Creep properties and microstructures of helium implanted AISI 316L electron-beam weld and parent material

Creep properties and microstructures of the “as-received” electron-beam welds and its parent material of the former Next European Torus (NET) reference material AISI 316L (now a candidate material for the International Thermonuclear Experimental Reactor (ITER) program) have been investigated at 873 K as a function of applied stress and pre-implanted helium concentration. The results show that helium embrittlement effects are more serious in the parent material than in the welds. The creep properties of the welds are almost unaffected by helium concentrations below 300 appm. Weld specimens with low helium concentrations (CHe < 100 appm) ruptured in the welded zone with a ductile “transgranular” fracture. In those with higher helium concentrations some regions of “intergranular” fracture were also found. Parent material specimens always ruptured in a mixed fracture mode. TEM observation revealed that in the welded zone δ-ferrite occupies about half of the boundary area. δ-ferrite partly transforms into precipitates (molybdenum rich carbides, M23C6 etc.) and austenite during high temperature treatments. Preferential helium bubble sites are grain boundaries, δ-ferrite—matrix interfaces, incoherent parts of twin boundaries and disloactions. Quantitative helium bubble size and number distribution results indicate that in the matrix helium bubble sizes are lager but the bubble densities are lower than at boundaries or interfaces. With increasing helium concentration helium bubble densities at the different sites do not change much, while the sizes increase with slopes of about 0.3 in a power law fit.