Influence of tempering on the mechanical properties of austempered ductile iron

This paper aims to study the influence of tempering on the mechanical properties of austempered ductile iron. A modified austempering treatment was carried out to obtain a microstructure mixture of ausferrite and tempered martensite by stopping the austempering reaction before stage 1 reaction was completed, and resuming tempering at 473 K for 7.2 ks after cooling to room temperature. The change in the microstructure and the related changes in mechanical properties of ADI after tempering were investigated. The results show that tempering at 473 K after austempering decreased the sensitivity of mechanical properties to austempering conditions. Independent of austempering temperature, tempering at 473 K also led to significant improvement in mechanical properties with increasing martensite content. Strength and toughness could be improved to exceed the maximum values obtained without tempering, and these strengthening effects were particularly obvious at martensite content below 5%. With tempering at 473 K, the required austempering time for obtaining the maximum values of mechanical properties could be shortened from 10.8 ks without tempering to 1.8 ks with tempering, and the effective austempering time interval was also extended.