Modification of microstructure to increase impact toughness of nanostructured bainite–austenite steel

To improve impact toughness of the nanostructured bainite–austenite steel, a heat treatment operation was developed to divide prior austenite grains by plates of martensite directly before isothermal transformation. In the investigation, nanostructured steel containing 0·55C, 1·95Mn, 1·82Si, 1·29Cr and 0·72Mo was used. It was found that a partial transformation to martensite achieved by cooling to 160°C followed by direct isothermal transformation to bainite at 225°C was the most promising treatment to improve Charpy impact energy of the investigated steel. For each testing temperature: ambient, 0, −20, −40 and −60°C, the specimens subjected to the developed treatment showed a higher averaged impact energy than the specimens subjected to the standard treatment.