ASTRID, MYRRHA, and ALFRED will most probably use austenitic stainless steels. This means that also large welded parts like the vessel will be made of this material. Therefore, understanding the influence of heavy liquid metal environment on welded materials is an important part in the qualification procedure of these reactors. During welding, the temperatures of the base metal around the weld reach levels where different microstructural transformations occur. To which extend these changes occur and their effect on the final properties of the weld (including melting and heat effected zone(s)) depend on parameters like alloy content, material thickness, filler metal, joint design etc. Nevertheless, regardless of all the influencing factors the ultimate objective of welding is to provide a sound joint with qualities equal or better than the base metal. In the current case the preservation of corrosion resistance, prevention of cracking and retaining the mechanical properties of the weld in heavy liquid metal environment is of high importance. For ensuring the performance of the weld the main concerns are avoidance of Cr depletion on grain boundaries due to carbide formation to maintain corrosion resistance as well as formation of delta ferrite and its influence on the mechanical properties. In the current context the influence of δ-ferrite is of special interest as it acts in welding of austenitic steels as a collection site for steel impurities such as S and P. Therefore, a typical 316L welding is produced with filler material that guarantees a certain minimum δ-ferrite content in the final weld.
In this PhD work, the welded material, including the heat effected zone and the base material will be characterized by mechanical testing methods like fracture toughness-, tensile- and hardness tests in different environments. Combining this with different microscopy techniques (optical-, focused ion beam-, scanning electron, electron back scatter, and transmission electron microscopy) as well as X-ray diffraction will lead to a thorough understanding of the influence of delta ferrite, precipitates and segregations on the properties of austenitic stainless steel in heavy liquid metal environment.