Effect of Helium Concentrations on Weldability of Irradiated Stainless Steel 304

Abstract

Radiation-induced degradation of the nuclear reactor materials such as austenitic stainless steels is a major concern to the aging nuclear fleet, and welding solutions will be required if repair of these components is deemed necessary. However, the weldability of the highly irradiated materials with conventional arc welding techniques is significantly diminished due to both the existence of helium in the materials and the associated intergranular helium-induced cracking (HeIC) in the heat-affected zone under the influence of high temperatures and high tensile stresses. In this study, improved laser welding techniques, including the novel Auxiliary Beam Stress Improved (ABSI) technique that proactively manages the stresses during laser repair welding, are applied to welding of boron-doped irradiated SS 304L, as well as SS 304 samples harvested from the National Research Universal (NRU) reactor. The helium concentration in these samples ranges from 3 to 44 atomic parts per million (appm). A variety of welding conditions were investigated to explore suitable welding techniques and parameters and to determine the upper helium concentration limit as a function of welding parameters. The experimental results showed that both the maximum fusion zone void size and total heat affected zone helium-induced crack length generally increased with increasing He levels. A proper combination of welding parameters, including laser power, weld travel speed, wire feed speed and ABSI, were shown to improve the welding quality, and potentially reduce the length and probability of the formation of grain boundary helium-induced cracking and voids within irradiated SS 304.