Mechanical Characteristics of Conduits for the ITER Central Solenoid Conductor

Abstract

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.