Influence of strain rate on dynamic transformation of austenite in an as-cast medium-carbon low-alloy steel

In this work, the effect of strain rate on the dynamic transformation (DT) of austenite to ferrite during high temperature forging of an as-cast medium-carbon low-alloy steel was investigated. Hot deformation experiments were carried out in the 1150 °C to 1200 °C temperature range (400-450 °C above Ae3) and 0.25 s−1 to 2 s−1 strain rate range using a Gleeble 3800® thermomechanical simulator. The critical strains for DT and dynamic recrystallization (DRX) were determined to be in the 0.08 to 0.18 strain range. A microstructural analysis was conducted using optical and electron microscopy. The kernel average misorientation (KAM) technique was applied to electron back-scattered diffraction (EBSD) images to quantify the internal misorientation of grains for the characterization of DT ferrite. Furthermore, it was found that an increase in strain rate decreased the amount of dynamically formed ferrite under the same applied strain and testing temperature. The obtained results were correlated with the influence of deformation parameters on carbon diffusivity and its impact on the growth of dynamically formed ferrite. It was found that an increase in the strain rate decreased the diffusion distance of carbon, which was the responsible mechanism for decreased ferrite formation at higher strain rates.