Research On Carbide Control And Fatigue Properties Of M50 Bearing Steel

M50 bearing steel is mainly used in the main shaft bearing of aero-engine due to its excellent high-temperature hardness,wear resistance,and fatigue performance.The serious problems of aero-bearings are their unstable performance and short service life.The performance of aero-bearing steel is a crucial factor affecting the service life of the bearing,which should be improved urgently.In the past few decades,metallurgical equipment and metallurgical technology have rapidly developed,which has significantly improved the metallurgical quality of steel.The inclusions and impurity elements in M50 bearing steel can be effectively controlled and the cleanliness has been made a giant leap when the VIM-VAR(Vacuum induction melting-vacuum arc remelting)process was applied.However,with the development of aviation technology and the harsh service environment,the demand for ultra-long service life and ultra-high stability of bearings becomes more and more urgent,and the homogeneity control of bearing steel becomes more and more significant.The hard and brittle primary carbides in M50 bearing steel are large-size and distributed inhomogeneously,which can cause stress concentration,accelerate the fatigue cracks initiation and propagation,and hurt the fatigue performance of bearing steel and harm the service stability of bearing.In response to these problems,the precipitation behavior during solidification process and evolution behavior during the hot working process of primary carbides in M50 bearing steel were analyzed,the formation mechanism and decomposition mechanism of primary carbide were explained,and the influence of carbide characteristics on fatigue probability was explored.Based on the research results,the idea to control primary carbides was put forward,and the industrial steel with excellent fatigue properties was successfully made.The main research content and conclusions are summarized as follows:(1)The characteristics of primary carbides in as-cast M50 steel and the thermodynamic calculation of carbide precipitation during equilibrium and nonequilibrium solidification were analyzed.The relationship between element redistribution and carbide precipitation during solidification was clarified.The nucleation and growth mechanism of primary carbides were explored.The results indicate that the formation of primary carbides is due to the segregation and enrichment of carbon and other alloy elements in the residual liquid phase during solidification.The nucleation and growth of carbides have a specific orientation relationship with the precipitated austenite.(2)The influence of the solidification cooling rate and rare earth(RE)element on the precipitation of primary carbides were studied.The influence mechanism of solidification rate on carbide precipitation was explored,and the effects of rare earth inclusions and solid solution rare earth on precipitation of primary carbides were analyzed.The results indicate that the increase of cooling rate could inhibit local micro-segregation,and reduces the equivalent size and spacing of carbide,which improves the homogeneity of billet based on the double refinement of carbides and dendrites.(La,Ce)10OS14 inclusion is formed in steels with RE elements addition,whose interface could be used as a substrate for nucleation of MC carbides.The RE in solid solution could inhibit the growth of dendrite,which increases the nucleation positions of carbides and reduces the size of carbides.In addition,the RE atoms can segregate at the carbide/matrix interface.(3)The influence of high-temperature diffusion annealing process on the evolution of primary carbide was studied,the pre-deformation high-temperature diffusion process was proposed,and the mechanism of decomposition and transformation of M2C and the formation of MC were clarified.The results reveal that the decomposion of M2C and formation of MC could happen when the samples are annealed at 1130℃ and 1160℃.When the annealing temperature reaches 1170℃,M2C carbides can be decomposed adequately,but MC carbide will grow rapidly.When pre-deformation high-temperature annealing treatment is carried out,M2C carbides could be decomposed completely,and large-size MC carbides are avoided after holding for 1h at 1160℃,which makes the primary carbides refined well.It is further found that few stacking atomic layers with FCC structure can form due to the segregation of V element at the interface and inside of M2C carbide,which can be considered as the nucleation core of MC carbides.The orientation relationship between MC and M2C is(111)MC//(0001)M2C.The nucleus of MC at the interface can grow into the matrix and inside M2C carbides at the same time.The nucleus inside M2C can grow by HCP-to-FCC transformation of M2C carbides.(4)The fracture behavior of primary carbides during tensile test was studied by in situ observation.The results show that M2C primary carbides will be fractured during tensile test,but carbide decohesion was hardly observed.Compared with MC carbides with small size and ellipsoidal shape,large-size and irregular M2C carbides have worse coordinated deformation ability with the matrix,and are more likely to cause stress concentration and then cracked.The crack is easy to be along(0001)crystal plane.(5)The billets with significant differences in carbide size and distribution were prepared by forging with different forging ratios,which were used to explore the effect of carbides on high cycle fatigue behavior.The results indicate that increasing the deformation can refine primary carbides and improve fatigue performance.When the forging ratio increased from 2.4 to 9.4,the fatigue crack source changes from primary carbides to matrix microstructure,and the L50 life of high cycle fatigue increased from 4.4 × 107 to 109 steadily.(6)Based on the above research results,the production process of M50 bearing steel was optimized,and the industrial bar was produced,its primary carbide characteristics and fatigue life was compared with that of billets produced by two conventional processes.The results show that the optimized process effectively refines the primary carbides,improves the homogeneity and increases the fatigue life.The L50 life of high cycle fatigue is increased by 1.6 times and 340 times,respectively,by comparing with two conventional billets.This effectively gurantees the safety of bearings.