Effect of the addition of nitrogen through shielding gas on tig welds made homogenously and heterogeneously on 300 series austenitic stainless steels

Tungsten inert gas (TIG) welding of austenitic stainless steels is a critical process used in industries. Several properties of the welds must be controlled depending on the application. These properties, which include the geometrical, mechanical and microstructural features, can be modified through an appropriate composition of shielding gas. Researchers have studied the effects of the addition of nitrogen through the shielding gas; however, due to limited amount of experimental data, many of the interaction effects are not yet reported. In this study, welds were made homogeneously as well as heterogeneously with various concentrations of nitrogen added through the shielding gas. The gas compositions used were 99.99%Ar (pure), 2.5% N₂ + Ar, 5% N₂ + Ar and 10% N₂ + Ar. Additionally, the welding process parameters were varied to understand different interaction effects between the shielding gas chemistry and the process variables such as filler wire feed rate, welding current, etc. Strong interactions were observed in the case of heterogeneous welds between the gas composition and the filler wire feed rate, with the penetration depth increasing by nearly 30% with the addition of 10% nitrogen in the shielding gas. The interactions were found to influence the bead geometry, which, in turn, had an effect on the mechanical properties as well as the fatigue life of the welds. A nearly 15% increase in the tensile strength of the samples was observed when using 10% nitrogen in the shielding gas, which also translated to a similar increase in the fatigue life.