Study On Strengthening And Toughening Process Of M50 Aerospace Bearing Steel By Combining Cold Rolling With Multiphase Heat Treatment

Recently,with the fast development of aviation industry,the aviation spindle bearings should be operated under the circumstance of high speed,high temperature and heavy load.Under this circumstance,it is required that the sliding linear speed DN value of the bearing[DN value=bearing inner diameter(mm)×bearing speed(r/min)]should reach 3.6×10~6mm?r/min.Meanwhile,the engine should be able to operate normally under the temperature of-250°C?315°C and bear the contact stress of 2?4GPa.Therefore,it is necessary to improve the manufacturing process of the aviation spindle bearings to meet the current service requirements.As a kind of high alloy bearing steel,the content of Mo and Cr in the alloy composition is about 4%,and the content of V is about 1%.The addition of these alloys can increase the hardenability of the steel,refine the grains and promote carbide precipitation during tempering.The high temperature hardness and contact fatigue properties of M50 steel can be enhanced by the secondary hardening during tempering,which makes M50 steel widely used in aviation bearing manufacturing.The traditional heat treatment process of M50 steel is martensite quenching-tempering(Q-T),and the high temperature hardness of M50 steel is improved by the formation of secondary carbides with dispersion distribution.However,the newly formed martensite is a kind of embrittle phase with high hardness and cracks are easy to generate at the defects,and propagate along the interface between martensite and carbides,resulting in low reliability and life of bearing.Therefore,how to improve the impact toughness of M50 steel is a major issue that needs further attentions.In this thesis,a new process of cold rolling combined with austempering was proposed to improve the strength and toughness of M50 steel.The evolution of microstructure and mechanical properties under different process parameters were studied.The relationship between microstructure and mechanical properties was analyzed in detail.Microstructure observation shows that the cold rolling can facilitate the dissolution of spheroidal carbides in the austenitizing process,and refines the prior austenite grains in the quenched specimens through recrystallization.Meanwhile,more carbon and alloy elements can diffuse into the austenite,which contributes to the decrease of Ms temperature and the retention of more austenite after quenching process.The prior austenite grain size of specimen without cold rolling and with 40%cold rolling reduction is 11.64?m and 7.41?m,respectively.The Ms temperature is 186°C and 166°C,respectively.It is found that the amount of bainite sheaves decreases with the increase of cold rolling reduction.Moreover,with the increase of cold rolling reduction(<20%),the B/M duplex microstructures become finer.However,when a higher cold rolling reduction(>20%)is applied,the amount of bainite sheaves decreases sharply,which weakens the refinement of the martensite during subsequent quenching.This will lead to the coarsening of the microstructures.Mechanical properties test indicates that an excellent combination of ultimate tensile strength(2535.7MPa)and impact toughness(128.14J)was achieved in specimen with 20%cold rolling reduction after austempered at 240°C for 1h and three cycles of tempering at 550°C,of which the ultimate tensile strength increases by about 10%whereas the impact absorb energy exhibits 2.3 times comparing with the Q-T specimen.The improvement of toughness is mainly due to the double refinement of austenite grains and microstructures by cold rolling,and thus the ultra-fine equiaxed ferrite can be obtained after tempering,which increases the propagation path for cracks.The increase of strength is due to the fact that the carbon rich retained austenite formed in the quenching process decomposes a large number of carbides through the para-equilibrium mechanism in the tempering process,which hinders the climb of dislocations.Consequently,small numbers of dislocations could be retained during the tempering,which increases the resistance of dislocation sliding in the tensile process.Therefore,the tensile strength of the microstructures is enhanced.The new heat treatment process proposed in this thesis has practical application value for improving the rolling contact fatigue life of aviation spindle bearings.