Evaluation of mass production results of cryogenic structural stainless steels for ITER toroidal field coil

量研機構はITER計画において主要機器の一つであるTFコイルの製作を担当している。TFコイルは４Kという特殊な環境で使用されることことから、構造材料には量研機構が開発・調整した特別なステンレス鋼が用いられる。特殊ステンレス鋼の総数は2000部材以上、総重量は5000トンを超える。量研機構ではこれらすべての材料に対し品質管理試験として室温機械試験、冶金試験を行うとともに、４K及び７７K引張試験特性を抜き取り検査することで、構造材料の品質を確保してきた。本発表では、品質管理検査並びに抜き取り検査の結果を評価することで、ITER TFコイル製作の成果の一つとして極低温用ステンレス鋼の特徴を明らかにするとともに、将来の核融合炉用極低温用構造材料開発の一助に資する。27th International Conference on Magnet Technology (MT27

Evaluation of mass production results of cryogenic structural stainless steels for ITER toroidal field coils

The National Institutes for Quantum Science and Technology (QST) has been developing new cryogenic structural materials, such as JJ1 stainless, and nitrogen-enhanced 316L stainless, known as 316LN, since the 1980s for increased tensile strength to comply with the ITER requirements. QST has also developed prediction curves for yield stress and tensile strength at temperatures from room temperature to 4 K based on the actual properties of these materials at room temperature to reduce the number of tensile tests conducted at 4 K, which are costly. After material trials and qualifications, the manufacture of material candidates began in earnest from 2012. In total, over 2400 materials (over 5000 tonne) were manufactured by six fabricators until manufacturing was completed successfully in 2020. The material properties of each material candidate were evaluated by QST and added into a database registry. Sampling tests, e.g., tensile tests were conducted at 4 K and 77 K. The correlations between the mechanical and material properties were evaluated to optimize material specifications of future superconducting magnets. The accuracy of QST\u27s prediction curves was also confirmed. This paper summarizes the evaluation of the material properties and the prediction accuracy of mass-produced materials for the ITER toroidal field coil cases