Speaker
Description
This work presents novel results on factors influencing austenite stability in carburised high carbon bearing steels. We have conducted a series of experimental and computational studies targeted at understanding these factors, including chemical composition, austenite grain size, stress/strain partitioning, and grain orientation.
In-situ neutron diffraction in steels with different nickel contents show that nickel increases austenite stability in the elastic regime but has no effect on the transformation rate; we also show that austenite grains with <200> direction parallel to the loading direction were the most susceptible to martensite transformation, and that these grains were most likely transformed into martensite with <211> direction parallel to the loading direction. Data from the neutron diffraction studies are coupled with a crystal plasticity finite element formulation to predict the mechanical responses of various multi-phase bearing steels, and estimate the stress/strain partitioning ratio as the austenite transforms into martensite.
Selected alloys were subjected to different heat treatments to study the relationship between the prior austenite grain size (PAGS) and the martensite start temperature (Ms); it is found that the Ms does not always increase with PAGS, as typically reported in literature for low/medium carbon steels, and that the resulting trends are due to competing effects of grain size and chemical composition. A limitation in the CPFE model is that the critical driving force for martensite transformation was originally assumed to vary linearly with the initial volume fraction of retained austenite. Predictions based on this method may be inaccurate if the austenite fraction falls outside the accepted range. Subsequently, the Ms, which is an indicator of austenite stability, is used to establish the critical driving force. Based on these results, correlations on the effect of chemical composition and PAGS towards austenite stability have been established.