| Aeroengine,known as the "crown pearl" of modern industry,is the "heart" of aircraft.The independent manufacturing of aeroengine is the symbol of industrial manufacturing power.Bearings support the core rotor system of the aeroengine.The working conditions are extremely harsh,which require high speed,high precision,high load and high reliability.They are the most critical parts to ensure the long service life of aeroengine.However,the traditional hot rolling forming of M50 bearing steel often leads to coarse grain and cutoff of streamlines.Meanwhile,in order to ensure the high-temperature hardness and dimensional stability of M50 steel,martensitic quenching and high temperature tempering are applied to realize secondary hardening and eliminate metastable microstructure,which results in a small amount of retained austenite(RA)and poor toughness.Restricted by the forming and heat treament of M50 bearing steel,aeroengine bearings seriously hinder the independent development of China’s aeroengine and the implementation of aviation power strategy.It is urgent to develop advanced manufacturing theory and technology of aeroengine bearings,so as to provide support for the development of manufacturing technology of bearings and the promotion of China’s aeroengine.In order to meet the urgent demand of independent research and development of long-life aeroengine main bearings,this paper puts forward an innovative idea of combining the cold rolling and multiphase heat treatment with electromagnetic regulation.By means of theoretical analysis and calculation,process experiment and numerical simulation,the evolution law of the microstructure and properties during cold rolling of M50 steel was studied,the effect of heat treatment parameters on the microstructure and properties of bainite/martensite duplex phase transformation was explored,the strengthening and toughening mechanism of microsturecture produced by cold rolling and bainite/martensite duplex phase transformation was clarified,and the mechanism of electromagnetic regulation on the microstructure and properties for the forming and heat treatment process was revealed.Furthermore,the regulation method of bearings by combining cold rolling and multiphase heat treatment with the electromagnetic field was invented.The research results are expected to provide a scientific basis for breaking through the key manufacturing technology of long life/high reliability aeroengine bearings.The main contents are as follows:(1)The microscopic deformation behavior of cold rolling was studied,the influence mechanism of cold rolling on austenitizing,carbide dissolution,martensite transformation and tempering process was explored,and the relationship between cold rolling and mechanical properties after heating treatment was clarified.The results show that the cold rolling makes the ferrite grains elongate along the rolling direction,the obvious misorientation appears in the grains,the proportion of large carbide particles decreases,and the <111> texture perpendicular to the rolling direction is formed.In addition,the cold rolling process can significantly refine the original austenite grain size and lath size,and promote the dissolution of carbides during austenitizing and precipitation of carbides during tempering.With the increase of cold rolling,the hardness and impact properties show no significant changes after tempering,while the tensile strength first increases and then decreases.The increase of tensile strength under small deformation is due to that the finer grain structure and more precipitated fine carbides hindering the movement of dislocations during tensile deformation.In addition,since the cold rolling can promote the dissolution of primary carbides and the precipitation of fine carbides,the self lubricating film are formed on the worn surface during wear.This can protect the matrix and reduce the ploughing wear caused by primary carbides,so as to improve the wear properties of M50 steel.(2)Through characterization and analysis of the microstructure and properties under various heat treatment conditions,the strengthening and toughening mechanism of bainite/martensite multiphase microstructure was revealed,and the relationship between heat treatment conditions,microstructure and mechanical properties was clarified.Finally,the optimization method of bainite/martensite multiphase heat treatment process was discussed.The results show that with the increase of isothermal temperature and time,the contents of bainite and RA gradually increase,while the hardness and tensile strength gradually decrease.However,the impact toughness does not show a simple proportional relationship with the austempering conditions.By means of process optimization analysis,it is found that the microstructure after tempering can be refined to the maximum extent,and the tensile strength and impact toughness can be both improved by introducing the proper content of bainite(10% ～ 20%).In addition,the content of RA should be strictly controlled below 35% after quenching,so as to decrease the coarsening degree of bainite and ensure that the massive residual austenite can be completely decomposed during tempering.This would avoid the deterioration of mechanical properties caused by a large number of blocky RA.(3)The interaction influence between cold rolling and multiphase heat treatment was studied.The effect of cold rolling on bainite transformation kinetics was revealed.The relationship between process conditions,microstructure and mechanical properties was clarified.The matching method of cold rolling and multiphase heat treatment process was proposed.The results show that cold rolling can significantly refine the bainite structure.The microstructure characterization and kinetic analysis together confirm that cold rolling can significantly shorten the incubation period of bainite transformation and accelerate the early formation of bainite.However,with the increase of austempering time,cold rolling begins to inhibit the bainite transformation.The refinement of austenite grain(increase of nucleation rate)caused by cold rolling is the main reason to promote the early formation of bainite,while cold rolling heredity affects the prior austenite(increase of dislocation density,carbon content and strength of austenite),while will inhibit the growth of bainite.Furthermore,the tensile strength and impact toughness of M50 steel were improved by optimizing the cold rolling and multiphase heat treatment process,and then the rated life of rolling contact fatigue test was significantly improved.(4)Based on the idea of electromagnetic assisted regulation of the microstructure and properties during forming and heat treatment process,the influence of electromagnetic shock on the microstructure and properties of cold rolled M50 steel was studied.The mechanism of voids healing,carbide refinement,residual stress and texture regulation under electromagnetic shock was revealed.The influencing mechanism of high magnetic field on microstructure and properties during tempering was also explored.The results of in situ SEM and CT show that the size of void decreases from 260 nm to 95 nm and the porosity decreases from 0.26%to 0.09%.The results of numerical simulation show that the the voids can be healed after electropulsing,and the higher the current density and the number of pulses,the more significant the voids healing is.In addition,the electromagnetic shock can not only reduce the residual stress cuased by cold rolling,but also homogenize the distribution of residual stress.During tempering,high magnetic field can significantly promote the decomposition of residual austenite and precipitation of cementite,and increase the magnetization of precipitated carbide.After high temperature tempering,the high magnetic field could inhibit the dislocation recovery,thereby promoting the dislocation strengthening effect.This will contribute to the improvement of the hardness and tensile strength in M50 steel. |
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