INFLUENCE OF SHIELDING GAS-COMPOSITION AND WELDING PARAMETERS ON THE N-CONTENT AND CORROSION PROPERTIES OF WELDS IN N-ALLOYED STAINLESS-STEEL GRADES

TIG welding using different heat inputs, are lengths and shielding gas nitrogen contents was performed, The aim was to evaluate the possibil ities to avoid nitrogen losses on welding or even increase the weld me tal nitrogen content and thereby improve the corrosion properties and, in the case of duplex grades, also: to improve the phase balance. Thr ee different nitrogen-alloyed duplex grades and one superaustenitic gr ade were investigated. The corrosion resistance in terms of CPT, Criti cal Fitting Temperature, of the super duplex material was found to be strongly correlated to the nitrogen content of the weld metal. In the case of the superaustenitic weld metal, the increased nitrogen content was found to be associated with an increased pore formation, leading to a lower corrosion resistance and thereby masking the positive effec t of the increased nitrogen content. In order to illuminate the nitrog en exchange reactions between the are, weld pool, and shielding gas, s tationary weld experiments were also performed. The results from these stationary trials indicated that the net weld pool nitrogen content c ould be qualitatively understood if the various fluxes involved in the nitrogen transport between the plasma are, weld pool and weld pool-sh ielding gas were considered. At short times; the weld pool was limited in area and the nitrogen content of the weld pool increased due to hi gh nitrogen activity in the are. At longer times the nitrogen escaped from the weld pool to the shielding gas. This flux became then the dom inating factor due to the increased weld pool area exposed to the shie lding gas. The situation then approached the equilibrium conditions th at were expected from the gas nitrogen activity and weld pool alloy co mposition according to thermodynamic calculations using the Thermo-Cal c database.