Study On Microstructure And Properties Of G20CrNi2MoSiAl Nanobainite Carburized Bearing Steel

Nanobainite carburized bearing steel is gradually used in wind power bearing and gear industries because of its high strength,high toughness and excellent wear resistance of the surface,and the excellent strength and toughness of the center.However,the use of nanobainite carburized bearing steel in the heavy-load trains bearing industry is still in the groping stage.With the rapid development of heavy-load train transportation in our country,the demand for heavy-load trains with larger axle loads has also become stronger.As one of the key components of heavy-load trains bearing,carburized bearing rings mainly play a role in supporting the weight and load of the train body.With the increase of the axle load of heavy-load trains,the traditional martensite carburized bearing steels are prone to early cracking under large impact loads due to their poor combination of surface strength and toughness,and can not meet the performance requirements of bearing rings with larger axle load.The new nanobainite steel can significantly improve the strength,toughness and wear resistance while maintaining the same hardness as the martensite steel with the same composition due to the very fine bainitic ferrite lath and the filmy retained austenite evenly distributed between the laths.Therefore,nanobainite carburized bearing steel with better comprehensive mechanical properties is considered to be the best material to replace the traditional martensitic carburized bearing steel.In this paper,on the basis of the traditional G20CrNi2Mo steel,through Si and Al microalloying,and the low temperature austempering process is further optimized,a nanobainite carburized bearing steel with better surface and center mechanical properties was obtained.By means of tensile,impact,frictional wear and contact fatigue life tests,combined with OM,SEM,TEM,EBSD,EPMA and XRD,the strength,toughness and microstructure of the carburized surface and center of optimized G20CrNi2Mo Si Al nanobainite carburized bearing steel under different heat treatment conditions was studied.The details are as follows:(1)Effect of deep cryogenic treatment on surface microstructure and wear resistance of nanobainite carburized steel.After carburizing,G20CrNi2Mo Si Al nanobainite carburized bearing steel was subjected to traditional quenching+tempering process(QT),low-temperature austempering treatment(AT)and low-temperature austempering+deep cryogenic treatment(ACT),respectively,and investigated the effects of retained austenite with different morphologies,stabilities and volume fractions on the wear resistance of nanobainite microstructure.The results show that ACT process can effectively eliminate the blocky retained austenite in the microstructure and increase the stability of filmy retained austenite,but has no effect on the nanobainite microstructure.Compared with QT process,the samples treated by AT and ACT processes have higher surface residual compressive stress and better wear resistance.Especially for the specimen subjected to ACT process,a multiphase microstructure composed of nanobainite,martensite and filmy retained austenite is formed near the surface,which significantly optimizes the wear resistance of carburized surface.After the samples were austempered for 8,12 and 24 h,respectively,and then followed by deep cryogenic treatement at-196°C for 4 h,the surface wear resistance of the samples was about 23%,52%and 93%higher than that of the QT samples,respectively.In addition,the results also show that deep cryogenic treatment can be combined with conventional low-temperature austempering treatment used for nanobainite,which can effectively eliminate the unstable blocky retained austenite,avoid brittle martensitic transformation,and it is expected to improve the surface wear resistance of nanobainite steel in a short heat treatment time.(2)Effect of intercritical quenching treatment on mechanical properties of martensite/ferrite dual-phase microstructure.The orthogonal experimental method was used to optimize heat treatment experiments including different primary quenching temperatures,secondary intercritical quenching temperatures and tempering temperatures,so as to optimize heat treatment process.At the same time,the effect of ferrite volume fraction and morphology in the center of the carburized steel on the mechanical properties and rolling contact fatigue life of the martensite/ferrite dual-phase microstructure was investigated by changing intercritical quenching temperature.The results show that after primary quenching at 900°C,secondary intercritical quenching at 805°C and low temperature tempering at 160°C,there is about 6.9vol.%continuous grain boundary ferrite in the center of G20CrNi2Mo Si Al nanobainite carburized bearing steel,and the comprehensive mechanical properties of the center reach the best.After heat treatment,the tensile strength of the center is 1495.8 MPa,the yield strength is1212.1 MPa,the elongation is 12.92%,the impact energy is 75.8 J and the hardness is 46.9HRC,which are about 2.4%,1.9%,2.5%,11.1%and 4.0%higher than those obtained under the traditional heat treatment process.In addition,when there is continuous grain boundary ferrite in the center of carburized bearing steel and the volume fraction is between 6.9-13.9vol.%,the contact fatigue life is approximately 76.1-85.0%higher than that of the sample quenched after traditional complete austenitization.(3)Effect of deep cryogenic treatment parameters on martensite multi-level microstructures and mechanical properties of a lath martensite/ferrite dual-phase microstructure.Through changing the deep cryogenic treatment time and the number of cycles,the changing tendency of microstructures and properties of the martensite/ferrite dual-phase microstructure under different deep cryogenic treatment parameters was studied.The results show that with the increase of deep cryogenic treatment time(QCT-h process)and cycle number(QCT-t process),the size of ferrite was refined and its distribution became more homogeneous;the hardness and impact energy were simultaneously increased.Especially in the QCT-t processed specimen,the microstructure refinement and the property improvement were significant.The impact energy of samples treated through 3-cycle DCT were 29.4%higher than that of the samples without DCT.Meanwhile,deep cryogenic treatment also has a significant refinement effect on the substructure of each level in the martensite multi-level microstructure,and the refinement effect of increasing the number of cryogenic cycles on the microstructure size is more obvious.The size of martensite packets(d_p),blocks(d_b)and laths(d_l)decrease with the increase of the total deep cryogenic treatment time.The Hall-Petch relationship is used to establish the relationship between the martensite multi-level microstructure and the impact energy,and according to the deflection degree of the crack propagation path at the interface boundaries with different crystallographic orientations,it is proved that the effective control unit of impact energy of martensite multi-level microstructure is martensite block.(4)A slow-cooling treatment developed based on the carbon diffusion process during nanobainite transformation in high-carbon steel.According to the changing trend of carbon content in retained austenite in high-carbon nanobainitic steel after austempering treatment at240°C for different time,the relationship between retained austenite transformation temperature and austempering time was established.Based on the relationship between carbon content,transformation temperature of retained austenite and austempering time,a slow-cooling treatment(SCT)with continuous decreasing temperature was developed.The results show that when the high-carbon nanobainitic steel was continuously cooled from 240°C to216°C at a cooling rate of 0.1°C/min,and then the sample was cooled to 120°C at a cooling rate of 0.2°C/min,and austempered at 120°C for 12 h,the microstructure with higher volume fraction of nanobainite,finer bainitic ferrite lath thickness and lower blocky retained austenite content can be obtained.Thus,the tensile strength,yield strength,elongation,impact energy and hardness of nanobainitic steel are significantly improved.Compared with the sample after the traditional austempering treatment(AT),the SCT sample has improved the tensile properties,impact energy and hardness to a certain extent,especially the yield strength and impact energy,which are about 11.2%and 48.7%higher than those of the AT sample respectively,and have obtained more excellent comprehensive mechanical properties.