Study On Fatigue Behavior And Reliability Growth Of High Temperature Bearing Steel

Higher requirements on fatigue and other properties of high temperature bearing steels were proposed to meet the design and application goals of bearing steels with long life and high reliability during the rapid development of a new generation of aerospace equipment in China.Not only improving fatigue life but also improving reliability is an inevitable trend in the development and upgrading of aerospace high-performance bearing products.The fatigue crack initiation mechanism,fatigue limit and fatigue life dispersion of M50 bearing steel were previously studied,and the fatigue crack nucleation model and crack propagation model of high temperature bearing steel were established,which laid the technical foundation for the preparation of high performance high temperature bearing steel.The effects of specimen size,notch stress concentration and temperature on fatigue crack initiation and propagation behavior were investigated to obtain the characteristic parameters of the defects causing fatigue crack initiation.The contact fatigue damage and fatigue failure behavior of bearing steel under cyclic contact stress and lubrication conditions were investigated to achieve growth in fatigue reliability.The results of room temperature rotating bending fatigue test of M50 bearing steel show that MgO-Al2O3 inclusions and primary carbides MC and M2C inside the specimen lead to fatigue crack initiation.Debonding at the interface between Al2O3 and the matrix and primary carbide cracking led to the formation of cavities under cyclic stress.Localized cyclic hardening at the inclusions-matrix interface or carbide-matrix interface caused an increase in the value of the stress intensity factor range up to crack propagation threshold ΔKth(5.00 MPa·m1/2),which led to cracking.Internal small cracks propagated at a speed of 8.01 ×10-10m/cycle with hysteresis in the fine granular area(FGA).When the stress intensity factor range of internal small cracks was lower than ΔKth for M50 steel,the initiation stage of fatigue crack was composed of crack nucleation caused by defects and hysteresis crack propagation in the FGA.The fatigue cracks propagated at the fish-eye area(FiE)when the stress intensity factor range at the front end of the FGA exceeded ΔKth.A fatigue crack nucleation model was established based on the true stress at the defect,size of the defect,and number of cycles.A fatigue life model was constructed based on fatigue crack nucleation and fatigue crack propagation processes.The results of rotary bending fatigue tests on high cleanliness M50 bearing steel showed that the fatigue strength of M50 bearing steel was related to the size of primary carbide MC,M2C.The fatigue limit of M50 bearing steel was related to the size of the primary carbide.With the size of primary carbide MC and M2C in M50 bearing steel less than 9.64μm,the type of fatigue crack initiation in M50 bearing steel changed from the coexistence of surface crack initiation and internal crack initiation to internal crack initiation.The results of rotary bending fatigue tests on M50 bearing steel with different specimen sizes at room temperature showed that the fatigue strength of a cylindrical specimen with a diameter of 6 mm decreased by 113 MPa compared to a funnelshaped specimen with a diameter of 3 mm.The volume of cylindrical specimens of 6 mm diameter subjected to stress increased 27 times,the number of fatigue defects Oxide inclusion increased to 5,and the number of carbides increased from 6096 to 164502.The results of rotary bending fatigue tests on notched specimens with stress concentration coefficient Kt=1.65 showed that the stress concentration in the range of 0-9.53 μm from the surface of the notch led to the initiation of cracks in carbide defects in the size range of 5.29-13.17 μm thus leading to a fatigue strength reduction of 420.03 MPa under cyclic stress.The stress concentration at the root of the notch caused fatigue cracks to initiate directly at the surface of the specimen,and the stress concentration at the root of the notch also caused fatigue cracks to initiate at the primary carbide near the surface,which led to a reduction in fatigue strength.The results of high temperature rotational bending fatigue tests on M50 bearing steel showed that the rotational bending fatigue strength at 300℃decreased by 240.03 MPa compared to that at room temperature.Primary carbides of size 5.67-13.88 μm inside the specimen were the main defect leading to the initiation of fatigue cracks at high temperatures.Internal primary carbides cracking led to the initiation of fatigue cracks and the formation of FiE caused by the faster rate of crack propagation.The crack closure effect caused by surface oxidation changed the crack initiation type from surface crack source and internal crack source to internal crack initiation.As the temperature increased,the crack propagation threshold increased and the long crack propagation rate increased.The initiation of surface cracks was more difficult at high temperatures,and the faster propagation of long cracks resulted in a shorter surface crack propagation life.The nitiation life of fatigue cracks accounted for the main part at 300℃.Rolling contact fatigue test results on M50 bearing steel showed that the rolling contact fatigue failure types were spalling due to pits,spalling due to Al2O3 inclusions,and spalling due to primary carbides.The stress concentration at the edges of dents with area sizes of 2155.65-10039.52μm2 causes fatigue cracks to be initiated at the trailing edge of the crater,under cyclic contact stress.The clustered Al2O3 inclusions with sizes of 4.16-11.98 μm occurred microscopic peeling to form microscopic peeling pits with areas of 4357.71-16626.92 μm2,and the stress concentration at the edge of the microscopic peeling pits led to fatigue cracks initiation at the trailing edge of the microscopic peeling pits.Primary carbides in the size range of 5.45-7.99 μm formed pitting pits,and the stress concentration at the trailing edge of the pitting pits led to fatigue crack initiation.Dark etching region(DER),white etching cracks(WEC)and butterfly organization were observed on the subsurface of M50 bearing steel.The DER appeared in the region of maximum shear stress located at a depth of 50 μm to 170 μm below the raceway.The density of dislocations in plate martensite increased,and carbon in martensite diffused along the dislocations under cyclic shear stress.Some of the martensite plates were fragmented by dislocation tangles into cellular ferrite and formed DER.Cracking of primary carbides in the size range of 5-15 μm located on the subsurface led to the initiation of fatigue cracks under the action of cyclic shear stress and fatigue cracks propagated in the direction of shear stress.The increased density of dislocations in the matrix around the crack and the diffusion of carbon led to the formation of butterfly organization.The butterfly organization expanded along the direction of maximum shear stress to form white etching cracks,under the continuous action of cyclic stress.