Exploration of the cold cracking risks and limits for welding repair applications of low-alloy ferritic steel nuclear components

Abstract

This project explores the cold cracking risks and limits for welding repair applications in low-alloy ferritic steel nuclear components. This work is part of a series of projects aimed at the codification of these repair applications using bead tempering techniques initiated by the French nuclear industry. The first part of the project comprised benchmarking of both the Tekken and the historically used implant welding test methods, testing a welding procedure selected to undertake the repairs. An assessment of the viability of this procedure was therefore also established. A computational weld modelling study was first undertaken to size the Tekken test mock-ups and improve understanding of their metallurgical and thermo-mechanical behaviour. The calibration and qualification of the models required experimental data. These were produced using various instrumented small-scale thermal, metallurgical, mechanical, and chemical tests. An acceptable match between the simulations and experimental results was established. For the welding procedure selected, it was found that the level evolution of the self-restraint conditions reaches a stabilisation above a thickness of 50 mm. Then, the benchmarking of the test methods was carried out experimentally. The Tekken method led to a positive assessment of the proposed repair welding procedure. In contrast, the implant method led to a negative appraisal. In order to inform the future development of a second, broader model allowing the risk of cold cracking to be predicted using a probabilistic criterion, an experimental study of the behaviour of the heat-affected zone exposed to hydrogen was also conducted. The results demonstrated a net loss of ductility directly attributable to the hydrogen. The hydrogen embrittlement of these microstructures was therefore quantified. Ways of mitigating these material property degradations were also identified, highlighting the benefits offered by the bead tempering techniques. This work was carried out at the University of Manchester and at the EDF R&D laboratories at Chatou, under the sponsorship of EDF, and involved cross-Channel collaborations with several partners, especially the University of La Rochelle and the Institut de Soudure (the French welding institute).

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	John Francis (Supervisor) & Michael Smith (Supervisor)