Tensile Property Evaluation of Cryogenic Structural Materials for the ITER TF Coil

Approximately 5,000 tons of cryogenic structural materials are used to manufacture the ITER TF coils. The materials are required to have a high yield strength at cryogenic temperature to ensure huge electromagnetic force. In a previous study, the yield strength at 4 K was predicted from the carbon and nitrogen (C+N) contents and yield strength at room temperature. Applying this prediction method, the chemical composition and yield strength at room temperature corresponding to the required yield strength at 4 K were standardized. Next, the specifications for the actual materials, including a margin for error, were determined. For this study, the authors perform tensile tests at 4 K using actual materials. A chemical analysis and tensile tests at room temperature using all of the actual materials are conducted. The tendency of the tensile properties at 4 K is evaluated considering these factors. Although the yield strength at 4 K is highly correlated with the C+N contents, the values predicted tend to be 11.2% higher than the experimental value. The cause for this discrepancy is presumed to be the effect of test speed used during tensile testing at room temperature and oversaturation of nitrogen. The yield strength of the actual materials is increases 56 MPa when the lower limit of C+N is raised 0.02. All materials satisfied the required values. The views and opinions expressed herein do not necessarily reflect those of the ITER organization.