PWSCC Crack Growth Mitigation With Inlay

Primary water stress corrosion cracking (PWSCC) is an issue of concern in the dissimilar metal welds (DMW) connecting vessel nozzles and stainless steel piping in PWR nuclear power plants. PWSCC occurs due to the synergistic interaction of several factors including tensile weld residual stresses, a corrosion sensitive weld metal (usually Alloy 82/182 weld metal) and a corrosive environment. Several mechanical mitigation methods to control PWSCC have been developed in order to alter the weld residual stresses on the nozzle. These methods consist of applying a weld overlay repair (WOR), using a method called mechanical stress improvement process (MSIP), and applying an inlay to the nozzle ID, the latter of which is the subject of this paper. An inlay consists of machining the pipe ID at the region of the DMW and applying a PWSCC resistant weld material at the machined region. The PWSCC resistant material is mainly Alloy 52/152, which has a higher chromium content compared with Alloy 82/182. The inlay is a corrosion resistant material, and the proposed application thickness (after final machining) is 3 mm. Therefore, once the crack grows through the inlay, the growth in the underlying A82/182 material is much faster. This leads to a complicated crack shape which is small at the nozzle ID and becomes larger in the original weld material and approaches a balloon shape. Here the weld residual stress state caused by the inlay is first discussed. Next, the effect of crack growth through the inlay and into the underlying Alloy 82/182 material is discussed. Finally, implications of inlay for mitigation and consideration of alternatives is discussed.