A model is developed for simultaneous prediction of the processing window and the hardenability (austemperability) of austempered ductile iron (ADI). A bainitic model for steels is calibrated for the upper and lower limits of the processing window according to experimental data from the literature. The austemperability is modelled by prediction of the point at which the pearlite curve nose in the TIT diagram is touched by the cooling curve at the centre of a 76.2mm ADI test bar. The outcome of the model presents the lower and upper limits of the processing window and austemperability results as a three-dimensional diagram. The model requires as its input the alloy composition, prior austenite grain size, the austenitising temperature, and austempering temperature. The result of the model is presented for a heavy section size ductile iron of composition (wt-%) Fe-3.36C-2.4Si-0.39Mn-0.25Cu-0.24Mo-0.04Ni-0.04Mg.