2003 Volume 43 Issue 7 Pages 1056-1062
The effect of stepped austempering heat treatment (900°C, 60 min; 380°C, 15-120 min; 320°C, 60-360 min) on the microstructural and mechanical properties of an unalloyed ductile iron having a chemical composition of 3.86% C, 2.1% Si, 0.4% Mn, 0.042% Mg, 0.059% S and 0.07% P was studied. These properties were compared with those obtained by single austempering heat treatment (900°C, 60 min; 380°C, 60-360 min). Microstructure, hardness, Charpy impact V-notch and tensile properties were evaluated after each heat treatment cycle. The average austenite carbon content and retained austenite content have been evaluated by X-ray diffraction. The fractography of tensile heat treated specimens was examined and evaluated using SEM. The results obtained showed that a stepped austempering treatment could be used to increase the strength without significant loss in ductility or impact energy compared with single austempering. The optimum combination of mechanical properties is obtained at first step austempering time of 45 min. For the other conditions, The UTS increases slightly by increasing the second step austempering time. The average austenite carbon content results indicate that stage II begins after 120 min of the second step austempering time which leads to reduction in ductility and impact toughness as a result of carbides precipitation. Decreasing the first austempering time, accelerates the stage I reaction in the second step treatment but causes a loss in the strength and hardness and an increase in ductility and impact energy. Analysis of fractured surfaces reveals that the fracture of stepped austempered ductile cast iron is a ductile fracture and this mode almost remains unchanged with different conditions.