Understanding and Optimising Post Weld Heat Treatment in Electron Beam Welds of A508 Ferritic Steel

Abstract

Nuclear reactor components, like reactor pressure vessels and steam generators, are manufactured by welding large forgings of SA508 steel together, traditionally by multi-pass arc welding. During the welding procedure, residual stresses are induced, leading to the acceleration of several degradation mechanisms and, risking the structural integrity of these safety-critical nuclear components. Post-weld heat treatment (PWHT) is known to reduce the residual stresses, however the parameters utilised are often based on judgement and are not yet fully standardised for electron beam welds, which are single-pass autogenous welding processes that do not involve pass-to-pass tempering. The aim of this PhD thesis was to understand how the post-weld heat treatment parameters of duration and soak temperature, affect residual stress relaxation, and optimise their use for electron beam welded SA508 nuclear reactor components. To achieve this aim, experimental measurement of residual stresses was conducted in parallel with modelling development and optimisation, in order to acquire predictions of stress relaxation during PWHT regimes provided by industrial partners. Five thick-section EB welds were manufactured from SA508 Grade 3 Class 2 low alloy steel. Four underwent four different PWHT schedules with differing hold temperatures and soak times, and one was retained in the as-welded (AW) state. The residual stresses in the as-welded condition were measured using incremental deep hole drilling and the contour method, while residual stresses after PWHT were measured using the contour method alone. 2D and 3D finite element models that successfully predict the stress relaxation during PWHT in SA508 electron beam welded steels were constructed and optimised through an extensive study of material property variability and creep behaviour. The experimental results concluded that the characteristic M-shape distribution, associated with solid-state phase transformation, was present for all the welds and all the methods, and that significant stress relaxation was evident after PWHT, with peak tensile stresses falling in the range of ~100-170MPa. The sensitivity studies showed that soak temperature is more significant than hold time, with the majority of stress relaxation occurring prior to reaching the soak, dominated by creep. Furthermore, it was noticed that the level of AW residual stress had only a limited effect on the final stresses after heat treatment. The PWHT predictions from the 2D and 3D FE models, validated the experimentally measured results, by predicting similar stress relaxation to ~116-183MPa, and suggested that a 2D generalised plane strain FE model is adequate to provide accurate predictions of PWHT, as the 3D model did not significantly improve the results despite the added computational cost.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Smith (Supervisor)