Chapter 8 | Heat Treatment Process Technologies—Hardening and Tempering Treatments

Abstract

The choice of appropriate hardening heat treatments profoundly influences the functional properties of rolling bearings. Through hardening heat treatments of 52100 type steels include martensitic or bainitic treatments and such heat treatments are applied when the static capacity needs to be maximized in point contact rolling bearings. Surface hardening is used when toughness is a primary consideration with case-carburizing, surface induction hardening and laser heating being the most familiar surface hardening technologies. The formation of near-surface compressive stresses is a beneficial factor in the selection of case carburization heat treatments. Case carburization can be combined with nitrogen enrichment to enhance the raceway tolerance to contaminated lubrication. Microstructures developed by appropriate austenitization, quenching and lower bainite transformation are essential considerations with respect to functional properties. Hardness is an overriding property consideration but the amount of austenite is an issue with respect to bearing component dimensional stability. Increased volume percentages of retained austenite goes hand-in-hand with high carbon plate martensitic surfaces zones of case carburized bearing components. Tempering after martensitic quenching is applied to transform austenite retained in the microstructure and in certain cases subzero treatments are employed. Surface induction heating and quenching is an efficient methodology for hardening of rolling bearing raceways. Hub unit bearing raceways are selectively austenitized and quenched in a few seconds whereas large-size rolling bearing rings are progressively heated and quenched using lengthier cycle times. Secondary hardening is applied to higher alloyed bearing steels such as M50 when high temperature hardness retention is required. Nitriding of the surface in combination with through or case carburization is a methodology for improving performance in hybrid bearings, i.e., steel rings and silicon nitride rolling elements. Distortion of rolling bearing rings is the curse of the hardening operation and, if not controlled, can have a negative influence on grinding costs.