and subsurface non-metallic inclusion cracking.The fatigue limit of the 32Cr3MoVE bearing steel after nitrided is 947 MPa,which is 19.4%higher than the fatigue limit of the unnitrided sample.The stress distribution is in the four-level stress gradient,and the cracking methods mainly include the cracking of the surface and the cracking of the non-metallic inclusions on the subsurface.At the same time,the surface roughness and nitrided compound layer have great influence on the fatigue life of unnitrided samples and nitrided samples.32Cr3MoVE nitrided bearing steel room temperature rated rolling contact fatigue life L10=0.357×108 cycles,median fatigue life L50=2.083×108 cycles,characteristic life Na=3.040×108 cycles,Weibull slope parameter?=1.050.Fatigue failure is mainly caused by surface pitting or scratch initiation and material subsurface initiation crack spalling.In the surface cracking mode,the surface forms an initial crack and the edge extends to the inside of the material by nearly 30°to the surface.When the crack propagates in the extended region,the crack propagation direction is substantially at an angle of 45°to the surface of the sample;in the subsurface cracking mode,the crack is initiated on the subsurface near the surface,and the expansion direction is parallel to the surface of the sample.Cracks were found to propagate along the venous tissue in both modes of initiation. |
PDF Full Text Request