AbstractThe long term aging behaviour of a large number of Mo bearing and Mo free heats of cast duplex stainless steels has been studied between 300 and 450°C in terms of hardness and impact Charpy toughness. The results demonstrate the importance of the ferrite, Cr, and Mo contents in controlling aging response. Owing to the complexity of microstructural evolution and related changes in fracture mechanisms, the extrapolation of higher (i.e. 450 and sometimes 400°C) towards lower (i.e. service) temperature is not straightforward. Aging kinetics cannot be accounted for with a single activation energy Arrhenius plot; instead, a range of energies increasing with decreasing temperature must be considered. In the lower temperature range, this energy is of the order of 250 kJ mol−1, in agreement with the metallic element controlled mechanisms of microstructural evolution.MST/1186