The Optimization of the Isothermal Transformation Dwell with Emphasis on the Matrix Structural Mixture of Austempered Ductile Iron

Abstract

The studied austempered ductile iron (ADI) was obtained by the heat treatment of an unalloyed nodular cast iron
consisting of austenitization (900 °C / 1h), an isothermal transformation at 380 °C and water cooling at the end. To study
matrix composition changes during the isothermal transformation, the dwell at the transformation temperature was
chosen in very wide range of 2 min to 540 min. The isothermal transformation starts by a creation of bainitic ferrite
needles resulting in an enrichment of the non-transformed austenite by carbon which increases its stability. For short
dwells (2, 5 and 10 min) the stabilization of the austenite is not sufficient yet, therefore the partial martensitic
transformation during cooling occurs. With increasing dwell the amount of the stabilized austenite increases and reaches
its maximum for the transformation dwell of 60 min. With further dwell lengthening the amount of the stabilized
austenite decreases again as a consequence of its continuing transformation into the bainitic ferrite. The maximum
fraction of the bainitic ferrite is reached for the longest dwells, i.e. for 270 min and 540 min. For the same dwells iron
carbides on the austenite/bainitic ferrite interface have been found.