Microstructural changes during simulated service of case carburized tapered roller bearings with different initial retained austenite profiles.

摘要

The near surface layers of the inner races or cones of 6½ x 12 type tapered roller bearings manufactured 8720M steel were analyzed as a function of both initial retained austenite profile and simulated service mileage (rolling contact revolutions). This analysis examined the microstructural changes produced in these layers by examining the retained austenite content, residual stress, texture, diffracting particle size, microhardness, cone bore, and metallographic structure. A postproduction cryogenic thermal treatment was used to alter the retained austenite profile on selected cones.; The results of this research have shown that postproduction cryogenic thermal treatment results in a reduction of the as-produced retained austenite profile due to its transformation to martensite, a reduction in the surface compressive residual stress in the martensite, an increase in the surface tensile residual stress in the austenite, and an increase in cone bore diameter. In addition, the diffracting particle size of the austenite was also reduced.; Assessment of as-produced and cryogenically treated test cones as a function of simulated service mileage indicated that cryogenically treated cones experienced less transformation of retained austenite during rolling contact loading but developed compressive surface residual stresses of similar magnitude as the as-produced cones. Subsurface residual compressive stresses were significantly less in the cryogenically treated cones, but cone bore growth was reduced.