Microstructural and Rotating-Bending Fatigue Behavior Relationship in Nanostructured Carbo-Austempered Cast Steels

Carbo-austempering processes have been established as a viable alternative to conventional carburized—quenched and tempered (Q&T) steels for components subjected to wear and contact fatigue conditions. However, little information is available about the rotating-bending fatigue performance of carbo-austempered nanobainitic steels. In this article, the rotating-bending fatigue behavior of a high silicon cast steel subjected to different carbo-austempering processes has been evaluated. The implemented heat treatments were designed to lead to nanobainitic microstructures in the case and to obtain different microstructures in the core, including multiphasic microstructures achieved by intercritical austenitization. The results were compared with the ones given by a conventional carburized-Q&T specimen. The microstructural features involved in crack propagation during the cyclic fatigue were evaluated by means of electron backscatter diffraction (EBSD), by scanning the premortem and postmortem samples. Results show that nanobainitic carbo-austempered cast steels exhibit better fatigue performance than traditional carburized-Q&T steels, which is explained in terms of the differences between both microstructures, the role of both ferrite and austenite crystals in crack propagation, the importance of prior austenite boundaries and block boundaries as crack deflectors, and how all these parameters finally have an effect on the fatigue behavior.