Influence of alloy element distributions on austempered ductile irons

Abstract

The influence of alloy element distributions on austempered ductile iron microstructure and austempering treatment was analysed by a cellular automaton model that considers the ausferritic and martensitic transformations. The initial microstructure is modelled as spherical graphite nodules inserted in an austenitic matrix, in which the alloy elements are distributed in a uniform or non-uniform way. The study is performed for different chemical compositions and graphite nodule sizes. Delays in the development of ausferritic transformation are produced by the increment of graphite nodule size and the presence of alloy element microsegregations. Moreover, microsegregation reduces the final volume fraction of ferrite platelets. The predicted retained austenite volume fraction is in good agreement with the experimental measurements reported in the literature.

Analysis of different approaches to model the austempering heat treatment of ductile irons

“…This model employs a cubic volume element with periodic boundaries, which is discretized by means of cells that have three possible states (graphite nodule, sheaf, and austenite block) in order to represent the ADI microstructure during the ausferritic transformation 9 . At the beginning of the transformation, the microstructure is formed by graphite nodules modeled as spheres randomly distributed into a metallic matrix that is formed by austenite with alloying element microsegregation.…”

“…The ADI microstructure is modified by the heat treatment variables such as the austenitizing and austempering temperatures, austempering time, among others, and the material characteristics such as the graphite nodule size, chemical composition, and existence of alloying microsegregation. In order to aid the heat treatment design to obtain ADI parts with the required mechanical properties, it is possible to perform numerical simulations by employing different types of models to compute the final phase volume fractions at the environment temperature 3,4 and the kinetics of phase change during the ausferritic transformation 2,5–10 …”