EFFECT OF AGING ON MECHANICAL PROPERTIES OF 316LN AT 4.2 K FOR FUSION APPLICATIONS

Abstract

Within the fusion magnet technology the low-temperature superconductor Nb3Sn is usually used for high magnetic field in the range of 12 T and 4.5 K. These superconductors are produced as round strands. Here Nb3Sn filaments are embedded in a round copper matrix with a diameter of about 0.8 mm. To allow magnet windings of several Mega-Ampere to produce the needed magnetic field, about 900 superconducting strands are cabled and compacted in a stainless steel conduit resembling the so-called cable-in-conduit-conductor (CICC). However, the mechanically brittle Nb3Sn superconducting phase is produced by the diffusion reaction by a long-term heat treatment. Therefore, the magnet winding containing the superconducting strands together with the stainless steel jacket has to undergo this heat treatment. In order to simulate the magnet manufacturing process, seamless tubes were compacted, bend and straightened and tensile stretched by 2.5 % at room temperature followed by the heat treatment necessary for the Nb3Sn formation. The aim of this study was to compare the microstructures and tensile properties at cryogenic operation temperature of two modified 316LN austenitic stainless steels with very-low carbon (≤ 0.013) in the as built conditions and after heat treatments. Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and X-ray diffraction were used to study microstructure. Deformation behaviour was investigated by tensile test at 4.2 K.