ROLE OF AUSTENITE IN PROMOTING DUCTILITY IN AN AUSTEMPERED DUCTILE IRON

Measurements of the austempering kinetics and mechanical properties of an alloy ductile iron after single and stepped austempering treatment s following austenitising at 920 degrees C are presented. The kinetic and tensile data are analysed to show how the thermal and mechanical s tability of the the austenite phase vary with austempering treatment. Both the thermal and mechanical instability of the austenite phase ale shown to limit the ductility achieved in single austempering treatmen ts at 375 and 400 degrees C to such an extent that the Charpy impact e nergy fails to satisfy the AST M standard AST M :1990. Steppen austemp ering 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 impa ct energy and the austenite carbon content for different austempering treatments after austenitising at 920 degrees C. (C) 1997 The Institut e of Materials.