A Study of the Key Parameters Affecting the Design of Optimized Weld Overlays

Abstract

The potential for primary water stress corrosion cracking (PWSCC) of large diameter austenitic nickel alloy components and their associated welds presents a particular problem for the nuclear industry due to a limited number of available options for mitigating or repairing large bore pressure boundary components such as reactor vessel, reactor coolant pump, and steam generator inlet or outlet nozzles. While a full structural weld overlay (FSWOL), as governed by ASME Code Case N-740, is commonly used to mitigate and repair small (4″) to medium (10″) bore piping assemblies employing Alloy 82/182 dissimilar metal welds, the large amount of weld metal that would be have to be deposited on large components (and the associated impact on outage schedule) makes this an unattractive strategy for managing the degradation of Alloy 600 type materials. An alternative design option, specifically developed for the mitigation and repair of large bore (30″) components, utilizes a thinner weld overlay whose thickness has been optimized to achieve a specific level of stress on the inside surface of the PWSCC susceptible material. According to ASME Code Case N-754, inside surface stresses should be limited to 10 ksi during the design phase of an optimized weld overlay (OWOL) in order to minimize the initiation or consequences of primary water stress corrosion cracking. With the increased inspection requirements of Code Case N-754 and the corresponding smaller crack growth design flaw size, and along with the reduced weld volume of an OWOL, as compared to a FSWOL, an optimized weld overlay is often the preferred technique for mitigating or repairing large bore piping components. This paper investigates the influence of various parameters on the effectiveness of an optimized weld overlay in satisfying its principle design objective, to reduce the inside surface stresses in PWSCC susceptible materials to no more than 10 ksi. Inherent design parameters are the thickness of the underlying pipe or weld, and the depth of any recorded or postulated weld repairs in the pre-overlay configuration of the welded joint. Explicit design parameters include the thickness of the overlay, the number of weld layers used to form the overlay, and the length of the overlay. Finite element analysis is used to calculate residual and operating stresses in a representative large bore reactor vessel coolant nozzle dissimilar metal weld for various combinations of design parameters. The overall objective of this study is to identify the key parameters influencing inside surface stresses, and thereby provide screening criteria for use in determining the applicability of the optimized weld overlay as a viable PWSCC mitigation or repair option for large bore primary pressure boundary components.