Work was undertaken to establish reliable time-temperature-transformation (TTT) diagrams for the cast duplex stainless steels CD3MN and CD3MWCuN. The latter contains higher Cr, Ni, and Mo contents compared to the former. The TTT diagrams for sigma-phase formation in both alloys were developed based on quantitative metallography results from optical and scanning electron microscopy. The kinetics of sigma-phase were found to be much faster in CD3MWCuN alloys than in CD3MN. An Avrami analysis was employed to assess transformation characteristics and to refine the TTT diagram determination. Theoretical prediction of phase equilibria using the thermodynamic software package ThermoCalc was compared to the amount of sigma phase observed experimentally from long-term heat treatments. The constructed TTT diagrams of the cast alloys were compared to wrought counterpart alloys.