SHIELDING GAS OXYGEN ADDITIONS AS A MEANS OF CURBING NITROGEN DEGASSING DURING THE AUTOGENOUS ARC WELDING OF NITROGEN-ALLOYED STAINLESS STEEL

This study examined the influence of oxygen additions (0.5 % to 2.0 %) to argon-rich shielding gas on nitrogen degassing during the autogenous arc welding of a high-manganese nitrogen-alloyed austenitic stainless steel, previously commercially available under the trade name of Cromanite. Autogenous arc welding of this steel in inert shielding gas results in considerable nitrogen losses from the weld pool, characterised by an unstable arc, spattering and violent metal expulsion from the weld pool. Oxygen additions to the shielding gas stabilise the arc and curb nitrogen-induced porosity, but at least 2.0 % oxygen (by volume) is required to maintain the weld metal nitrogen content at the level of the parent material prior to welding. The beneficial effect of oxygen additions to the shielding gas is attributed to the formation of a solid MnCr(2)O(4) spinel phase on the weld pool surface during welding, which retards nitrogen degassing by reducing the area available for the adsorption of nitrogen atoms prior to their recombination to form N(2). This layer has a granular, irregular appearance and presents a less effective barrier to nitrogen degassing than the continuous, uniform liquid slag layer that forms when the Cr-Ni 300-series austenitic stainless steels are welded in oxygen-containing shielding gas mixtures.