Stress distribution in a 316L(N) steel narrow gap TIG model weld for Gen IV nuclear applications

Abstract

The stress distribution in a model weld developed for nuclear application has been determined non-destructively by means of neutron diffraction, in the frame of the Horizon 2020 Project Generation IV Materials Maturity (GEMMA). The investigated sample is a narrow gap Tungsten Inert Gas (TIG) austenitic 316L(N) steel weld, prepared following RCC-MRx Code specifications. Two lines perpendicular to the welding direction, at the middle of the sample, were scanned at 6 mm and 14 mm depth; additional measurements were carried out in the middle of the weld, down to 16 mm depth. At 6 mm depth and within ± 5 mm distance from the weld centre, marked tensile stress gradients are found, with the residual stresses reaching maximum values up to 400 MPa in the longitudinal direction. At 14 mm depth, the stresses decrease to around 200 MPa for the longitudinal component and get compressive for the transverse and normal components, down to −200 MPa for the transverse one, with smoother stress gradients around the weld. The in-depth measurements inside the weld confirm that the main integrity concern for the investigated sample may arise from the tensile longitudinal stresses. Additional micro-structural information has been obtained by qualitative comparison of diffraction line profiles in the weld and in the base metal. These experimental results are discussed with reference to the expected service conditions of such welds and to their capability to fulfill Gen IV safety goals and requirements.