In the last few years, the effect of Nitrogen as a strong stabilizer for Austenite have been understood which results in improvement on mechanical properties and corrosion resistance of the weld joint in the duplex stainless steels. In the current paper, the investigation of Nitrogen addition to the shielding gas on microstructure, mechanical properties, and corrosion behavior of the welded sections of UNS S32750 super-duplex stainless steel was performed. The GTAW process with DCEN polarity was chosen accompanied by ER 2594 filler electrode. To study the effect of Nitrogen, pure Ar, Ar + 2% N 2 , and Ar + 5% N 2 , gases were used as the shielding gas. Heat input was controlled in the range of 0.5-2 Kj/mm. microstructure of the samples were analyzed using optical microscopy, scanning electron microscopy, and X-ray diffraction techniques. In order to evaluate the distribution of alloying elements in Ferrite and Austenite phases, Energy Dispersive Spectroscopy (EDS) was used both before and after welding. Mechanical properties were investigated by means of micro-hardness, room temperature tensile strength, and Charpy impact techniques. Also, cyclic polarization test was used in 3.5 wt% NaCl solution at room temperature to study the corrosion behavior. The results showed that by increasing the Nitrogen content of the shielding gas from 0 to 5%, the amount of Austenite phase increased from 35 to 50 volume percent. A Cr rich phase was identified in pure Ar and Ar + 2% N welded specimens. Due to the higher Ferrite content and existence of Chromium Nitride phase in the Heat Affected Zone (HAZ) of all samples, highest average hardness value was attributed to this zone. Average hardness value of the sample with pure Ar gas was 240 HV which drop down to 220 HV due to increasing amount of Austenite phase. Highest tensile strength was recorded in Ar+5% N 2 sample. All samples were softly fractured from base metal except from the sample with pure Ar shielding gas. In the impact test, highest impact resistance was recorded for 48 J in Ar+5% N 2 specimen and only 40 J impact energy was recorded in the absence of N 2 which is lower than that of the base metal. In the cyclic polarization test, pitting potential was increase from 1.07 V to 1.18 V by increasing the N 2 content. Resistance to pitting growth which is determined by the size of Hysteresis loop was enhanced with presence of N 2 and reached to its peak in Ar+5% N 2 specimen. The weakest corrosion behavior was attributed to pure Ar shielding gas which demonstrated a low ability to re-passivation. Keywords: Super-duplex Stainless Steel, Nitrogen, Pitting Corrosion, Welding, mechanical Properties
