Microstructural Evolutions and Properties of Partially Austenitizing and Austempered Ductile Irons

Abstract

In this study, commercial FCD700 ductile irons are austenitized in the intercritical region to observe their effects upon the microstructures and mechanical properties of austempered ductile irons (ADIs). The austenitizing temperature is chosen such that ferrite, austensite, and graphite coexist in the microstructures. Strength of 974 MPa, 16.4% elongation, and impact energy of 166 J are achieved by the ADI austenitized at 1103 K and subsequently austemperted at 673 K. These properties are in contrast to 790 MPa strength, 8.2% elongation, and 42 J impact energy in as-cast FCD700. The properties are also superior to the traditionally austempered ductile iron with 967 MPa strength, 12.7% elongation, and 124 J impact energy. Combinations of the optimum ductility and strength are achieved by 8–19% of proeutectoid ferrite and over 20% of retained austenite in the intercritically treated microstructures. Nanoscale twinned and lenticular martensite forms within the micro-size blocky retained austentite islands during plastic deformation leading to the transformation induced plasticity. The intercritically austenitizing and austempering treatments provide unique combination of strength and elongation for commercial ductile irons.