Microstructure and Mechanical Properties of ITER Correction Coil Case Material

The modified 316LN austenitic stainless steel was selected as ITER correction coils case material to provide structural reinforcement to the winding pack. Considering the case structure, high-assembling accuracy and other strict requirements, 316LN in special extruded form has been developed. In the present study, the microstructure and mechanical properties of the material were investigated. The microstructure of the 316LN material was analyzed by means of the optical microscopy, transmission electron microscope, and X-ray diffraction. It was observed that the material presents fine grain size and a single austenitic phase. Moreover, the intergranular corrosion resistance of the 316LN was evaluated and the results indicated that it exhibited a remarkable intergranular corrosion resistance. The tensile properties of materials were measured at both room and cryogenic temperatures. The 0.2% offset yield strength (R
_p0.2
), ultimate tensile strength (R
_m
), and elongation at break (A) at 4.2 K were determined to be higher than 800 MPa, 1500 MPa, and 40%, respectively. Furthermore, the J-integral fracture toughness of the 316LN was tested through means of an unloading compliance method at 4.2 K and the plane strain fracture toughness K
_ιc
converted from the J-integral are well above the specified value of 180 MPa√m. In addition, the fatigue test and fatigue crack growth rate of the 316LN stainless steel were also investigated at 4.2 K. According to these results, all requirement of the case material including uniform microstructure, excellent corrosion resistance, and good mechanical properties established by the ITER IO are confirmed.