Role of Bulky Retained Austenite in Austempered Ductile Iron

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.