Austenitic transition layer in a carburized bearing steel and formation mechanism

CSS-42L is a new generation carburized bearing steel for the critical service environment of aero-engine applications. Owing to the complex composition and heat treatment, this steel exhibits unique microstructural features at the subsurface, one of which is the formation of soft austenitic transition layers. So far, a plausible mechanism of this phenomenon is still absent. In this study, a systematic investigation into such austenitic transition layer formation process is conducted. By using various characterisation techniques along with thermodynamic analysis, it is found that the contents of Cr and C in the matrix are dominated by the formation of different types of carbides along depth. With increasing depth, the undissolved carbides vary from M7C3 and M23C6 to M6C and to nil. This is postulated to result in a drastic decrease of martensitic start (Ms) temperature in the M6C region, leading to the formation of austenitic transition layers. The simulations of both microstructure and Ms temperature evolution along depth during heat treatment are performed for different austenitic temperatures and compared against the experimental observation, with good agreement obtained. This study reveals the mechanism of austenitic transition layers for the first time and provides useful instructions for the further microstructural design of such high-performance bearing steels.