Nondestructive Characterization of Austempered Ductile Irons

Recently developed austempered ductile irons have good mechanical properties. Comparing the austempered ductile irons with the normal ductile irons the austempered ductile irons have about two times or higher tensile strength than the normal ductile irons at the same level of elongation as shown in Fig.l [1]. Austempered ductile irons are made from ductile cast irons by austempering heat-treatment. Ideally, during austempering, the ductile cast irons are cooled rapidly from the austenitizing temperature to the temperature range for bainite formation, avoiding the zone of pear-lite formation. However, wide spread use of austempered ductile irons have not yet occurred. The main problem is in the difficulties of identifying and controlling the austempering heat-treat — ment parameters to achieve optimum properties consistently [1]. Those heat-treatment parameters are austenitizing temperature, austempering temperature, and austempering holding time. Austempering reaction of ductile cast irons is usually divided into two stages. First stage is toughening stage. Austenite decomposes into bainitic ferrite and carbon riched austenite. These two phase mixture of bainitic ferrite and austentie is the reason for the outstanding mechanical properties of austempered ductile irons.