FRACTURE TOUGHNESS MEASUREMENTS AND ASSESSMENT OF THIN WALLED CONDUIT ALLOYS IN A CICC APPLICATION

The Series‐Connected Hybrid Magnets under construction at the NHMFL use Cable‐in‐Conduct‐Conductor (CICC) technology. The 4 K mechanical properties of the conduit are extremely important to the performance and reliability of the magnets. We have measured tensile and fracture toughness of two candidate conduit alloys (Haynes 242 and modified 316LN) in various metallurgical states, with emphasis on the final state of production. To assess the material in its final production state, non‐standard specimens are removed directly from the round‐corner rectangular conduit and tested after exposure to a simulated $Nb3Sn$ reaction heat treatment. Non‐standard middle‐tension (MT) fracture toughness specimens enable toughness evaluation of the base metal, welds and weld/base transitional region in the as‐fabricated conduit with final dimensions not suitable for conventional fracture toughness specimens. Although fracture toughness tests of the thin walled conduit fail to meet ASTM test validity requirements they provide a qualitative evaluation and estimate of the fracture toughness of the conduit and the welds.