PSI - Issue 41

Chouaib Zeghida et al. / Procedia Structural Integrity 41 (2022) 384–393 Zeghida Chouaib et al. / Structural Integrity Procedia 00 (2022) 000–000

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The stability of the part-through or through-wall crack is checked by comparing net-section stress with the flow stress of the material. The failure criterion for pipe leakage used in the M-PRAISE code was a = h, where h is the wall thickness and a is the crack depth (Guedri et al., 2012; Guedri, 2013a-b).

Fig. 1. Geometry of the part-through circumferential crack considered.

2.2. Effect of Welding Residual Stresses Residual stresses play a very important role in the initiation and growth of stress corrosion cracks. Hence, they are important in the analysis of the reliability of sensitized welds in BWR piping, and PRAISE includes extensive provisions for their consideration (Harris, 1992). Hence, residual stresses in austenitic piping materials are concentrated upon, because this is the material used in recirculation lines of BWRs. Weld-induced residual stresses have shown considerable scatter and dependence on pipe site. They also exhibit a relatively complex spatial variation (Khaleel et al., 2009). Residual stresses can be deterministically defined by an axisymmetric linear through-wall variation (which will not necessarily be self-equilibrating) or by coefficients in a polynomial curve fit to stress intensity factors due to residual stresses as a function of crack size. Welding residual stresses can be defined by random distribution. They were found to have considerable scatter and to differ significantly depending on the size of the line. A description of the spatial distribution of the residual stresses in the longitudinal pipe direction in the heat-affected zone is developed along with a characterization of the randomness of the residual stresses. As an additional consideration, the stress corrosion cracking portion of PRAISE was exercised and compared with field observations of failure (and successes) in BWR piping. Since residual stresses are one of the major uncertainties in PRAISE inputs, the weld-induced residual stresses were adjusted by a constant factor for a given pipe size. The axial component of the as-welded residual stresses in the heat-affected zone is of interest because stress corrosion cracks form in the heat-affected zone, and the circumferential cracks are of concern (Harris, 1992; Khaleel et al., 2009). 3. Proposal of Welding Residual Stress Mitigation Several techniques for controlling or altering residual stresses on the inside surface of girth welded pipes are presently being investigated within the pressure vessel and piping industry. These include Induction Heating for Stress Improvement (IHSI), last pass heat sink, welding heat sink welding and backlay welding (Rybicki and McGuire, 1982; Shimizu et al., 1984; Schmidt et al., 1997). These techniques are based on the principle that the residual stresses are influenced by the thermal history of the pipe. In each process, a heat sink, such as running water, is applied on the inside of the pipe while the exterior is heated. It is the means of exterior heating which distinguishes one process from the other.