Qualification of structural stainless steel products for the ITER correction coil cases

The ITER Correction Coils (CCs) consist of three sets of six coils, Bottom (BCC), Side (SCC) and Top Correction Coils (TCC), respectively. The CCs rely on 10 kA NbTi cable-in-conduit conductor. Each CC winding pack is enclosed inside a 20 mm thick 316LN stainless steel case, providing structural reinforcement against the electromagnetic loads arising in the winding pack. The material of coil cases shall feature ready weldability both by laser and conventional techniques, high strength and toughness at 4 K. Material production involves not only hot rolled plates of different thicknesses, but also heavy gauge extrusions. An adapted steelmaking route including Electroslag Remelting (ESR) combined with a hot transformation step involving redundant multidirectional forging of the slabs and of the billets was adopted. This route allowed the stringent material specification to be fulfilled and permitted to achieve cleanliness, fineness of the structure and homogeneity on the final products. An extensive follow-up of the steel manufacturing through systematic non-destructive and destructive examinations was carried out in order to ascertain the soundness and the homogeneity of the final products. The progresses accomplished throughout the steel manufacturing are highlighted. The properties achieved on the final products that were enabled by the selected manufacturing route are discussed.