Study and selection of most appropriate filler rod for GTAW of S32750 super duplex steel joints: A comprehensive study on microstructural, mechanical and corrosion properties

Abstract

In the present study, the effect of using different filler metals on the microstructure, mechanical and corrosion properties of gas tungsten arc welding (GTAW) of S32750 super duplex steel joints has been investigated. Three different filler rods of 316 L, S32202, and S32750 containing different concentrations of ferrite and austenite stabilizer elements were used to fabricate GTAW S32750 joints. From the microstructural studies, it was found that elongated structure of austenitic-ferritic transforms into a dendritic γ-austenite structure within an α-ferrite matrix, when S32202 and S32750 were used. While, the microstructure of sample welded by 316 L was consisted of dendritic δ-ferrite phase in the austenite grain boundaries. Owning the higher percentage of α-ferrite phase, the sample welded by S32202 filler rod showed the highest microhardness and tensile strength. The best combination of tensile strength (826 MPa), ductility (27%) and absorbed energy prior to fracture (51 J) was observed in the sample welded by S32750 filler rod due to the higher percentage of γ-austenite phase or a decrease in the percentage of the α and δ-ferrite phase in the weld microstructure. The Tafel polarization test showed that the highest and lowest corrosion resistance belonged to the samples welded by S32750 and 316 L, respectively. The presence of higher percentage of γ-austenite phase caused an increase in the corrosion resistance of the weld metal, while the presence of higher percentage of α and δ-ferrite caused a significant decrease in the corrosion resistance.