Residual Stress Analysis of a Bimetallic Weld Subjected to Stress Improvement and Weld Overlay Repair

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.