Determination of the formation mechanism of sigma and CHI phases in the duplex stainless steel in fusion welding with modeling

Duplex stainless steels have high corrosion resistance and excellent mechanical properties. Because of these specifications, duplex stainless steels are used in several applications such as architecture, petrochemistry, off-shore, desulfurization units and desalination. Duplex stainless steels are characterized by ferritic and austenitic phase structure, offering a combination of corrosion resistance and mechanical properties. Depending on intended use, such as heat treatment or welding processes, ferrite/austenite phase balance will change. Also during the cooling, formation of intermetallic phases affectly mechanical properties and corrosion resistance, occurs. In this thesis, to determine the mechanism of the secondary phase precipitation, SAF 2205 duplex stainless steels welded with flux cored wire. During the welding process, thermal distributions are determined by finite element method by using ANSYS Workbench Academic Research. The cooling rate that occurs during the welding process calculated theoretically and compared with the modeling results to determine the heat transfer efficiency. In welding, the heat affected zone occurs by the effect of the heat input. As a result of this, the rapid microstructural changes can be induced by phase transformation during the cooling. The secondary phases precipitate from the relevant temperatures in the heat affected zone. For the calculation of the secondary phase precipitation rate, the simulation of the heat affected zone was performed by the heat treatments. The Johnson-Mehl-Avrami-Kolmogrov (JMAK) equations used for determination of the precipitation mechanism.