Measurements of austempering kinetics and mechanical properties are presented as a function of austempering time over the range 1-4320 min for different combinations of austempering temperature (275, 315, 370 and 400 degrees C) and austenitizing temperature (870, 900 and 950 degrees C) for a ductile iron of composition 3.5% C, 2.6% Si, 0.48% Cu, 0.96% Ni, 0.27% Mo and 0.25% Mn. The austempering kinetics are used to calculate processing windows for the three austenitizing temperatures. The mechanical properties are analysed to show that the processing windows accurately predict the austempering times over which the mechanical properties satisfy the ASTM standard. The analysis shows the role of austenitizing temperature, austempering temperature and time in optimizing the mechanical properties.