Cryogenic mechanical properties of high-manganese steel weldments

The structural alloys used in fusion reactor magnets of the next generation are required to have a 4.2 K yield strength and fracture toughness combination superior to that of alloys used currently. An alloy of nominal composition 18Mn-5Ni-16Cr-0.22N after proper thermomechanical treatment approaches the JAERI projected requirements and exceeds the proposed U.S. requirements for 4.2 K yield strength and fracture toughness in the base metal. However, the properties in the welded condition are still uncertain. This work investigates the effect of autogenous GTAW on the cryogenic properties of this alloy. It was found that the 4.2 K yield strength of the weld can be increased through the addition of nitrogen to a 75% He/25% Ar shield gas; specifically, a yield strength comparable to that of the base metal was achieved for a 6 vol% nitrogen addition. Fracture toughness data are still preliminary; however, autogenous welds made on this alloy approach the U.S. requirements for both yield strength and KIc