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.
Highlights
Large-scale superconducting magnets are often built applying the cable in conduit conductor (CICC) technology [1]
We examine the influence of cold working and long-term aging microstructure and mechanical properties at 4.2 K of 316LN stainless steel
Tensile tests were performed at 4.2 K on sub-size specimens which were cut by electrical discharge machining with a width of 12.5 mm and a thickness of 2 mm (ASTM E8M standard), with a displacement rate of 0.5 mm/min and a gauge length of 50 mm using a universal servo-hydraulic testing machine [19] equipped with a cryostat system
Summary
Large-scale superconducting magnets are often built applying the cable in conduit conductor (CICC) technology [1]. Depending on the necessary electrical current of such superconducting cables and the magnetic field at the conductor, this technology implements the possibility to optimize the inductance and a force flow cooling with Helium to a temperature of about 4.5 K. For magnetic fields lower than 10 T NbTi alloys are typically used as superconductors, e.g. in the Large Helical Device (LHD) [2], in Wendelstein-7X [3] and in the newly built Japan Torus JT60-SA [4]. Going to even higher magnetic fields above 11 T Nb3Sn has to be used. The largest magnetic confinement experiment under construction using this alloy is ITER [5].
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