High-temperature stability of retained austenite and plastic deformation mechanism of ultra-fine bainitic steel isothermally treated below Ms

The mechanical properties of the sample and the stability of retained austenite were studied by designing two kinds of ultra-fine bainitic steel with different heat treatment methods (austempering above and below Ms (martensite start temperature)), which were subjected to tensile tests at 20 and 450 °C, respectively. The results show that compared to room temperature (20 °C) tensile properties, the uniform elongation of the sample at high temperature (450 °C) significantly decreased. Specifically, the uniform elongation of the sample austempered above Ms decreased from 8.0% to 3.5%, and the sample austempered below Ms decreased from 10.9% to 3.1%. Additionally, the tensile strength of the sample austempered above Ms significantly decreased (from 1281 to 912 MPa), and the sample austempered below Ms slightly decreased (from 1010 to 974 MPa). This was due to the high carbon content (1.60 wt.%), high mechanical stability, low thermal stability for the retained austenite of the sample austempered below Ms. Besides, the retained austenite decomposed at high temperatures, the carbon content and transformation driving force were significantly reduced, the transformation rate increased, and the phase transformation content reduced.