Chemical Component Design,Microstructure And Properties Control Of Nanobainitic Steels Used For High-power Wind Power Bearing

In this dissertation,the heat treatment process of a bainitic carburizing steel used for wind power spindle bearing was optimized,and the chemical composition was further optimized.The microstructures and mechanical properties in surface and center of the G23Cr2Ni2Si1 Mo steel,which was finally determined as the material used for the spindle bearing ring,were investigated in detail,and compared with those of the classical martensitic carburizing G20Cr2Ni4 steel.The rolling contact fatigue performances of carburized and high-C nanostructured bainitic steels and its influencing factors were investigated.And the influence of the undissolved spherical carbide on nanostructured bainitic transformation behavior in bearing steel is also discussed.The heat treatment process of a bainitic carburizing steel used for wind power spindle bearing was optimized through the investigations about the bainite phase transformation kinetics of carburized surface layer,the spheroidizing annealing process,the microstructure and hardness in surface,as well as the mechanical properties in the core of the G20Cr2Ni4 SiMo steel and the G20Cr2Ni2 SiMoAl steel.The chemical composition was further optimized and the materials used for the spindle bearing ring were determined.A multiphase microstructure composed of nano-scaled bainite,martensite,undissolved spherical carbides and retained austenite was obtained on the top surface of carburizing G23Cr2Ni2Si1 Mo steel after low-temperature austempered at 200?C,with a dual-phase microstructure of fine martensite lath and carbon-enriched retained austenite in the center of steel.As compared with the conventional oil quenching and tempering(OQ)state,the low-temperature austempering specimens showed a higher surface residual compressive stress and a thicker residual compressive stress layer,and the wear resistance of the top surface of specimens austempered for 4 h,8 h and 24 h was increased 40%,58% and 12%,respectively.Moreover,the impact toughness of the center of these austempered specimens is also improved 20.8–33.3% as compared with that of the OQ specimen.Nanostructured bainite and dispersed carbide particles were obtained in the surface layer of the G23Cr2Ni2Si1 Mo steel based on carburization and succedent low-temperature austempering at 200?C.As compared with the conventional carburizing G20Cr2Ni4 martensitic steel,the carburizing G23Cr2Ni2Si1 Mo bainitic steel showed a much higher wear resistance and rolling contact fatigue performance.Moreover,the hardenability and the comprehensive mechanical properties of the G23Cr2Ni2Si1 Mo steel are better than those of the G20Cr2Ni4 steel.Then,it can be deduced that the carburizing G23Cr2Ni2Si1 Mo bainitic steel is hopeful to produce heavy section steel structural components,especially for components that bear impact load and need high wear resistance and high rolling contact fatigue performance during their service life.The influence of undissolved carbide on nanostructured bainitic transformation behavior in bearing steel has been investigated by electron probe(EPMA).Results show that the more volume fractions of the undissolved carbides in the microstructure,the greater ups and downs of the carbon concentration in the matrix.The greater difference of carbon concentration in the matrix,is not only beneficial to bainite ferrite nucleation in lean carbon area,so as to shorten the incubation period of the bainite transformation;but also is beneficial to the diffusion of carbon atom,thus promoting the bainite ferrite to grow up.Therefore,the more volume fractions of the undissolved carbides in the microstructure,the faster rate of the nanostructured bainite transformation,the less total time to complete the transformation.Under the same austempering temperature,the volume fractions of nanostructured bainite were basically the same,even if the volume fractions of the undissolved carbides in the microstructure were different.The nanostructured bainitic microstructures were obtained at the surfaces of a carburized steel and a high-C steel.The rolling contact fatigue(RCF)performances of the two alloy steels with the same volume fraction of undissolved carbide were studied under lubrication.Results show that the RCF life of the carburized nanostructured bainitic steel is superior to that of the high-C nanostructured bainitic steel in spite of the chemical composition,phase constituent,plate thickness of bainitic ferrite,hardness,and residual compressive stress value of the contact surfaces of the two steels under roughly similar conditions.The excellent RCF performance of the carburized nanostructured bainitic steel is mainly attributed to the following reasons: finer carbide dispersion distribution in the top surface,the higher residual compressive stress values in the carburized layer,the deeper residual compressive stress layer,the higher work hardening ability,the larger amount of retained austenite transforming into martensite at the surface and the more stable untransformed retained austenite left in the top surface of the steel.