Role of Cavity Formation in Crack Initiation of Cold-Worked Carbon Steel in High-Temperature Water

Crack initiation behavior was studied to understand the underlying processes during the incubation period for prediction of crack initiation after long-term exposure in high-temperature water. Tests were performed using blunt-notched compact tension-type specimens of cold-worked carbon steel (ASTM A106 [UNS K03006]) exposed under static load condition in hydrogenated pure water and in air in the range of temperatures between 320°C and 450°C. Five important patterns were observed. First, intergranular cracking was observed in both water and air, even in static load conditions when steel specimens had been cold-worked. Second, 1/T-type temperature dependencies of initiation times were observed for cold-worked (CW) carbon steel (CW carbon steel), and the crack initiation time in an operating plant (Point Lepreau Nuclear Generating Station, Point Lepreau, New Brunswick, Canada) seemed to lie in the extrapolated line of the experimental results. Third, cavities were identified at the grain boundaries at the bottom of a notch (highly stressed location) before cracks initiated both in water and air. The cavities seem to result from the condensation of vacancies and affected the bond strength of grain boundaries. Consequently, the bond strength is assumed to be weakened during the incubation period. Fourth, the rate of formation of cavities in water was more than 10 times the rate in air. This suggests that the rate of diffusion of vacancies may be enhanced by the absorption of hydrogen, which results from the reduction of water on the surface. Fifth, an excellent correlation was observed between the rates of crack growth and cavity formation both for stress corrosion cracking (SCC) and creep cracking. This suggested that the rate limiting process of crack growth is the formation of cavities at grain boundaries both in creep cracking and for SCC in high-temperature water. Finally, to assess the mechanism of intergranular stress corrosion cracking (IGSCC) initiation and growth at high-temperature water, the diffusion of vacancies driven by stress gradients was studied using a specially designed compact tension (CT)-type specimen. As a model for IGSCC in cold-worked carbon steel in high-temperature water, the formation of cavities from the collapse of vacancies seems to be the best interpretation of the present data.