Weld overlays have been used to provide repair and mitigation to stress corrosion cracking (SCC) susceptible butt welds in nuclear power plant piping. Among the several advantages associated with weld overlays are the beneficial compressive residual stresses that are developed in the inner portion of the component after application of the overlay. These compressive stresses can provide significant mitigation against SCC in these welds. To determine the residual stresses resulting from the weld overlay process in analytical modeling, a weld repair during original fabrication of the butt weld is typically assumed before application of the weld overlay. If the fabrication records are available, the details of the weld repair can be simulated in the analysis. However, in most cases, the weld records are not easily accessible and in instances where they are available, the quality and completeness of the information are questionable. As such, various conservative assumptions are made on the extent of the weld repair to be simulated in the analytical modeling. In this paper, the residual stress results of an axisymmetric finite element simulation of a bimetallic weld subjected to an inside surface weld repair followed by a weld overlay repair are presented. Three through-wall weld repair sizes (25%, 50% and 75% of the wall thickness without the overlay) assumed to be full 360° around the circumference were considered in the study. The results indicate that for all three weld repair cases, the inside of the configuration is very tensile after the weld repair indicating that regardless of the size of the weld repair, SCC is a possibility. The post weld repair stress distribution of the 50% and the 75% repair cases are similar indicating that an assumed 50% repair is fairly representative of repairs that can be assumed for analysis purposes. The application of the overlay resulted in favorable compressive stresses on the inside portion of the configuration for all the three weld repair cases indicating that regardless of the size of the initial weld repair, the application of the weld overlay provides mitigation against SCC.

Bimetallic welds associated with nozzle-to-safe end welds typically involve the use of Alloy 82/182 weldments. These weld materials are susceptible to intergranular stress corrosion cracking (IGSCC) in boiling water reactor (BWR) environment in the presence of tensile stresses. To mitigate IGSCC in these welds, stress improvement using either mechanical stress improvement process (MSIP) or induction heating stress improvement (IHSI) has been applied to convert the tensile stresses on the inside surface of the components to favorable compressive stresses on several of these welds at many BWR plants. The stress improvement applications to most of these welds were performed at the time when UT inspection technology for detecting and sizing flaws was at its infancy. As such, with improved modern day UT technology, it is not uncommon to detect flaws in these previously stress improved welds. Typically, weld overlay repairs using IGSCC resistant Alloy 52 weld metal are implemented on these welds when flaws are detected. Even though IGSCC resistant material is used for the design of the overlay, it is desirable to have adequate compressive residual stresses on the inside surface of the configuration after the overlay repair to provide further resistance against IGSCC. This paper describes a weld residual stress evaluation performed for a nozzle-to-safe end bimetallic weld that was previously stress improved with MSIP, and in which a flaw was identified during inspections. Four operating cycles were performed after application of MSIP. To repair the flaw, a weld overlay repair was implemented on this weld. The analytical process closely simulated the history of operation of this weld including the assumption of a weld repair during the original weld fabrication process. A thermal analysis was performed using a two-dimensional finite element model to simulate the welding process of the repair followed by one heatup and cooldown cycle, the weld overlay, and final operating heatup and cooldown. A non-linear, elastic-plastic stress analysis was then performed to calculate the residual stress state at various stages. The MSIP loading was simulated by pressure applied to the outside surface of the safe end, and iterated in order to produce the measured residual reduction in pipe circumference as measured in the field following the application of MSIP. The post stress improvement and the post weld overlay residual stresses at normal operating conditions resulted in beneficial compressive stresses on the inside of the configuration, assuring that crack growth into the weld overlay is highly unlikely.

Weld overlays have been used to repair or mitigate stress corrosion cracking (SCC) in both boiling water reactor (BWR) and pressurized water reactor (PWR) nozzle-to-pipe dissimilar metal welds (DMW). One of the contributing factors to SCC is the high tensile residual stresses produced during the fabrication of the original butt weld, especially when local weld repairs were present during the welding process. In analytical simulations to determine the post weld overlay residual stresses, complete simulation of the original butt weld, weld repair and the overlay is desired. However, to reduce the computational effort, it is commonly assumed that the weld repair stresses overwhelm the original butt weld residual stresses such that the original butt weld need not be simulated and only the weld repair is simulated before the application of the overlay. Questions have also been raised as to why the butt weld and/or the weld repair need to be simulated since it is assumed that both of these fabrication processes would be overcome by the weld overlay process. This paper investigates three fabrication sequences in order to determine their effect on the post weld overlay residual stresses: (1) the butt weld is simulated followed by a weld repair and then the weld overlay is applied; (2) the butt weld is simulated followed by the weld overlay with no consideration of a weld repair; (3) the butt weld is not simulated but a weld repair is assumed and the weld overlay is applied. Five different nozzle-to-pipe size configurations were used in the study to determine the effect of pipe size on the three fabrication sequences described above. The investigation indicates that the post weld overlay residual stresses for Cases 1 and 3 are similar and hence simulation of the weld repair alone (without the butt weld simulation) prior to simulating the weld overlay is a reasonable assumption. However, not simulating the weld repair (corresponding to Case 2) may provide different residual stress distribution.

The net section plastic collapse equations currently used for piping flaw evaluation in ASME Section XI, IWB-3600 are derived based on thin shell theory assuming properties for a single material. Because of the wide use of weld overlays to repair piping flaws in both BWRs and PWRs, there is a need to investigate the effect of multi-layered materials on the net section plastic collapse equations since weld overlays are typically applied with the weld material being different than the underlying base material. In addition, the larger section of the weld overlay provides additional load-carrying capability. A question also arises as to the application of appropriate loads and Z-factors if the underlying material is low-toughness material and the weld overlay has high toughness, not requiring consideration of thermal expansion stresses and Z-factors. The inherent assumptions that are made may lead to over-sizing of the overlay which can increase the welding time during the overlay implementation and subsequently the outage time. In this paper, an approach is developed for inclusion of thermal expansion loads and Z-factors for a weld overlaid circumferentially cracked section. Limit load equations are derived assuming a two layered material, both with and without taking advantage of the greater area and section modulus associated with the weld overlay. The equations for the allowable applied loads include consideration of both limited and complete circumferential flaw extent into the compressive zone of the section. The formulations are then applied to the design of weld overlays which are used to repair flawed piping. The evaluation shows that there can be significant difference in the allowable piping stresses as compared to those based on use of simplified uniform material assumptions.