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

Abstract

The 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.