The Japan Atomic Energy Research Institute (JAERI) has successfully developed new cryogenic steels for the superconducting magnets of the Fusion Experimental Reactor (FER) in collaboration with 4 steel companies. JAERI required a strength-toughness combination (σ_y>1 200 MPa, K_lc>200 MPa√m, called JAERI box) that was beyond the capabilities of the existing austenitic stainless steels for cryogenic use. These requirements were determined from stress analyses, properties of the superconducting materials, crack propagation analyses, coil operation, etc. The new cryogenic steels were developed using tension, Charpy impact, and fracture toughness tests, step by step. The largest 4-K testing machine in the world, of maximum load 100 t, was installed at JAERI in order to evaluate the mechanical properties of large specimens and to investigate specimen size effects. The newly developed steels supplied from industrial heats (5-50 t) were named "Japanese Cryogenic Steels" (JCS), and their mechanical properties satisfied requirements mentioned the above. These steels have been already used in superconducting coils, such as JAERI's Demo Poloidal Coils (DPC).
This paper reviews the development of the JCS for the superconducting magnets of the FER.