The potential for SAGBO cracking in a Nb3Sn wind-and-react solenoid using a CICC with Incoloy 908 jacket and glass fabric insulation

Abstract

The 45-T Hybrid Magnet System being constructed for the National High Magnetic Field Laboratory (NHMFL) in collaboration with the Francis Bitter National Laboratory is composed of a resistive insert and superconducting outsert. Two of the outsert solenoids use Nb3Sn and will be wound with cable-in-conduit conductors (CICCs) using the wind-and-react technique. A candidate for the jacket material is Incoloy 908, but an obstacle to this choice is its sensitivity to Stress Accelerated Grain Boundary Oxidation (SAGBO). The conditions for SAGBO-residual stress, elevated temperature, and exposure to oxygen (even in extremely low levels)-are difficult to avoid in coils fabricated by the wind-and-react process. The glass fabric insulation, applied during winding, is at high temperature, both a source of oxygen (primarily in the form of absorbed water vapor) and, being highly compacted by the winding, an impediment to its removal. In attempt to solve this problem NHMFL has developed a pre-baking procedure in vacuum at 730 C, to be applied to the glass fabric before to insulate the coil. In order to test the efficiency of pre-baking and to check the existence of possible other features that also lead to SAGBO, a dummy coil, wound with a CIC conductor jacketed with Incoloy 908, has been manufactured. The coil has been heat treated in very high purity helium atmosphere. The results are reported.