Cryogenic structural materials having high strength and high toughness at 4 K are one of essential elements to fulfill the performance of toroidal field (TF) coils for fusion facilities, in which a large electromagnetic force is generated. The National Institutes for Quantum and Radiological Science and Technology (QST) has developed cryogenic structural materials for ITER TF coils and central solenoids (CS). Two review papers that holistically describe the history of development of cryogenic structural materials at QST for over 30 years, as well as their applications to ITER TF coils and CS, were published in TEION KOGAKU in 2013. This review paper complements those papers, focusing on the 4 K mechanical properties of the base and weld metal of the cryogenic structural materials and material code, which are necessary for applying these materials to the TF coils. The effects of carbon and nitrogen on the strength, relation between strength and toughness, fatigue properties, technical background of material code published by the Japan Society of Mechanical Engineers (JSME), and so on are explained in this paper. The status and vision of the materials development towards TF coils for future fusion facilities are also briefly described.

The ITER Central Solenoid (CS) conductor conduit requires high yield strength (> 850 MPa at 4 K), fracture toughness (> 130 MPam^1/2 at 4 K), and smaller thermal expansion characteristics and fatigue crack growth rate than those of conventional stainless steel. High manganese austenitic stainless steel, JK2, has these characteristics. However, the fracture toughness of JK2 deteriorates after Nb₃Sn heat treatment at 650ºC. Low carbon and boron-added JK2 (JK2LB) has been developed to improve the fracture toughness after heat treatment. Additionally, cold-working in the conductor manufacturing process and the coil winding process deteriorate fracture toughness. The nitrogen and boron contents of JK2LB have been optimized taking the impact of cold-working on mechanical performance into account. Accordingly, the mechanical performance of the conduit produced meets ITER requirements. Finally, a total of 6,200 conduits have been manufactured. This paper introduces the development process for JK2LB, and the mechanical characteristics of JK2LB are summarized.

JJ1 steel was developed through joint research between the Japan Atomic Energy Research Institute (JAERI, now the National Institute for Quantum and Radiological Science and Technology, QST) and Japan Steel Works, Ltd. (JSW). This steel became the structural material for the superconducting magnet coils of fusion reactors in the early 1980s. JJ1 steel is a non-magnetic steel that demonstrates good mechanical and metallurgical properties at liquid He temperatures (4 K) as well as good weldability. JJ1 steel is being used as the structural material for the Toroidal field (TF) coil of the International Thermonuclear Experimental Reactor.

High nitrogen austenitic stainless steel forgings with heavy thickness and complex configuration are used for the ITER TF coil case. In the forging process for high nitrogen austenitic stainless steel, it is generally difficult to simultaneously obtain good ultrasonic attenuation, the required mechanical properties and the desired near-net shape. Accordingly, JCFC undertook development of an innovative forging method to form the near-net shape of the product, through consideration of possible methods and laboratory testing. By applying JCFC’s innovative method in trial manufacturing commissioned by QST, it was confirmed that the resulting forgings had material properties that satisfy ITER requirements and were also formed into a near-net shape. Subsequently, JCFC manufactured the actual forgings.

Approximately 5,000 tons of cryogenic structural materials are used to manufacture the ITER TF coils. The materials are required to have a high yield strength at cryogenic temperature to ensure huge electromagnetic force. In a previous study, the yield strength at 4 K was predicted from the carbon and nitrogen (C+N) contents and yield strength at room temperature. Applying this prediction method, the chemical composition and yield strength at room temperature corresponding to the required yield strength at 4 K were standardized. Next, the specifications for the actual materials, including a margin for error, were determined. For this study, the authors perform tensile tests at 4 K using actual materials. A chemical analysis and tensile tests at room temperature using all of the actual materials are conducted. The tendency of the tensile properties at 4 K is evaluated considering these factors. Although the yield strength at 4 K is highly correlated with the C+N contents, the values predicted tend to be 11.2% higher than the experimental value. The cause for this discrepancy is presumed to be the effect of test speed used during tensile testing at room temperature and oversaturation of nitrogen. The yield strength of the actual materials is increases 56 MPa when the lower limit of C+N is raised 0.02. All materials satisfied the required values. The views and opinions expressed herein do not necessarily reflect those of the ITER organization.

Full austenitic stainless filler metal, FMYJJ1(12Cr-14Ni-10Mn-5Mo-0.13N), for Tungsten Inert Gas (TIG) welding has been developed to realize high strength and toughness at the temperature of liquid helium, and is to be applied for manufacturing the toroidal field coil case (TFCC) of the ITER. Before applying this filler metal in the actual production of a TFCC, 30 weld joints were trial-manufactured in order to confirm welding quality, including weldability, metallurgical properties, and mechanical properties at room temperature and liquid helium temperature. This paper provides a summary of weld joint mechanical properties, such as tensile strength, fracture toughness, fatigue crack growth and fatigue properties, which were obtained in the qualification phase at the temperature of liquid helium. In addition, in order to clarify correlations between welding control parameters and mechanical properties, the results of comparisons with other parameters are provided.
   The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

The hot isostatic pressing (HIP) diffusion bonding process is the one of rationalization processes to manufacture radial plates of superconducting magnets used in fusion reactor because of its small deformation and applicability to complex shapes. In order to establish the design and manufacturing processes, trials using small and actual-size prototypes have been carried out. The prototypes were manufactured using FM316LNH hot-rolled plates having a grain diameter of approximately 74 μm. The HIP bonding process makes the grains coarse and the tensile and fatigue strength are reduced. Meanwhile, forged FM316LNH has been applied to manufacture TF coils for the ITER. Based on the Hall-Petch relationship, since that material has a coarser grain diameter, approximately 125~177 μm, when this forged material is applied as base material for the HIP bonding process, a further reduction in mechanical properties can be observed. Accordingly, small prototypes with a diameter of 121 μm were used in the HIP bonding process in order to confirm HIP bonding quality, tensile properties at 4 K, 77 K and room temperature, and fatigue strength at 4 K. In addition, the relation between mechanical properties and grain size were summarized and a method for predicting tensile properties and fatigue strength based on the tensile properties and grain size was proposed.
   The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.