Formation of Cavities Prior to Crack Initiation and Growth on Cold-Worked Carbon Steel in High-Temperature Water

The rate of growth of stress corrosion cracking (SCC) was measured for cold-worked carbon steel (ASTM A106 [UNS K03006]) in hydrogenated pure water under static load condition. Four important patterns were observed. First, intergranular stress corrosion cracking (IGSCC) was observed even in the static load condition in case that material was cold-worked. Second, to assess the role of creep, rates of creep crack growth were measured in air, argon, and hydrogen gas environments using cold-worked carbon steel in the range of temperatures between 270°C and 475°C; intergranular creep cracking was observed in gas environments. Similar 1/T temperature dependencies for IGSCC and intergranular creep crack growth were observed for cold-worked carbon steel. Similar fracture morphologies and 1/T dependencies suggest that creep is important in the growth of IGSCC of cold-worked carbon steel in high-temperature water. Third, cavities were recognized at the grain boundaries on the periphery of both the SCC and creep crack tips. The cavities seem to play a role as crack embryos before cracks advance. Fourth, the initiation of cracks was examined on cold-worked carbon steel in water and in air at 360°C using specimens of blunt-notched compact tension-type specimen. Cavities were observed at grain boundaries after 8,082 h exposure in water and 8,033 h exposure in air at the bottom of the notch before crack initiation. Cavities observed in specimens exposed to pure water were more numerous than that in air at the same temperature and time of exposure. This result suggests that diffusion and condensation of vacancies in high stress fields occurs during the incubation period in high-temperature water. Finally, to assess the mechanism of IGSCC initiation and growth in high-temperature water, the diffusion of vacancies driven by stress gradients was studied using a specially designed CT specimen. As a model for IGSCC in cold-worked materials in high-temperature water, the formation of cavities from the collapse of vacancies seems to be the best interpretation of the present data.