Effect of activated flux and nitrogen addition on the bead geometry of borated stainless-steel GTA welds

Borated stainless steels (304B) are used in nuclear power plants as control rods, shielding material, spent-fuel storage racks and transportation casks as they have a high capacity to absorb thermal neutrons.In this study, bead-on-plate welds were made on 10-mm-thick 304B plates using gas tungsten arc welding with Ar and Ar+2% nitrogen as the shielding gases, activated-flux GTA and electron-beam welding processes.The effects of the activated flux and nitrogen addition to the weld metal through the shielding gas, on the microstructure, bead geometry and mechanical properties were investigated.Activated-flux GTA welding and electron-beam welding substantially enhanced the depth of penetration and the aspect ratio compared to the other processes.Full-penetration welds were obtained in a single pass using activated-flux GTA and EB welding.The fusion-zone (FZ) microstructure of an activated GTA weld exhibits a columnar dendritic structure with eutectic borides in interdendritic regions, while a fine equiaxed dendritic structure was noticed in EB welds.GTA, nitrogen-added GTA and activated-flux GTA welds exhibited a partially melted zone adjacent to the fusion zone, with the activated-flux GTAW process resulting in a significantly thinner partially melted zone (PMZ).No PMZ was noticed in the EB welds.All the welds exhibited a high joint efficiency and impact toughness equal to those of the base material.It is concluded that the activated-flux GTA and EB welding processes are advantageous due to the use of a low heat input and failure location.