Microstructure and rolling contact fatigue life of austenitic nitrocarburized STB 2 high carbon chromium bearing steel

Microstructures and rolling contact fatigue properties of STB 2 high carbon chromium bearing steel were investigated by means of electron microscopy, hardness tests and rolling contact fatigue tests. In order to examine the influence of the heat treatment process on microstructures and rolling contact fatigue, two kinds of heat treatment processes, quenching/tempering (QT) and austenitic nitrocarburizing were performed on STB 2 steel. Rolling contact fatigue life of the nitrocarburized steel was 3.7 times longer than that of the QT treated steel under a clean lubrication condition and was 1.5 times longer under a contaminated lubrication condition. The amount of retained austenite in the nitrocarburized steel was found to be larger than that in the QT treated steel. As the tempering temperature was increased from 150°C to 300°C, the decrease in hardness for the nitrocarburized steel was smaller than that of the QT treated steel. This means that the nitrocarburized steel is suitable as bearing material for high temperature applications. Very fine Fe₄(Fe(CN)₆)₃ carbonitride of less than 100nm in diameter was found in the nitrocarburized steel. An improvement in rolling contact fatigue life in the nitrocarburized steel was attributed to the combination of the formation of very fine Fe₄(Fe(CN)₆)₃ and retained austenite.