Friction stir welding is a manufacturing process with several positive outputs for duplex stainless steel joints, such as improved mechanical resistance and maintenance of optimal phase fraction. Duplex steels are vastly used in naval and petrochemical operations, being commonly manufactured and joined via welding processes. Thus, there is high demand for straightforward methods of thermal evaluation of friction stir welded joints. In this sense, numerical models are a practical tool for assessing the joints’ welding condition. This work focused on the development and application of a more complete coupled numerical model for friction stir welding of UNS S32205 plates in order to correlate processing conditions with microstructure evolution. A peak temperature of 1213 ºC at the joint’s central line was observed. Distributions of temperature and material flow through the joint cross-section indicate that a more intense material flow at retreating side favors coarser grain size. Simulation results indicated that the strain rate plays a more intense effect in microstructure development compared to the welding peak temperatures. The coupled numerical model was additionally used to obtain temperature profile of the tool, which was thermally stable even after standing temperature values greater than 1200ºC. Even after 18 cm of welding procedure, deterioration of tool was not observed, leading to sound welded joints.