Cryogenic properties of new austenitic stainless steel for fusion reactor superconducting magnets

A non-magnetic austenitic stainless steel has been developed, which has an adequate combination of yield strength and fracture toughness at liquid helium temperature, as a candidate structural material for fusion reactor superconducting magnets. The salient feature of the steel is its low-temperature toughness preserved after aging equivalent to the wind-and-react heat treatment of Nb 3Sn superconducting coil. The steel also keeps its toughness after electron beam welding. Its composition is characterized by addition of vanadium and nitrogen to 25Cr-14Ni-0.5Mo austenitic stainless steel. Microscopic observations have suggested that finely dispersed precipitates, presumably vanadium carbonitrides, are responsible for the excellent cryogenic mechanical properties.