Weldability of Neutron Irradiated Stainless Steels by YAG Laser and Low-Heat-Input TIG Welding Techniques

Type 304 and 316L stainless steels (SS) were irradiated at ambient temperature in the U.S. Advanced Test Reactor up to 2×10²⁵n/m² (E>1MeV). Weldings were performed under various weld heat-input levels from 1 to 10 kJ/cm for specimens containing up to 29 appm helium (He). After the welding, cross-sectional observation and mechanical property examinations such as tensile and side bend tests were carried out. In the case of the welding with a heat-input of 1 kJ/cm, no weld cracks were found in irradiated 304SS with all of He concentrations tested, and in the case of 2 kJ/cm, no weld cracks were also done in up to 9 appmHe 304SS. On the other hand, 316LSS contained only 0.5 appmHe was cracked by welding at 2 kJ/cm. These results clearly suggested that susceptibility of weld cracking in irradiated 316LSS was remarkably higher than that in 304SS. The mechanical properties were in compliance with their acceptance criteria in the weld joints without cracks. The results of SEM and TEM observations showed these weld cracking was attributed to He cavities formed along grain boundaries, which were made from transmuted He atoms aggregated by tensile stress and heating during welding. In the same He concentration and weld conditions, the cavity density in 316L was about one order magnitude higher than that in 304SS. The computer simulation method for irradiated SS welding was developed based on the He aggregation model during welding. The sound repair-welding conditions, which could be applied actual plants, were proposed using the simulation method.