Development of low carbon and boron added 22Mn-13Cr-9Ni-1Mo-0.24N steel (JK2LB) for jacket which undergoes Nb3Sn heat treatment

Japan Atomic Energy Research Institute has developed a low carbon and boron added 0.03C-22Mn-13Cr-9Ni-1Mo-0.24N-0.003B steel (JK2LB) for a conductor jacket of the central solenoid for the International Thermonuclear Experimental Reactor in collaboration with Kobe Steel Ltd. The feature of JK2LB is to have enough ductility and toughness after reaction heat treatment to produce Nb₃Sn. Tensile properties, fracture toughness, and fatigue crack propagation rate were measured at 4 K by using samples taken from an intermediate billet and the final jacket of JK2LB, which were produced with a mass production line. The average elongation and fracture toughness at 4 K after the heat treatment are 33% and 93 MPa√m for the final jacket, and 30% and 121MPa√m for the intermediate billet, respectively. The mechanical properties of weld metals were also evaluated and all data satisfied the ITER targets. It was also clarified that improvement of ductility and toughness were caused by reduction of chromium carbide precipitations due to low carbon and boron addition.