The research on high-strength CICC jackets with YS > 1500 MPa at 4.2 K for future fusion applications

Developing cryogenic structural materials with high strength and toughness is a challenge for the high-field superconducting magnet of the future fusion reactor. 0.2% proof stress should be over 1500 MPa and the fracture toughness KIC should be better than 130 MPa·m1/2 at 4.2 K for future cable-in-conduit conductors (CICC) jackets. Thus the cryogenic structural materials of modified 316LN, JK2LB, Incoloy 908, and JJ1 developed by ITER do not meet the requirements. Based on Nitronic 50 (N50) super-austenitic stainless steel material, the modified N50 (N50H) developed in China shows excellent low-temperature mechanical properties by completely eliminating δ ferrite and strictly controlling the carbon content of the alloy. Meanwhile, the jackets of CICC were prepared with N50H material for the first time, and some R&D work has been done. The N50H jackets have been compacted to final dimensions, bent to a radius of r = 2000 mm, straightened, and aging treated. The yield strength of N50H circle-in-square jackets is greater than 1550 MPa, and the fracture toughness KIC is better than 150 MPa·m1/2 at 4.2 K. The fatigue crack growth rate (FCGR) of the N50H jacket is lower than that of the modified 316LN, JK2LB and close to Incoloy 908. This study will present experimental data of modified 316LN, JK2LB, Incoloy 908, JJ1, and N50H. Moreover, this paper discusses the feasibility of N50H as a high-magnetic field cryogenic structural material for next-generation fusion reactors.