Enhanced dissolution of nitrogen during gas tungsten arc welding of steels

Abstract

Although nitrogen concentrations at levels much higher than Sieverts' Law predictions during the arc welding of iron and steel are well established, there is currently no commonly accepted methodology to determine this concentration quantitatively. The nature and concentrations of various species in the plasma phase above the weld pool surface are therefore investigated in the present work using both theoretical and experimental techniques. A comprehensive thermodynamic analysis of the nitrogen containing plasma phase of a gas tungsten welding arc shows that ionised species dominate close to the electrode, whereas neutral monatomic and diatomic nitrogen are the primary species near the metal surface at plasma temperatures as low as 5000 K. When oxygen is added to a nitrogen containing plasma, the resulting nitrogen concentration in the weld metal is further enhanced. Definitive proof is provided for a mechanism in which nitrogen and oxygen species interact in the plasma phase at temperatures below 6000 K, resulting in a significant increase in the concentration of monatomic nitrogen. Furthermore, at plasma temperatures as low as 5000 K, the equilibrium monatomic nitrogen partial pressure is sufficiently high to cause nitrogen saturation in the weld metal. Emission spectroscopy of glow discharge plasmas validates both the species density calculations and the presence of NO in the nitrogen and oxygen containing plasmas.