Assessment of production, materials and welds applicable at cryogenic temperatures to different components of iter magnets

Resumen

In the framework of the cooperation agreement between CERN and ITER, a significant task concerns Materials and metallurgy studies. As it will be explained later, there are converging technologies applicable to both projects, thus the interest of this collaboration. Materials, production processes, testing techniques have to be identified, defined and/or qualified for several components of the ITER Toroidal Field (TF), Poloidal Field (PF), Central Solenoid (CS) and the correction coils (CC) magnets. These components go from structural components for magnet supports to He inlets and outlets, essential for the cooling of the superconducting high field magnets. The assessment of production, materials, and welds passes through dedicated campaigns of tests that for specific components include fracture mechanics tests at cryogenic temperatures, carried out in partner laboratories under CERN supervision. The definition, the follow-up and the interpretation of destructive and non-destructive tests and results in charge of the Metallurgy and Metrology section (EN-MME-MM) for ITER is of paramount importance at this stage of the production of the ITER magnet system. The knowledge acquired in the understanding of material behavior at conditions close to operation ones is essential for a wide variety of cryogenic systems using stainless steel as structural material. The test campaign enclose mostly a very specific and unique mechanical testing known as J – tests, in which the fracture toughness of austenitic stainless steel welds is ascertained at cryogenic temperature. The obtained results being size independent and thus, a material property, are directly exploitable for a fracture mechanics’ approach to design as well as for finite element modeling. The ITER magnet system is based on the ‘cable-in-conduit’ conductor (CICC) concept, which consists of various types of stainless steel jackets filled with superconducting strands. The jackets are thus conferring the structural integrity to the conductors, providing high strength and fracture toughness to counteract the high stress imposed by, amongst others, electro-magnetic loads at cryogenic temperature. Austenitic stainless steel is the material of choice for a wide variety of applications, especially in the highly demanding fields of nuclear fusion and high-energy physics. Its use as structural material for cryogenic applications is broadening due to its weldability, high strength and fracture toughness at 4 K. If a cryogenic system is conceived for ultra high vacuum (UHV) applications for synchrotron applications, in addition to the above-mentioned properties, the reduction of synchrotron radiation - induced hydrogen outgassing rate is one of the most challenging constraints to be faced. Some of the connections of these components are done by welding, and thus, the properties of the welded joints under close-to-service conditions need to be established. Additionally, for some of the transitions, non – destructive testing is not applicable due to limitations of space. Thus, when a full inspection of the welded components is not possible, it becomes of critical interest to assess its fracture toughness under close-to-service conditions (if a fracture mechanics’ design approach is to be adopted). Material and mechanical properties at cryogenic temperature can be found in literature, but very little is published when it comes to welds. The thesis investigates fracture toughness behavior at cryogenic temperature of AISI 316L and AISI 316LN welded joints. Tungsten Inert Gas (TIG) welds using three fillers adapted to cryogenic service is studied and analyzed: EN 1.4453, EN 1.4455 and the exotic JK2LB. Additionally, the effect of several heat treatments is presented: the Nb3Sn reaction heat treatment (650o C for 200 hours) and two variants of the so – know vacuum firing (650o C for 24 hours and 950o C for 2 hours). The reduction of fracture toughness of the welds is evaluated. Brittle secondary phases are generally held responsible of the loss of ductility and toughness which is to be expected both before and more after post - weld heat treatments. Their quantification becomes thus essential in order to explain the negative impact in fracture toughness of their appearance during welding, and after unavoidable thermal treatments. The main objectives of this thesis are, in a first instance, develop a methodology to reliably quantify secondary phases that occur, in occasions, very scarcely in austenitic stainless steel welds. Additionally, a comprehensive assessment of the best material combination (base + filler) is presented, which will point the right direction for future developments in the field of welding filler materials. Special attention is put to the properties after several post weld heat treatments. For this purpose, the thesis is dedicated to the study of the behavior at cryogenic temperatures on the materials applicable to the above-mentioned components. Fracture toughness tests of different structural welds are performed, analyzed and interpreted. Different combinations of base material and filler material are considered. The impact of post weld heat treatments in the mechanical properties at low temperature is assessed together with a necessary correlation of the latter with the microstructure of the welded joints. Innovative methods to effectively quantify secondary phases are proposed, implemented and discussed, and advanced material characterization is used to identify the secondary phases present in the welds.