A Comparative Study of Fracture Toughness at Cryogenic Temperature of Austenitic Stainless Steel Welds

The ITER magnet system is based on the "cable-in-conduit" conductor (CICC) concept, which consists of stainless steel jackets filled with superconducting strands. The jackets provide high strength, limited fatigue crack growth rate and fracture toughness properties to counteract the high stress imposed by, among others, electromagnetic loads at cryogenic temperature. Austenitic nitrogen-strengthened stainless steels have been chosen as base material for the jackets of the central solenoid and the toroidal field system, for which an extensive set of cryogenic mechanical property data are readily available. However, little is published for their welded joints, and their specific performance when considering different combinations of parent and filler metals. Moreover, the impact of post-weld heat treatments that are required for Nb3Sn formation is not extensively treated. Welds are frequently responsible for cracks initiated and propagated by fatigue during service, causing structural failure. It becomes thus essential to select the most suitable combination of parent and filler material and to assess their performance in terms of strength and crack propagation at operation conditions. An extensive test campaign has been conducted at 7 K comparing tungsten inert gas (TIG) welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB, applied to two different base metals, AISI 316L and 316LN.The authors would like to express their thanks to Dr. Arman Nyilas (in memorian) for the contributions made in this work

Design of load-to-failure tests of high-voltage insulation breaks for ITER's cryogenic network

Advances in Cryogenic Engineering ‐ Materials: Proceedings of the International Cryogenic Materials Conference (ICMC) 2015 28 June to 2 July 2015, Tucson, AZ, USAThe development of new generation superconducting magnets for fusion research, such as the ITER experiment, is largely based on coils wound with so-called cable-in-conduit conductors. The concept of the cable-in-conduit conductor is based on a direct cooling principle, by supercritical helium, flowing through the central region of the conductor, in close contact with the superconducting strands. Consequently, a direct connection exists between the electrically grounded helium coolant supply line and the highly energised magnet windings. Various insulated regions, constructed out of high-voltage insulation breaks, are put in place to isolate sectors with different electrical potential. In addition to high voltages and significant internal helium pressure, the insulation breaks will experience various mechanical forces resulting from differential thermal contraction phenomena and electro-magnetic loads. Special test equipment was designed, prepared and employed to assess the mechanical reliability of the insulation breaks. A binary test setup is proposed, where mechanical failure is assumed when leak rate of gaseous helium exceeds 10−9P a · m3 /s. The test consists of a load-to-failure insulation break charging, in tension, while immersed in liquid nitrogen at the temperature of 77 K. Leak tightness during the test is monitored by measuring the leak rate of the gaseous helium, directly surrounding the insulation break, with respect to the existing vacuum inside the insulation break. The experimental setup is proven effective, and various insulation breaks performed beyond expectations

Assessment of residual stresses in ITER CS helium inlet welds fatigue tested at cryogenic temperature

Proceeding of: 27th International Cryogenic Engineering Conference and International Cryogenic Materials Conference 2018 (ICEC-ICMC 2018), September 3-7, 2018, Oxford, United KingdomThe ITER Central Solenoid (CS) consists of six independent wound modules. The cooling of the cable-in-conduit conductor is assured by a forced flow of supercritical He at 4.5 K supplied by He inlets located at the innermost radius of the coil. The inlets consist of a racetrack-shaped boss welded to the outer conduit wall through a full penetration Tungsten Inert Gas (TIG) weld. They are critical structural elements submitted to severe cyclic stresses due to the electro-magnetic forces acting on the coils. The weld contour is shape-optimised and locally processed by Ultrasonic Shot Peening (USP), conferring large compressive residual stresses on a subsurface layer of several millimetres thickness to improve fatigue strength. The distribution of the residual stresses and the effect of USP on microstructure and mechanical properties is assessed, with reference to the results of a cryogenic fatigue test campaign, performed on peened and as-welded inlets for comparison

The Effect of Specific Manufacturing Characteristics on PF ITER Full-Size Joint Performance

The development of new generation superconducting magnets for fusion research, such as the ITER experiment, is largely based on coils wound from so-called “Cable-In-Conduit” Conductors (CICCs). CICCs consist of various types of stainless steel jackets, densely filled with compacted superconducting strands, which are cooled by supercritical helium. The design of the various magnet systems, and in particular the ITER Poloidal Field (PF) coils, imposes the use of electrical joints to connect unit lengths of the CICCs. The electrical joints are delicate, electrical resistive components, carefully designed to provide efficient high current transfer while avoiding heat generation. The PF joints are subjected to fast varying magnetic fields that induce currents which, combined with the Joule heating in the resistive joints due to transport current, increase the temperature of the helium. Various characteristics, including electrical performance and mechanical behavior, have been addressed in the past in order to optimize manufacturing for satisfactory joint operation. Here an extensive post-mortem characterization of pre-qualification full-size PF joints is reported. Void fraction, twist pitch, and the current path connection are investigated in order to understand their effect on electrical performance and tune the manufacturing processes