Numerical Simulation and Experimental Results of in situ Heat Treated Austempered Ductile Iron

Generally, a three-step heat treatment consisting of austenitizing, quenching and austempering is necessary to produce ADI. Using an in situ heat treatment the nodular cast iron casting was ejected at a temperature above 870°C and directly quenched to 300–400°C for austempering. Because the austenitization is no longer necessary in this method a significant reduction of energy and production time can be achieved. To guarantee the casting to be ejected with a well defined temperature distribution a permanent mould casting process was used.For predicting the microstructural evolution during the solidification and the subsequent ausferrite reaction during austempering a simulation model was developed for ternary Fe-C-Si alloys. For the simulation it was assumed that eutectic grains are formed below the eutectic temperature and that their growth rate is controlled by carbon diffusion through the austenite shell. Carbon and silicon concentrations at the phase interfaces are calculated from the Fe-C-Si phase diagram obtained from the thermodynamic phase equilibrium calculation tool, ChemAppTM. The pursuing solid state transformation is described by the growth of the ferrite shell into the austenite, which is controlled by carbon diffusion in a quasi-steady state. The distribution of temperature, nodule counts, nodule sizes and phase fractions in the casting were calculated from mass conservation. The transformation from austenite to ausferrite was calculated by the modified Avrami equation with the additivity rule. The simulation results were verified by experimental results. Mechanical properties show interesting potentials for application.