A52M/SA502 Dissimilar Metal RPV Repair Weld: Evaluation of different techniques

As part of nuclear power plant ageing management, the increased probability of a need of repair welding must be taken into account along with the increase of plant lifetime. An essential prerequisite for successful and safe repair welding is that the applied welding procedures have been properly validated and qualified prior to their use. For instance, if no post-weld heat treatment can be performed and the desired tempering effect has to be based on temper-bead technique, a user needs to scan among several available repair welding procedures. A decision has to be made which of the procedures provides the maximum desired tempering effect with the case in question. This research is a part of a larger experimental effort developing repair welding techniques, and is a part of the Finnish Nuclear Power Plant Safety Research Programme SAFIR2022. The currently studied experimental repair welding case is a low-alloy steel mock-up with an austenitic cladding. Repair welding is assumed to represent a 'worst-case' scenario where a postulated linear crack-like defect exists beneath the cladding and might extend across the interface into the reactor pressure vessel steel side. This postulated defect will be removed by machining, and the thereby machined groove will be filled by repair welding using a nickel-base super alloy 52M filler metal by cold metal transfer-gas metal arc welding with a robotic arm. In this paper, different repair welding techniques and alternatives are shortly surveyed based on existing literature. Overall, published documentation was sparse. While only few studies were considered relevant in terms of established links to actual repair cases of under-cladding defects in reactor pressure vessels, others were mainly for modelling and simulation purposes without e.g. cladding groove preparation or the use of irradiation-embrittled material. Most of these procedures were based on the use of nickel-base alloy filler metal in the combination with temper-bead welding technique, with the aim at omitting both preheating and post-weld heat treatment. The main challenge in the repair weld design is to optimise all relevant welding parameters, including the thermal efficiency of temper-bead welding, in order to obtain a sound, defect-free weld with controlled reactor pressure vessel steel heat affected zone maximum hardness. In the simulations presented in the paper, the goal was to compute the resulting deformations, strains and stresses induced by the repair process and make a-priori estimates of the effectiveness of different repair techniques based on the numerical predictions. The numerical analyses allow the comparison of the procedures and enable selecting the one with most efficient combination of weld thermal cycles in terms of tempering and normalisation effects. The prediction of prevailing residual stresses is also important when further application of the component is considered. The paper is followed by Part II, in which the topics of experimental evaluation and material characterization of the repair weld are presented.