Mechanical characterization by tensile tests on miniaturised specimens extracted from an in-bore welded DN200 AISI316L pipe

In the DEMO maintenance strategy, several pipes must be welded from the inside using in-bore systems, due to the limited space available. This constraint necessitates the development of bespoke solutions for both the welding process and the quality assessment of the welds. From the perspective of quality assurance, a key step is to verify whether the welded material meets the required mechanical performance. Given the different operational context compared to the ITER application, the development of a new remote handling strategy and a dedicated design for the in-bore tools is still in progress. Nonetheless, the qualification process has already begun to validate the selected welding process parameters. In this work, the mechanical behaviour of the welded material is compared to that of the base material by extracting miniaturised specimens from the weld bead of a 16 mm-thick AISI 316 L pipe. The welding was carried out using multiple passes of a Tungsten Inert Gas (TIG) torch, and specimens were obtained via wire electro-discharge machining from three distinct locations within the weld: the top (close to the internal pipe diameter), the middle, and the bottom (near the external diameter). This sampling strategy allowed for the investigation of potential variations in mechanical properties arising from the different thermo-mechanical histories experienced across the weld thickness. Tensile tests were performed on miniaturised specimens, and the results were analysed in terms of strength, ductility and fracture behaviour. These findings were compared with hardness measurements conducted on both the base material and the corresponding regions of the weld bead. Hardness profiles and 2D mapping revealed a through-thickness gradient with localised hard zones. The miniature tensile tests showed that all weld regions exhibited higher strength and lower ductility than the base material, while the tensile properties of the different regions of the fused zone were largely overlapping. Although a weak trend between hardness and yield strength can be recognised, hardness testing alone does not provide an exhaustive description of the mechanical behaviour of the fused and heat-affected material, confirming that miniature tensile tests remain essential for weld quality assessment in future qualification campaigns. These results provide a first mechanical basis for the qualification of in-bore TIG welds in DEMO-relevant service connections and support the future use of miniature tensile testing on limited-volume or activated material.