A Microscale Model for Ausferritic Transformation of Austempered Ductile Irons

Abstract

This paper presents a new metallurgical model for the ausferritic transformation of ductile castiron. The model allows predicting the evolution of phases in terms of the chemical composition,austenitization and austempering temperatures, graphite nodule count, and distribution ofgraphite nodule size. The ferrite evolution is predicted according to the displacive growthmechanism. A representative volume element is employed at the microscale to consider thephase distributions, the inhomogeneous austenite carbon content, and the nucleation of ferritesubunits at the graphite nodule surface and at the tips of existing ferrite subunits. Theperformance of the model is evaluated by comparison with experimental results. The resultsindicate that the increment of the ausferritic transformation rate, which is caused by incrementsof austempering temperature and graphite nodule count, is adequately represented by thismodel.