Effectively improving the hardness-strength-toughness of carburized bearing steel via nanoprecipitates and fine grain structure

Cryogenic treatments are fast and efficient methods used to improve the mechanical properties of metal components by promoting retained austenite (RA) transformation. However, the relationship between the evolution of RA with the gradient changes in content and hardness-strength-toughness improvements during this period are unclear. The present study investigated the beneficial effects of a cryogenic treatment on the microstructural evolution and mechanical properties of a gradient-structured carburized M50NiL bearing steel. The results revealed that the RA average sizes, the densities of the high-angle grain boundaries (DHAGBs) and low-angle grain boundaries (DLAGBs), and the hardness of the carburized layer changed with increasing distance from the surface. Compared with the untreated samples, the cryogenically treated samples demonstrated a 30% greater effective hardened layer thickness as well as 15.6% and 22.9% higher strength and toughness, respectively. The synergistic enhancements of the hardness, strength, and toughness provided by the cryogenic treatment were attributed to the increased nanoprecipitate content and finer grain structure generated in situ during transformation and further decomposition of the RA, which resulted in reductions of up to 83.6% in the RA content. This work provided new ideas for innovative enhancements of gradient-structured materials with superior mechanical properties.