Optimization of JK2LB chemical composition for ITER Central Solenoid conduit material

The Japan Atomic Energy Agency (JAEA) has developed a cryogenic structural steel to be used in large superconducting magnets for a fusion machine, and the results of this development will be utilized in the International Thermonuclear Experimental Reactor (ITER). Low carbon and low boron JK2 (JK2LB), which has high strength and fracture toughness at 4 K and thermal contraction from room temperature to 4 K which is lower than that of conventional 316LN steel has been developed as the conduit material for the ITER Central Solenoid (CS) conductor. In order to achieve the ITER requirements (0.2% yield strength ⩾ 1000 MPa, fracture toughness KIC (J) ⩾ 130 MPa m) for CS conduit material, chemical components such as carbon, nitrogen and boron, were optimized. In addition, since the CS is to undergo 6 × 104 load cycles during its lifetime with a maximum principal stress of 490 MPa, fatigue crack growth assessment of the CS conduit was performed. As the result, JK2LB containing nitrogen of 0.2%, boron 15–40 ppm, and low carbon was found to achieve the strength and fracture toughness requirements. For the welding of JK2LB, a filler wire of JK2LB with a low nitrogen content of 0.13% was developed and fracture toughness of more than 130 MPa m was confirmed in the weld metal. Measured fatigue crack growth rates of the base and weld metal at 4 K are low enough to achieve the required CS coil operation cycle lifetime.