A Review of Austempering for Bearing Applications

Bainitic bearings, produced through an austempering process, have shown great potential as an alternative to traditional, martensitic bearings. Bainite, specifically lower bainite, which is developed through low-temperature isothermal heat treatment, is a steel microconstituent consisting of carbon supersaturated bainitic ferrite and uniquely oriented, nanosized carbides. In bearing-quality steels, bainite can exceed hardness values of 60 HRC. Due to a uniform transformation, bainitic bearings avoid the formation of tensile residual stresses; the result is significantly reduced distortion and quench cracking compared to martensitic bearings. Because the carbon content of the retained austenite in bainite is higher than the retained austenite in martensite and its morphology is predominately of the film type, the retained austenite is more thermally stable in cold environments, providing designers an opportunity to relax retained austenite volume fraction specifications and to benefit from its mechanical advantages. At the same hardness, bainite tends to have competitive rolling contact fatigue properties and improved fracture toughness when compared to tempered martensite. However, due to the extended transformation time required, austempering heat treatments may be more expensive than traditional martensitic heat treatments. Therefore, researchers have been working on novel austempering variations to shorten the total heat treatment time or significantly increase bearing life. This paper reviews the literature on austempering for bearing applications, compares the microstructure of bainite to tempered martensite and how it impacts bearing performance, and introduces novel austempering variations that have superior rolling contact fatigue lives that may be cost-competitive with traditional martensitic heat treatments.