EFFECT OF SECOND STEP AUSTEMPERING TEMPERATURE AND TIME ON THE IMPACT PROPERTIES OF TWO-STEP AUSTEMPERED SG-CAST IRON

This study investigates the effect of a novel two-step austempering temperature on the impact properties of austempered spheroidal graphite (SG) cast iron. The study shows that the impact properties of a two-step austempered SG-cast iron are significantly influenced by the second step austempering temperature and time. At a given second step austempering time, decreasing the second step temperature enhanced the resistance of the present material to the crack propagation as well as the impact toughness. The maximum energy absorbed during the crack propagation and the fracture processes are obtained at shorter second step austempering time. It has also been found that the maximum crack initiation energy in the stepped austempering is achieved at different second step austempering time. It occurs at 22 min for the 360°C treatment and at 82 min for the 320°C. However, the crack initiation energy of the former is higher than that of the latter.