Super duplex stainless steel is a kind of duplex stainless steel that has over 40 pitting resistant equivalent number. UNS S32750 is a common super duplex stainless steel. That has a lot of applications in oil industries. Therefore, joining of these steel by welding is important and a most common problem in welding of these materials is decrease in corrosion resistance and mechanical properties after welding. In this research, UNS S32750 has been joined by friction stir welding (FSW) and gas tungsten arc welding (GTAW) method. The tool that used in this research is made of WC with 16mm shoulder diameter and 5mm pin diameter. For GTAW method the AWS ER2594 has been used as the filler metal. Having a lower heat input for both methods, X-ray diffraction showed that no harmful phases such as Sigma and Chi had been formed in the weld zone. The microstructure indicated grain refinement with reduced size in the stir zone. Moderate microhardness of base metal was 285 Vickers but in the stir zone microhardness increased to 360 Vickers. The cyclic polarization test determined that potential and current density of joint metal by friction stir welding method was similar to base metal. For GTAW welded specimens, the microstructure showed that final austenite phase was grown in ferrite matrix with dendritic structure. Hardness in weld zone of GTAW welded specimen decreased to 250 Vickers because of increase in austenite volume percentage. However, because of decrease in austenite volume percentage and precipitation of chromium carbides, the hardness increased in heat affected zone (HAZ). The microstructure figure revealed that ferrite percentage in stir zone didn’t decrease very much, in comparison to GTAW, because in FSW process the heat input is much lower. The cyclic polarization test on GTAW welded specimens showed that increasing in heat input, increases the pitting current density. Additionally, increasing heat input in GTAW process results in more decrease in hardness and tensile strength because of increase in grain size of weld zone. Keywords: Super duplex stainless steel, Friction Stir Welding, Gas Tungsten Arc Welding, Microstructure, Mechanical properties.
