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

Nakajima, Hideo; Hamada, Kazuya; Takano, Katsutoshi*; Okuno, Kiyoshi; Fujitsuna, Nobuyuki*

In ITER, the conductor for the Central Solenoid (CS) uses Incoloy 908, which requires special environment control to prevent cracks caused by Stress Accelerated Grain Boundary Oxidation (SAGBO) during Nb3Sn heat treatment. Use of a stainless steel jacket can simplify the design and fabrication because the special caution is not required for SAGBO. JAERI has already developed high manganese, JK2, whose thermal contraction from 300K to 4 K is almost the same as that of Incoloy: thus no change in the mechanical design of the CS is necessary. However, during the heat treatment, phosphorus enhances embrittlement of JK2. Carbon reduction and boron addition was considered to be a possible solution to mitigate the phosphorus effect. Jackets of low carbon and boron added JK2(JK2LB) were produced and tensile properties, fracture toughness, and crack propagation rate were measured at 4K. Elongation and fracture toughness at 4K after the heat treatment are 33% and 91 MPam for the final jacket, which satisfy the ITER targets. JK2LB can be applied to the ITER CS.