Effect of Austempering Parameters on Microstructure and Tensile/Impact Behaviours of Micro-Alloyed TRIP-Assisted Steel

The article is aimed at studying the effect of austempering temperature below and above Ms temperature on the phase-structural state and mechanical properties of 0.2 wt.% C TRIP-assisted steel micro-added with Nb, V, Mo, Cr. The samples were austenitized at a temperature close to the Ac3 point (900 °C) and held at 300 °C (below Ms), 350 °C (close to Ms) and 400 °C (above Ms) for 5-20 min. The work was performed using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction, and tensile/impact testing. It was found that austempering at the aforementioned modes ensures the multiphase structure consisting of carbide-free bainite, tempered martensite, ferrite and retained austenite (in different combinations). The optimal was austempering at a temperature close to Ms which provided an advanced complex of tensile properties (PSE of 23.9 GPa×%) and V-notched impact toughness (95 J/cm2). TRIP-effect contributed to these properties while the strain hardening process tended to be prolonged with increasing the austempering temperature.