Microstructure and mechanical properties of austempered medium carbon steels

Microstructures and mechanical properties for medium carbon steels including practical spring steels austempered at 400^°C and 300^°C were investigated. The larger strain was applied, the larger the fraction of retained au stem te became martensite through strain induced transformation. 0.2% proof stress was increased by the increase of the fraction of bainitic ferrite. Uniform elongation was increased by increasing the fraction of retained austenite. Good strength-ductility balance with tensile stress over 1800MPa for high strength spring was achieved in austempered SUP12 at 300°C composed of three phases of retained austenite, martensite and lower bainite.