Nitrogen Control During the Autogenous ARC Welding of Stainless Steel

Abstract

This study deals with nitrogen absorption and desorption during the autogenous welding of stainless steel, investigating the influence of the base metal nitrogen and surface-active element concentrations and nitrogen partial pressure in the shielding gas. The weld nitrogen concentration increases with shielding gas nitrogen content at low nitrogen partial pressures, but at higher partial pressures nitrogen absorption is balanced by N₂ evolution. This steady-state nitrogen content is not influenced significantly by the base metal nitrogen content in low sulphur alloys, but in high sulphur alloys, an increase in the initial nitrogen concentration causes higher weld nitrogen contents over the entire range of partial pressures evaluated. The weld metal saturation limit is reached at progressively lower shielding gas nitrogen contents as the base metal nitrogen level increases. It is postulated that less nitrogen is required in the shielding gas to reach the saturation limit in the high sulphur alloys because an appreciable fraction of the nitrogen already present in the base metal is prevented from escaping by a higher level of surface coverage. A kinetic model can be used to describe this behaviour. The desorption rate constant decreases with an increase in sulphur content, but the absorption rate constant is not a strong function of the sulphur concentration. The higher rate of nitrogen removal at the onset of steady-state behaviour causes higher-nitrogen alloys to require more supersaturation prior to bubble formation.