Simultaneous prediction of austemperability and processing window for austempered ductile iron

Abstract

A model is developed for simultaneous prediction of the processing window and austemperability of austempered ductile iron (ADI). The processing window represents a frame of time and temperature in which ADI satisfies optimum mechanical properties defined by ASTM A897M:1990. Austemperability is the maximum section size of ductile iron that can be austempered without formation of pearlite during the austempering process. The outcome of the model presents the processing window and austemperability as a three dimensional diagram (processing - austemperability window). The processing window boundaries are estimated according to a model for prediction of the time for ausferritic reaction in ADI. The austemperability of ductile iron is predicted according to the estimated pearlite curve of the TTT diagram and a mathematical model that simulates conduction of heat in a solid cylinder. The heat transfer model is calibrated for a ductile iron of composition (wt-%) 3.63C, 2.4Si, 0.39Mn, 0.4Mo, 0.25Cu, 0.04Ni, 0.04Mg. The model for the processing - austemperability window is validated for a ductile iron of composition (wt-%) 3.41C, 2.46Si, 0.36Mn, 0.18Mo, 0.25Cu, 0.036Mg at 285, 380, and 400 ° C austempering temperatures. Results show that the material satisfies ASTM A897M:1990 standard for the chosen experimental points within the processing - austemperability window without formation of pearlite in the microstructure.