ESSC & DUPLEX 2019

In duplex stainless steels (DSS) the desired microstructure after solution heat treatment is a ferritic matrix with austenite islands, maintaining approximately equal amounts of both phases. However, during several manufacturing processes, such as welding, the formation of deleterious phases could happen, and the one of greater influence is the sigma phase, resulting in loss of corrosion resistance and toughness. The present study worked on developing DICTRA ® simulation models that could describe the volume fraction of sigma as a function of the aging time of DSS, obtained in previous experimental works of these research team. Simulation models analysed have planar or spherical symmetry with different dimensions for ferrite and austenite. It is concluded that the simulation model which best describes the formation of sigma phase is a spherical configuration with austenite in the centre, surrounded by ferrite, placing sigma as active phase between the two former phases with negligible thickness. It was assumed ferrite and austenite volume fractions and compositions as the same obtained in Thermo-Calc ® equilibrium calculations of the solution treatment temperature, and sigma phase composition as obtained in equilibrium simulation at the aging temperature studied.