Demonstration of full scale JJ1 and 316LN fabrication for ITER TF coil structure

Japan Atomic Energy Agency has manufactured JJ1, strengthened 316LN (ST316LN) forgings, ST316LN hot rolled plates to demonstrate for mass production of the materials for ITER Toroidal Field (TF) coil structure. Tensile, fracture toughness and fatigue test samples were taken from the products and tested to demonstrate mechanical properties of mass production material. Averaged value of 0.2% yield strength for the JJ1, ST316LN forgings and the ST316LN hot rolled plate are 1126 MPa, 1078 MPa and 1066 MPa, respectively. The JJ1 and ST316LN forgings showed uniform ultimate tensile strength (UTS) distribution along the thickness direction. In the ST316LN hot rolled plate with thickness of 200 mm, UTS showed some scattering caused by coarse gains which was locally observed at one quarter thickness. However, the mixed grain condition would be solved by modification of forging process, followed by a hot roll process. Measured fracture toughness KIC(J) was more than 200   MPa m for all products. A fatigue characteristics (S–N) of ST316LN have been measured at 4.2 K and design S–N curve were determined. It was confirmed that ST316LN has enough margins for a cyclic operation of 384 MPa with 3 × 104 cycles on the TF coil case. Therefore, the ITER mechanical requirements for TF coil structure material will be achievable in actual JJ1 and ST316LN manufacturing.