| In recent years,the operating mileage and speed of high-speed railways have increased continuously,which induces higher requirements for its key components.As the key of high-speed railways,the axle box bearings subject to the load from the locomotive body and transmit the load to the wheelset,withstanding shocks simultaneously.Therefore,higher quality is required for the reliability and safety of the axle box bearings.The failure of the axle box bearing mainly is the peeling of the outer ring,which occurs in the load-bearing area of the bearing outer rings.The maintenance mileage of the axle box bearing is 1.2 million kilometers.The long maintenance mileage makes the crack initiation is difficult to find.Therefore,it is necessary to study the failure mechanism of the load-carrying area in the bearing outer ring.In this thesis,the bearing outer rings of 0 km and 1.2 million km are investigated.First,static materials test systems is used to conduct tensile tests on tensile samples of bearing outer rings in different circumferential directions to distinguish loading area.Then,use the HVS-1000 microhardness tester to characterize the hardness of outer ring;X-ray diffraction(XRD)and Leica metallographic microscope are used for microstructural characterization;then electron backscattered diffraction(EBSD)tests are conducted to analyze grain boundary angle,grain size,dislocation density,etc.;finally,the atomic force microscope(AFM)is applied to revealing indentation dents of nano-indentation tested surface of bearing.The macroscopic mechanical properties are associated with the microstructural characteristics.The results could be summarized as follows:(1)The average ultimate strength of the bearing outer rings at 0 km is 2068 MPa.After the bearing outer ring services 1.2 Mkm,the ultimate strength of the bearing outer ring in the load-bearing area decreased to 1696.02 MPa,about 18.4% lower compared to the bearing at 0 km.Meanwhile,the maximum ultimate strength of the bearing in the non-loading bearing area increased by about 3.23% to 2135.13 MPa.(2)After the bearing outer ring services 1.2 Mkm,the average Vickers hardness of the bearing area increased from 712.5 HV to 725.4 HV,while the maximum Vickers hardness of the non-bearing area was 726.9 HV,and the minimum Vickers hardness is679.9 HV.(3)Through the observation of the metallographic structure,it is found that after the bearing outer ring services 1.2 Mkm,the number and length of cracks in the loadbearing area of the bearing outer ring increase,and the number and length of cracks in non-bearing parts increase slightly,there was a decrease in the number and diameter of carbides after 1.2 million kilometers’ operation;and in some local areas of the nonload-bearing area of the outer ring,there was an increase in the number and diameter of cementite after the bearing outer ring services 1.2 Mkm.(4)After the bearing outer ring services 1.2 Mkm,the bearing area of the bearing outer rings is subjected to cyclic contact stress,which increases its dislocation density,changes its crystal orientation,and reduces its strength. |
