Super-long Life Mechanism And Microstructure-Properties Control Of High Performance Carburizing Bearing Steel

Bearing is the brace of all sorts of mechanical parts. The working condition of bearing is probably ultra-high temperature, cryogenic, strong corrosion and ultra-high vacuum, also may be strong impact, high-abrasion, etc. The structural characteristics and working conditions of bearing require bearings components must have high hardness, wear-resistance, contact fatigue strength, toughness, dimension stability, corrosion resistance and cold & heat working properties.Taking the super-pure steel 20CrNi4Mo as tested materials, which is refined by the measure of VIM & VAR double vacuum, this paper obtains a best heat-treatment process by studying the microstructures, mechanical properties of the steel treated by different heat-treatments. Then, through carburizing heat-treatment, we get high surface hardness, tiny martensitic variants, fine dispersed carbides, and controlled austenite ranks and content. All of the good properties can satisfy the demand of ultra-long contact fatigue life and high rotating & bending fatigue strength.Researches show that with the increasing of quenching and tempering temperature and the number of tempering, as well as the use of acryogenic technology, the strength and hardness of carburizing bearing steel increases and the impact ductility decreases. Quenching at 910℃and tempering twice at 180℃,the mechanical properties of bearing steels can reach:HRC45.2, Rm-1450MPa,Rp0.2-1240MPa, KU2-105J, austenite quantity control in less than 1%. The better match of strength and toughness of the steel is attributed to grain refinement, ultra-fine martensitic variants, and large number of fine dispersed carbides. The dimensional stability of the steel is due to the control of the content of residual austenite.We use dimethylmethane(C3H8) as carburizing agent, and carburize at 910℃in the carbon potential of 1.2%for four hours, then let the carbon diffuse in the carbon potential of 09%-1.0%for six hours. On this condition, the microstructures of carburized layer are implicit crystal martensite, residual austenite and a few carbides; the microstructure of the center is lath martensite. After carburizing, the depth of the carburized layer is 1.68 millimeter, the grade of both martensite and carbide is 2, the grade of austenite is 1-2, and the surface hardness is HV743.The excellent microstructure distribution and carburized layer characteristic of the steel after carburizing, is the guarantee of long contact fatigue life and high rotating & bending fatigue strength for bearing materials. The contact fatigue test of the carburizing steel shows this steel have a rating life L10 for 0.86×107, and a median life L50for 5.01×107. The studies of the morphology of rollaway nests and the microstructure of fatigue pits shows that the contact fatigue failure mainly originate in the oxide inclusions and the bulk carbon in the surface and the subsurface. We can conclude that the key to super-long life of high-performance carburizing bearing steel are high surface hardness, uniform microstructure and fine dispersed carbides distribution.The rotating & bending fatigue text shows that the fatigue limit of the carburizing steel is 901MPa. The fatigue fractures of the steel indicate that the fatigue crack is mainly caused by the oxide inclusions in the surface and subsurface. In order to increase the rotating & bending fatigue life of the steel, we must strictly control the sizes and quantities of the oxide inclusions through optimizing metallurgical processes.