Comparison of the welding behavior of P/M borated and I/M borated stainless steel

Borated stainless steels are being used to control neutron criticality in reactors as control rods, shielding material, spent fuel storage racks and transportation casks. Borated stainless steels fabricated via powder metallurgy (P/M) route (grade A) exhibit superior neutron absorption, enhanced toughness and ductility compared to cast ingot metallurgy (I/M) steels (grade B), due to fine and uniform dispersion of borides. In this study, welds were made using both 304B grade A and grade B materials by gas tungsten arc welding and activated gas tungsten arc welding processes. The effect of the activated flux on the bead geometry, microstructure and mechanical properties was investigated and compared. The heat input required for P/M steel material to produce full penetration welds has been found to be significantly higher compared to I/M steel, while all welds exhibited high joint efficiencies. Metallographic examinations revealed that no porosities and shrinkage cracks have been observed either in the fusion zone or in partially melted zone of the weldments. The fusion zone structure of 304 P/M type consists of a higher amount of eutectics with less number of austenite dendrites compared to I/M type. Both P/M and I/M steel welds exhibited partially melted zones adjacent to fusion zone. P/M steel welds were found to fracture in the partially melted zone, where a significant loss in hardness was noticed due to irregular boride eutectics, while in case of I/M steel welds, failure occurred in base material. It has been concluded that the P/M steel 304B grade A exhibits good GTA weldability compared to grade B material.