Electron beam weldability of nitrogen strengthened austenitic structural materials for fusion magnets

In this work the application of the electron-beam (EB) technique for the joining of structural materials used in fusion technology is reported. The selected structural material for the casings of the superconducting torus TESPE is a fully austenitic AISI 316 LN type stainless steel (SS). EB-welding of this type of steel brings serious problems because of the depletion of the dissolved nitrogen, resulting in metallurgical damage of the joints. The variation of the welding parameters and their effect on weld quality are studied. The major weld parameter for avoiding weld pool porosity is the heat input. For a 15 mm thick butt joint (L-type) the welding parameter is optimized by different EB-weld tests. It is demonstrated that a 0.3 mm foil, used as a filler material, was capable of dissolving excess nitrogen during the welding period resulting in a significant reduction of nitrogen depletion in the weld metal. (author)