ENHANCED DISSOLUTION OF NITROGEN DURING GAS TUNGSTEN ARC-WELDING OF STEELS

Although nitrogen concentrations at levels much higher than Sieverts' Law predictions during the arc welding of iron and steel are well esta blished, there is currently no commonly accepted methodology to determ ine this concentration quantitatively. The nature and concentrations o f 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 f or a mechanism in which nitrogen and oxygen species interact in the pl asma phase at temperatures below 6000 K, resulting in a significant in crease in the concentration of monatomic nitrogen. Furthermore, at pla sma 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 valida tes both the species density calculations and the presence of NO in th e nitrogen and oxygen containing plasmas.