Role of austenite in promoting ductility in an austempered ductile iron

Abstract

Measurements of the austempering kinetics and mechanical properties of an alloy ductile iron after single and stepped austempering treatments following austenitising at 920°C are presented. The kinetic and tensile data are analysed to show how the thermal and mechanical stability of the austenite phase vary with austempering treatment. Both the thermal and mechanical instability of the austenite phase are shown to limit the ductility achieved in single austempering treatments at 375 and 400°C to such an extent that the Charpy impact energy fails to satisfy the ASTM standard A897M: 1990. Stepped austempering treatment improves both the thermal and mechanical stability of the austenite phase and increases the impact energy to such an extent that the ASTM standard is satisfied over a wide range of austempering conditions. A well defined relationship is shown to exist between impact energy and the austenite carbon content for different austempering treatments after austenitising at 920°C.