The Study On Microstructure Evolution And Fatigue Behavior Of High Nitrogen Bearing Steel

40Cr15Mo2VN as a new type of high nitrogen bearings steel,its organizational evolution at high temperature insulation and high temperature tempering test,the rolling contact fatigue test and the bending bending fatigue test at room temperature and 200℃were studied.The main conclusions are as follows.The carbides in the pre-treatment of high nitrogen bearing steel is mainly along the grain boundary distribution,and the size of carbide is 0.4-0.8μm.When the temperature is less than 750℃,the number of carbide field area decreases gradually and the percentage of field area gradually increased,and the carbide coarsening characteristic value r*=0.8μm.The size above the characteristic value further growth with the holding time.When the temperature reaches 800℃,M₂₃C₆ and Cr₂N carbide precipites at the same time;900℃insulation,the steel in the field of carbide field area and the number of field area decreases,the mass fraction decreases,the carbide forming elements back to the matrix.The long rod carbide of the high nitrogen bearing steel self-sphere spheroidal growth in less than 750℃during the insulation process.When the maximum number of carbide roundness increases from 0.65 to 0.80,the effect of carbide spheroidization is obvious at700℃for 10h.The organization of high nitrogen bearing steel after high temperature tempering treatment is the tempering martensite,and the carbide is mainly along the grain boundary distribution.The nano-carbidebetween precipites of the strip,and slab width concentrated in the 180-260nm.The Carbides types are mainly M₂₃C₆ and VC,and inclusions types mainly incliudes Al₂O₃,Al₂O₃-SiO₂-CaO（silicate）,MnS,and MnS-Al₂O₃ etc.The inclusion sizes concentrates below 6μm,inclusions appear irregular shape.In contact fatigue test,the rated contact fatigue life and the mid-value life are as follows:L₁₀=0.197×10⁸ times and L₅₀=1.274×10⁸ times,and the slope parameterβ=1.006.The failure modes of contact fatigue are the surface of the peeling off and internal non-metallic inclusions peeling off.Fatigue crack propagation was mainly carried out along the direction of the carbide carbides.It is easier to provide the main crack propagation channel,when the carbide size is greater than 2μm,spacing within 5μm.The presence of dense inclusions changed,in the direction of the maximum stress in the direction of 45°expansion,so that in the direction of non-metallic inclusions in the lateral expansion.The flexural fatigue strength of high nitrogen bearing steel at 200℃is 883MPa,which was 17%lower than that at room temperature,which indicated that the temperature at 200℃is more likely to cause crack initiation.Rotational bending fatigue failure type were surface cracking and internal nonmetallic inclusions.The nominal stress amplitude and the predictes fatigue limitσ’/σ_w ratio of the rotary bending fatigue fracture specimen are all greater than one.σ’/σ_w ratio at the same level,200℃under the conditions of fatigue life than room temperature by an order of magnitude.When the inclusions size is controlled within the critical value associated with the service temperature,the high nitrogen bearing steel can obtains better fatigue limit strength.