INFLUENCE OF NITROGEN GAS OF THREE DIFFERENT PROCEDURES OF GTAW WELDING ON FINAL METALLURGICAL AND MECHANICAL PROPERTIES OF A 2 W SDSS PIPING

摘要

Duplex stainless steels (DSS) are finding increased application on offshore process plants in Brazil. Welding them is always a challenge. Pure argon and argon/low-nitrogen gas mixtures are available for welding them with GTAW process. The use of the same kind of gases on shielding and backing is usual. If no nitrogen is present in the gas mixtures there may be loss of nitrogen from the weld metal or HAZ. Three different welding procedures were tested for a 2% tungsten superduplex parent metal to verify the loss of nitrogen from weld metal and HAZ. First procedure used a 2%N-88%Ar-10%He as the shielding mixture and pure Ar as the backing gas while the second one used pure Ar as shielding and pure nitrogen as backing gas. A third procedure with no nitrogen for shielding or backing was also carried out for comparisons. Chemical analyses, corrosion test and determination of austenite/ferrite content were done to check metallurgical and chemical properties. Mechanical tests were performed to compare mechanical properties of the weld metal and HAZ. ASME Code, section IX, plus Norsok standard M-601 were referenced. Results indicate good mechanical behavior, corrosion resistance and austenite/ferrite phases balance in both procedures that used nitrogen as one of the gases. The procedure using only pure argon showed a decrease of austenite content in weld metal and heat affected zone. However, it fulfilled all the requirements. They didn't present secondary phases or very significant variances on austenite presence even on weld metal and HAZ.