Thermochemical treatment of bearing steels

Abstract

Bearings are applied to provide frictionless movements and to withstand the applied load. The critical part of the bearings is the surface which determines the bearing life. The surface mechanical properties like hardness and microstructural features play a major role in that. These surface properties are modified by thermochemical treatments. One of the thermochemical treatments is the carburizing process where carbon concentration is increased along the surface to modify the surface properties. In this thesis work, three different bearing steel grades are carburized for two different case depths using a vacuum furnace and gas quenched to room temperature. In order to get the desired properties, an additional rehardening process is carried out at a temperature lower than the carburizing temperature. This project addresses the variation in grain size and chemical elements along with hardness and microstructure. Initially the case depths are verified with the Vicker`s micro-hardness test. The results of grain size measurement showed significant reduction in grain size after the rehardening process which can be correlated with the microstructure. But there was a substantial pick of carbon during the rehardening process which was observed during chemical analysis. Other important observations in chemical analysis include the manganese effusion from the surface and variation in chromium and nickel elements along the case depth.