In this investigation, the characteristics of bulky retained austenite in an austempered ductile iron are evaluated in two tempered conditions. which were intially tempered at 200oC for 1h before cooling to room temperature, and then tempered at 350oC for 1h. The result shows that the hardness within retained austenite areas is distributed unevenly with a range from 423 HV to 897 HV, which is attributed to the transformation from austenite to martensite during austempering. Also, the mechanism regarding the quenched marteniste formation is discussed. The poor fatigue resistance of ADI is hypothesized to be due to the amount of austenite transformed to martensite.

An unalloyed ductile cast iron with a multiphase structure is designed by a novel austempering process. The designed austempering treatment consists of initial rapid quenching to 180°C after austenizing at 890°C for 20min, and finally austempering at 220°C for 240min. A multiphase structure comprising lenticular/needle-like prior martensite, fine needle bainitic ferrite and film retained austenite is obtained. The excellent mechanical properties, with a tensile strength of 1530MPa and an elongation of 3.1% can be achieved by controlling the matrix microstructure of 12% prior martensite, 15% retained austenite with 1.64% carbon content, and 73% bainitic ferrite. This is mainly attributed to prior marteniste which can promote refinement of multiphase colonies.