Detailed Simulation of an Overlay Repair on a 14” Dissimilar Material Weld

In nuclear reactors, ferritic low alloy steel heavy section components are connected with austenitic stainless steel piping systems. Despite a special manufacturing procedure to ensure a good resistance of the joint, several experiences from the field confirm sensitivity to fatigue and corrosion in this type of junction. Overlay welding is a process widely used to mitigate dissimilar material weld (DMW) stress corrosion cracking by replacing inside tensile stresses by compressive stresses. Taking into account the costs generated by mock-up manufacturing, predictive Finite Element (FE) residual stress calculations are of great interest to prove the effectiveness of the overlay welding and to adjust the parameters of the process and particularly the overlay thickness. This paper presents residual stress computations performed by Framatome-ANP on a 14” pipe geometry, resembling many mid size DMW in the US. Considering 2D axisymmetric hypotheses, the analysis simulates each elementary step of the mock-up manufacturing procedure. In particular, the pass-by-pass welding simulation reproduces the deposit of each bead by thermo-metallurgical and mechanical calculations. Thanks to residual stress measurements carried out on 2 mock-ups (with/without overlay), the numerical approach has been validated and highlights the beneficial overlay effect. However, some discrepancies raise various problems: the backing ring modelling, the machining heating effect, the experimental scatter and the weld material hardening. The simulation being able to analyze the influence of an overlay layer going up to 1 time the original pipe thickness, further work on the stabilization of the residual stress fields obtained here after the deposit of 4 or 5 layers, may lead to a better adjustment of the overlay thickness and to a cut in the operation costs too.