In-situ Neutron Diffraction Analysis of Crystal Plasticity of Retained Austenite in Bearing Steel

The presence of retained austenite in bearing steels can have a significant effect on the mechanical response of the material at varying length scales. Since retained austenite is metastable and cannot be produced independently without martensite/carbides, the key physical data on elastic-plastic behaviour is absent. In order to address this issue, this study investigates the transformation behaviour of retained austenite using in-situ neutron diffraction during uniaxial tensile testing of through hardened A485 (Grade 1) bearing steel. The study showed that retained austenite is remarkably stable within the elastic limit, and the transformation suddenly initiates once the true stress exceeds the macroscopic yield point. Analysis of the lattice strain and of the evolution of peak intensities showed dramatic and preferential transformation of the retained austenite, especially, grains with the <200> direction parallel to the loading direction (LD) underwent most obvious phase transformation. Analysis further revealed that these austenite planes were most likely transformed into martensite with <211> direction parallel with the LD.