Effect of multi-step austempering treatment on the microstructure and mechanical properties of a high silicon carbide-free bainitic steel with bimodal bainite distribution

The effect of multi-step austempering treatments on the microstructure and mechanical properties of a novel medium carbon high silicon carbide-free bainitic steel was studied. Five different isothermal treatment processes were selected, including single-step isothermal treatments above martensite start temperature (at 350◦ C and 370◦ C, respectively), and three kinds of two-step routes (370◦ C + 300◦ C, 370◦ C + 250◦ C, and 350◦ C + 250◦ C). In comparison with single-step austempering treatment adopting a two-step process, a microstructure with a bimodal-size distribution of bainitic ferrite and without martensite was obtained. Bainitic transformation was studied using dilatometry both for single-step and two-step routes and the specimens were completely characterised by electron microscopy (SEM and TEM), X-ray diffraction (XRD) and standard tensile tests. The mechanical response of the samples subjected to two-step routes was superior to those treated at a single temperature.