Abstract
Austempered ductile cast iron (ADI) has many excellent mechanical properties, which are influenced by heat treatment conditions (time, temperature) and chemical compositions. However the effects of these conditions on the fracture toughness have not been properly darified. In this study, the fracture toughness of ADI was investigated from the viewpoint of austempering heat treatment conditions (austenized conditions and isothermal transforming conditions). The main results obtained are as follows ; Under the austenized conditions, the fracture toughness decreases in either too high or too low treating temperature. In high treating temperature, austenite grains grow into big crystals and in low treating temperature, the amount of retained austenite decreases and free ferrite remains in matrix structure. The second reaction of bainite occurs and fracture toughness decreases when isothermal transformation increases. The best isothrmal transforming condition is 648K-3.6ks, in which high fracture toughness and tensile strength are obtaind.
