Microstructure and Mechanical Properties of Austempered Medium Carbon Steels

Abstract

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 austenite 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 1800 MPa for high strength spring was achieved in austempered SUP12 at 300°C composed of three phases of retained austenite, martensite and lower bainite.