Cold work study on a 316LN modified alloy for the ITER TF coil conduit

In cable-in-conduit conductor (CICC) magnets, such as the ITER TF coils, the conduit is the primary structural component. This function creates requirements for 4 K strength, toughness, fatigue crack resistance, and ductility after exposure to the superconductor's reaction heat treatment. The tensile ductility of a steel is a quality factor related to fatigue and fracture resistance that can be evaluated more economically with tensile tests rather than fatigue and fracture tests. Here we subject 316LN modified base metal and welds to a range of cold work from 0% to 20% and a subsequent Nb3Sn reaction heat treatment to evaluate the effects on the tensile properties. With the addition of cold work, the 4 K yield strength increases while tensile elongation decreases in both the base metal and weld. The results are compared to previously published data on the same alloy to evaluate the use of tensile ductility parameters as a materials qualification specification in magnet design.