Physical Properties of a High-Strength Austenitic Stainless Steel for the Precompression Structure of the ITER Central Solenoid

The ITER central solenoid (CS) consists of six independent coils kept together by a precompression support structure that must react vertical tensile loads and provide sufficient preload to maintain coil-to-coil contact when the solenoid is energized. The CS precompression system includes tie plates, lower and upper key blocks, load distribution and isolation plates and other attachment, support and insulating hardware. The tie plates operating at 4 K are manufactured starting from forgings in a high-strength austenitic stainless steel (FXM-19) with a stringent specification. Moreover, forged components for the lower and upper key blocks have to be provided in the same FXM-19 grade with comparably strict requirements. FXM-19 is a high-nitrogen austenitic stainless steel, featuring high strength and toughness, ready weldability, and forgeability. It features as well higher integral thermal contraction down to 4 K compared with the very high Mn steel grade selected for the CS coil jackets, hence providing an additional precompression of the CS modules during cooling down. The results of an extensive characterization of the physical properties of FXM-19, extended down to 4 K or lower, are reported here. The provided results allow requirements for the file for new materials described in French code RCC-MRx to be fulfilled, as well as to complete the material properties in the ITER database, where measurements were reported for room temperature and above only

Advanced Examination Techniques Applied to the Assessment of Vacuum Pressure Impregnation (VPI) of ITER Correction Coils

The ITER Magnet System includes a set of 18 superconducting correction coils (CC) which are used to compensate the error field modes arising from geometrical deviations caused by manufacturing and assembly tolerances. The turn and ground insulation are electrically insulated with a multi-layer fiberglass polyimide interleaved composite, impregnated with epoxy resin using vacuum pressure impregnation (VPI). Adequate high voltage insulation (5 kV), mechanical strength and rigidity of the winding pack should be achieved after impregnation and curing of the insulation system. VPI is an effective process to avoid defects such dry spots and incomplete wet out. This insulation technology has also been developed since several years for application to large superconducting coils and more recently to ITER CC. It allows the coils to be impregnated without impacting on their functional characteristics. One of the critical challenges associated with the construction of the CC is the qualification of the VPI insulation. Sections issued from representative VPI test samples with real scale side correction Coil (SCC) cross-section have been delivered and characterized at CERN. High resolution micro-optical inspections have been carried out on large areas through digital microscopy. The aim was to identify lack of impregnation, areas of pure resin and void entrapments. The areas near the filling fibre glass rope received special attention. High precision dimensional and geometrical assessments have been performed with the help of image analysis. Compression and pull-out tests have been also carried out. Finally, high-resolution 3D-computed tomography has been applied for a full volumetric inspection of the sections, enabling the reconstruction in three dimensions of the VPI samples and allowing to fully detect, confirm, and image the volume defects already identified by micro-optical observations

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

Examination and Assessment of Large Forged Structural Components for the Precompression Structure of the ITER Central Solenoid

Large structural forgings of complex shape are required for several components of the precompression structure of the ITER Central Solenoid, consisting of a stack of six electrically independent modules and featuring a total height of 18 m and a diameter of over 4 m. The precompression structure allows the vertical tensile loads to be reacted and adequate preload to be maintained, in order to insure the contact between the modules during plasma operation. Several components of the precompression structure such as tie plates, lower and upper key blocks, lower and upper components are machined from open die forgings of an unprecedented combination of complex shape and large size. The selected material is FXM-19, a high strength nitrogen bearing austenitic stainless steel. A specific manufacturing schedule including redundant multidirectional forging is applied in order to achieve the required properties and microstructure of the final parts. The paper summarises the lessons learned from the series production of the components. The achievement of a fine and homogeneous microstructure, which is of paramount importance for final inspectability of the parts and to obtain the mechanical properties, is particularly challenging taking into account their large size. It requires a perfect mastering of the whole manufacturing process, from the steelmaking route, based in some cases on sequential remelting of electrodes from different master heats to create large Electroslag Remelted (ESR) ingots, to the sequence of the thermomechanical steps, from the initial upsetting of the ingots to the final solution annealing of the as-forged parts. Non-Destructive Examinations are based on stringent acceptance criteria. A fine microstructure is indispensable to allow full volumetric inspection with sufficient lateral resolution. Indeed, inspectability of the full thickness of the parts by Ultrasonic Testing, compatible with the criteria imposed by the technical specification and the structural requirements of the single components, is only possible in absence of unrecrystallised areas or excessive grain growth

Microstructure and Mechanical Properties of ITER Correction Coil Case Material

The modified 316LN austenitic stainless steel was selected as ITER correction coils case material to provide structural reinforcement to the winding pack. Considering the case structure, high-assembling accuracy and other strict requirements, 316LN in special extruded form has been developed. In the present study, the microstructure and mechanical properties of the material were investigated. The microstructure of the 316LN material was analyzed by means of the optical microscopy, transmission electron microscope, and X-ray diffraction. It was observed that the material presents fine grain size and a single austenitic phase. Moreover, the intergranular corrosion resistance of the 316LN was evaluated and the results indicated that it exhibited a remarkable intergranular corrosion resistance. The tensile properties of materials were measured at both room and cryogenic temperatures. The 0.2% offset yield strength (Rp0.2), ultimate tensile strength (Rm), and elongation at break (A) at 4.2 K were determined to be higher than 800 MPa, 1500 MPa, and 40%, respectively. Furthermore, the J-integral fracture toughness of the 316LN was tested through means of an unloading compliance method at 4.2 K and the plane strain fracture toughness Kιc converted from the J-integral are well above the specified value of 180 MPa√m. In addition, the fatigue test and fatigue crack growth rate of the 316LN stainless steel were also investigated at 4.2 K. According to these results, all requirement of the case material including uniform microstructure, excellent corrosion resistance, and good mechanical properties established by the ITER IO are confirmed

Advanced examination techniques applied to the qualification of critical welds for the ITER correction coils

The ITER correction coils (CCs) consist of three sets of six coils located in between the toroidal (TF) and poloidal field (PF) magnets. The CCs rely on a Cable-in-Conduit Conductor (CICC), whose supercritical cooling at 4.5 K is provided by helium inlets and outlets. The assembly of the nozzles to the stainless steel conductor conduit includes fillet welds requiring full penetration through the thickness of the nozzle. Static and cyclic stresses have to be sustained by the inlet welds during operation. The entire volume of helium inlet and outlet welds, that are submitted to the most stringent quality levels of imperfections according to standards in force, is virtually uninspectable with sufficient resolution by conventional or computed radiography or by Ultrasonic Testing. On the other hand, X-ray computed tomography (CT) was successfully applied to inspect the full weld volume of several dozens of helium inlet qualification samples. The extensive use of CT techniques allowed a significant progress in the weld quality of the CC inlets. CT is also a promising technique for inspection of qualification welds of helium inlets of the TF magnets, by far more complex to examine due to their larger dimensions