Metallurgical assessment of large size tensioning components for the precompression structure of the ITER central solenoid

•An adapted fabrication and processing route including vacuum induction melting (VIM) and electroslag remelting (ESR) or vacuum arc remelting (VAR), combined with hot transformation steps involving where applicable redundant multidirectional forging, was successfully applied to the production of inconel 718 large size tensioning components for the precompression structure of the ITER central solenoid.•Material production involved a wide variety of gauges and product shapes, whose final properties are very consistent thanks to a repeatable and well mastered production process.•The material of tensioning components is submitted to a tight specification. Products feature an outstanding cleanliness, fineness and homogeneity of the microstructure, as confirmed by extensive macro and microscopic observations, which enable final properties to meet or exceed specification requirements. The precompression system of the ITER Central Solenoid (CS) consists of nine identical units designed to provide an axial precompression force of 210 MN at room temperature to the stack of the six independently energised modules of the CS. This load allows the modules to be maintained in contact with each other in the vertical direction during operation. Precompression is achieved by using tensioning components, by pretensioning at room temperature the tie plates of the precompression system. In addition, such components allow maintaining the necessary preload during the operation of the CS at cryogenic temperature. Tensioning components consist of large diameter tensioners (up to M190) and threaded rods (up to M110), as well as of different assemblies including multi-jackbolt tensioners, washers and jackbolts. They are manufactured from double aged forged or hot rolled Inconel 718, an age-hardenable, high strength nickel based superalloy. Stringent specification requirements apply to the threaded rods and the other components of the tensioning system. They were extensively characterised through measurements of metallurgical, physical, mechanical properties and non-destructive examinations. The paper provides a summary of the results of such wide campaign of evaluation based on testing heavy gauge products of a broad range of sizes and shapes, fully representative of the series production. The results are assessed against the requirements of the technical specification and applicable national and international standards.