| With the rapid development of the aviation industry,the thrust-weight ratio of aero-engine has been increasing(grade 15-20),which puts forward higher requirements for the strength and service temperature of bearing materials.High Cr-Mo-V bearing steel(the third generation of CSS-42L)has the properties of high strength heat resistance and high surface hardness,which is very competitive in the future application of aviation bearing steel.However,at present,there are few systematic studies on this type of steel,especially in the influence of Cr,Ni and other alloy elements on the strength and toughness of this type of steel and mechanism.In this paper,based on Cr-Mo-V bearing steel with high Cr type,the influence of alloy composition Cr and Ni content on its microstructure and properties was studied.The microstructure was characterized by means of SEM,TEM and XRD.The influence of Cr and Ni content(13.5%Cr,10%Cr/2%Ni,8%Cr,4%Ni)on mechanical properties was studied by hardness,tensile and impact tests.At the same time,the effect of holding time on the properties of test steel after Cr and Ni content optimization is studied.The rotating bending fatigue performance of 4%Ni test steel was studied,and the crack initiation and propagation mechanism was revealed.The results show that with the decrease of chromium content,the tensile strength and yield strength of the test steel increase by 113 MPa and 344 MPa respectively.After solution treatment at different temperatures,the yield strength of 13.5%Cr test steel changes obviously,while that of 10%Cr test steel and 8%Cr test steel changes little.The microstructure observation shows that with the decrease of chromium content,the carbide content in the microstructure increases and the residual austenite content decreases.The residual austenite content of 10%Cr steel and 8%Cr steel is only 1.34% and 0.3%.The decrease of residual austenite and the increase of carbide increase the strength of the steel.The results show that with the increase of Ni content,the yield strength of the test steel decreases obviously.The yield strength of the steel containing 4%Ni decreases significantly from 1670 MPa at 1040 ℃ to 1459 MPa at 1080 ℃ through solution treatment at different temperatures.It is found that the content of precipitated phase in 4%Ni steel is higher than that in 2%Ni steel(same as 10 Cr steel).The residual austenite content increases with the increase of Ni content,which leads to the decrease of yield strength.The toughness of steel increases with the increase of Ni content.At 500 ℃,the tensile strength of 13.5%Cr steel and 4%Ni steel is slightly increased,and the impact absorbed energy is more sensitive with the increase of aging time.The impact absorbed energy of 13.5%Cr steel decreases by 50.4% from 2 h to 48 h,and that of 4%Ni steel decreases by 33.8% from 2 h to 48 h.With the extension of aging time,nanoscale precipitated phase can be obviously observed.After holding for 48 h,precipitated phase clusters in the test steel are obviously formed,and the size has grown to about 26.36 nm.The clusters can effectively nail the dislocation,thus increasing the strength of the steel.With the extension of aging time,a large number of precipitated phases at grain boundaries and within grains precipitated,which promoted cleavage fracture and resulted in the decrease of impact absorption energy with the extension of aging time.The ultimate fatigue strength of 4%Ni steel at room temperature is 1020 MPa.Through the observation and analysis of the fracture,the results show that the material is sensitive to surface defects,and surface cracking accounts for 60 %.Especially under the stress of1120～1200 MPa,surface cracking is the main method.The size and location of inclusions have great influence on the fatigue life,so the number and size of inclusions should be reduced as much as possible. |
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