Investigation of Materials and Welds for the Precompression Structure of the ITER Central Solenoid

The Central Solenoid (CS) is the backbone of the ITER magnet system. It consists of six independent coils held together by a vertical pre-compression structure that must react tensile loads and provide sufficient preload to maintain coil-to-coil contact during all stages of plasma operation. Material selection and specifications applicable to the structural components of the pre-compression structure are particularly demanding. These include large forgings manufactured from a high strength austenitic stainless steel (FXM-19) with a stringent specification in terms of microstructure and maximum allowed magnetic permeability. Stringent requirements are also imposed on all welded joints. In particular, the attachment welds of the cooling pipes to the structure are subject to challenging restrictions in terms of weld imperfections and geometry. They must induce limited distortion of the components and are submitted to inspections carried out in accordance with the most severe acceptance levels of applicable standards. The results of extensive characterizations based on non-destructive and destructive evaluation of samples from different components and welds are reported in this paper, particularly focusing on the quality achieved at the microstructural and macrostructural level. The influence of the microstructure on the properties and on the inspectability of the material by non-destructive examinations is discussed.