Mechanical Properties of Welded Joint at Cryogenic Temperature for Manufacturing of ITER TF Coil Structure

The ITER toroidal field (TF) coil structure is a huge welding stainless steel structure with a height of 16 m and a width of 9 m. The TF coil structure has a function to ensure the enormous electromagnetic force, whereas in ITER operation temperature (4 K). FMYJJ1, which is full austenitic stainless steel welding material having high manganese contents, is applied for manufacturing TF coil structure. However, it was reported that some micro cracks were observed on the reheating zone in the thicker welded joint during trials. Low ductility of deposited metal at 1000 °C causes these microcracks. In addition, segregation of sulfur in grain boundary decreased ductility at high temperature. Hence, reducing sulfur content is effective to reduce microcracks. Two types of FMYJJ1 were developed by applying two reducing sulfur content methods. In the previous study, applicability of these improved FMYJJ1 was tested within welded joints having 20-mm thickness. In order to confirm the validity of applicability of them, the Japan Atomic Energy Agency manufactured welded joints with 200-mm thickness, which is the representative thickness of the TF coil structure. When ductility is increased, there is a concern that yield strength is decreased. Hence, confirmation of the mechanical properties is needed. Therefore, side bend test, tensile test, and fracture toughness test at 4 K were performed. As the result, it was confirmed that the crack sensitivity was improved, and it was also confirmed that mechanical properties at 4 K satisfied ITER design requirements.