Weldability of Duplex Stainless Steels - Thermal Cycle and Nitrogen Effects

Much development work has been performed worldwide on the welding of duplex stainless steels (DSSs), where losses in base metal nitrogen during welding have been compensated by filler metal and nitrogen additions to the shielding gas.However, some heat-affected zone (HAZ) microstructural changes have not always responded consistently to nitrogen mixtures added to the shielding gas.In this work, DSS weld microstructures were investigated as a function of weld thermal cycles and shielding gas nitrogen content.Physical simulations, actual gas tungsten arc welding (GTAW) in controlled atmospheres, and entrapped nitrogen measurements were used to relate to the weld austenite-to-ferrite (A/F) phase ratios.Results showed the thermal cycles had a stronger effect on the weld A/F ratio than shielding gas nitrogen content.Physical simulations and actual GTAW trials also showed the A/F ratio changes in the opposite way in the fusion zone than in the HAZ.Reheating was also found to have an important effect on the microstructure.The above findings should allow fabricators to better design the welding process with DSS.Better control of nitrogen-containing shielding gas mixtures, along with heat input limitations, should result in more consistent weld properties and wider use of DSS by industry.