PSI - Issue 41

Chouaib Zeghida et al. / Procedia Structural Integrity 41 (2022) 384–393

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Zeghida Chouaib et al. / Structural Integrity Procedia 00 (2022) 000–000

1. Introduction Experience has shown that austenitic piping is susceptible to IGSCC in the weld root area under BWR service conditions. Besides non-optimized materials, the residual stresses which are an inherent result of conventional welding processes are also responsible for this susceptibility. In the past, procedures were developed and used with the objective of reducing the tensile stresses in the root area. The present state of knowledge attributes three factors that must be present simultaneously, to causing IGSCC. These factors are (1) a corrosive environment, (2) a material susceptible to cracking, and (3) a tensile state of stress. The BWR piping systems contain numerous girth welds. The IGSCC has been observed in the sensitized zones near the weld fusion lines on the inside surfaces of the pipes (Boutelidja et al., 2019). At these locations, there is a corrosive environment in contact with a sensitized material which is susceptible to cracking. In addition, the welding process inherently causes residual stresses in the pipe. The magnitude and distribution of residual stresses depend upon the welding process and pipe geometry. In general, tensile residual stresses are harmful since they contribute to fatigue damage and stress corrosion cracking. On the other hand, compressive residual stresses are usually beneficial since they reduce total stresses, minimizing fatigue crack initiation and propagation, and increase wear and corrosion resistance. However, depending on the loading mode and the mechanical behavior of each material, residual stresses act in a different manner and therefore deserve detailed attention (Todinov, 2007; Bolognesi Donato and Magnabosco, 2014). Remedies for IGSCC are based on the observation that all three factors are required simultaneously for cracking to occur. Thus, the absence of any one of these factors should contribute to alleviating cracking. The focus of this paper is on removing tensile stresses in the areas of observed cracking. Although residual stresses as well as thermal and mechanical loading can contribute to the tensile state of stress, the welding residual stresses are of primary concern here (Rybicki and McGuire, 1982; Khaleel et al., 2009; Guedri et al., 2009; Guedri, 2013a-b ) . The purpose of this paper is to apply probabilistic fracture mechanics to the analysis of the influence of remedial actions on structural reliability limited to IHSI . The increased need for high performance or very high degrees of reliability or both have led to an increased interest in probabilistic analysis of structural reliability. This paper describes the stress corrosion cracking model used in the pc-PRAISE (Harris, 1992) for simulating the initiation and growth of IGSCC cracks. This model is based on laboratory data from IGSCC tests in combination with the calibration of the model using field data from pipe-cracking experience. (Khaleel et al., 2009; Guedri, 2013a-b) have improved on the prior calibrations by making adjustments to the modeling of plant loading/unloading cycles in addition to adjustments to residual stress levels, and (Priya et al., 2005; Guedri, 2012) concluded that equations used in PRAISE to calculate the stress intensity factors in order to simulate crack propagation need modification. The computational analysis for welding and IHSI treatment has four steps: (1) temperature analysis for welding, (2) residual stress analysis for welding, (3) temperature analysis for IHSI and (4) residual stress analysis for IHSI. These steps are outlined in the following sections. 2. Evaluation of piping reliability 2.1. Probabilistic IGSCC Model In this section, the methodology recommended in PRAISE for modeling IGSCC in stainless steel pipe is presented and all cracks are two-dimensional semi-elliptical interior surface cracks, generally circumferentially oriented, as shown in Figure 1. PRAISE separates the overall time to pipe leaks into three steps (Harris, 1992 ) : a) Time to initiate a very small crack, b) Time spent growing small cracks at an initiation velocity, c) Time spent growing larger crack at fracture mechanics velocity to become through—wall cracks. The growth of very small cracks that have just initiated cannot be treated from a fracture mechanics standpoint. In this study, cracks can fail the pipe by either breakage or leakage. The part-through initial stress corrosion cracks considered can grow and become unstable part-through cracks or stable or unstable through-wall cracks.