Unified Interpretation of Crack Growth Rates of Ni-base Alloys in LWR Environments

Abstract

Primary water stress corrosion cracking (PWSCC) of vessel penetrations (VP) fabricated from nickel based alloys such as alloy 600 and alloy 182 weld metal has created a great demand for elucidation of the cracking mechanism and for development of life prediction technologies. The generalized FRI crack growth rate (CGR) formulation was proposed, based on a deformation/oxidation mechanism and a theoretical crack tip strain rate equation derived by the authors. The effects of crack tip oxidation and crack tip mechanics and of their interactions on crack growth can be quantified. Experimental and actual plant data of CGR for alloy 600 in PWR primary water, which are sometimes scattered in CGR-K diagrams, are interpreted with the generalized CGR formulation, emphasizing the effects of temperature, K, yield strength and variations of K with time. It is suggested that it is essential to determine the type of dependency of CGR on K for accurate flaw disposition. The generalized formulation provides a unique parameter for interpreting CGRs as well as a unified method for predicting CGRs within a narrow scattered band even under various testing parameters, which is the basis for accurately predicting component life.