Failure analysis of a heavy gauge fastener of the ITER toroidal field gravity support system

Abstract

The Gravity Supports (GS) of the ITER Toroidal Field (TF) coils are positioned at the bottom of the machine and are situated under the 18 toroidal field coils. Each support includes 26 high strength bolts with M60 and M85 gauge. The TFGS will sustain a total load of about 11,000 tonnes of dead weight of the magnet system. Moreover, they will be exposed in operation to large electromagnetic forces and possible seismic dynamic loads. The bolts are manufactured from forged rods of double aged UNS N07718 (also known as Inconel® 718), a high strength nickel base superalloy. Stringent material specification and quality controls requirements apply to the threaded bolts and the other components of the GS. A complete break of one M85 bolt was discovered, that occurred about 40 days after its installation and preloading, prior to the application of any operating stress. Half of the part blew out from the bolthole and was found lying horizontally on the GS top area. Following the incident, a comprehensive failure analysis was carried out, based on a combination of non-destructive and destructive examinations including advanced techniques such as computed microtomography, immersion ultrasonic testing (UT) and fracture mechanics. The delayed rupture was understood as due to a combination of cavities not closed by the forging operations associated to a continuous network of brittle secondary phases at the grain boundaries, resulting in a local lack of ductility, poor impact toughness and mechanical properties locally lower than specified. As a consequence of the incident, UT procedures were developed to confirm that the remaining bolts, including the installed ones, are free of cavities using sufficiently conservative criteria and fit for purpose prior to operation. In particular, in-situ axial inspections allowed installed bolts that could contain relevant imperfections to be identified and replaced.