Assessment of Residual Stresses in ITER CS Helium Inlet Welds Fatigue Tested at Cryogenic Temperature

S Sgobba,I Aviles Santillana,D Everitt,C Jong,K Freudenberg,S Langeslag,P Fernandez Pison,P Libeyre,S Castillo Rivero

doi:10.1088/1757-899x/502/1/012095

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.

Highlights

The ITER Central Solenoid (CS) consists of six modules, each featuring 40 pancakes
The present paper summarises the results of the quantification of the residual stresses conferred by Ultrasonic Shot Peening (USP) on the inlet weld toe and its effect on the microstructure and fatigue performance of the inlets
In – depth hardness profiles performed under the peened region of the inlet weld toe confirm that the applied USP is able to harden a subsurface layer of 1.5 mm thickness

Summary

Introduction

The ITER Central Solenoid (CS) consists of six modules, each featuring 40 pancakes. The cooling of the CS Cable In Conduit Conductors (CICC) using a 45 kA Nb3Sn conductor is provided by a forced flow of 4.5 K supercritical helium entering through helium inlet nozzles and exiting at outlets [1]. In order to guarantee hydraulic performance while minimizing stress concentrations the inlets are designed as racetrack-shaped bosses welded to the outer conduit jacket through a 7.2 mm thick, full penetration Tungsten Inert Gas (TIG) weld with an imposed minimum connecting radius of curvature of 6.3 mm (figure 1) in order to guarantee a smooth transition from the boss to the jacket. The material of the jacket and the nozzle is the high-strength austenitic stainless steel JK2LB grade specially developed for the CS conductor jacket featuring a yield strength above 1000 MPa at 4 K and fracture toughness of more than 130 MPa m following the Nb3Sn reaction heat treatment (650°C–200 h) [3].

Methods

Findings

Conclusion