Dependence of Stress Corrosion Cracking of Alloy 690 on Temperature, Cold Work, and Carbide Precipitation-Role of Diffusion of Vacancies at Crack Tips

The growth rate of stress corrosion cracking (SCC) was measured for cold-worked, thermally treated Alloy 690 (UNS N06690, CW TT 690) and cold-worked, solution-treated Alloy 690 (CW ST 690) in hydrogenated pressurized water reactor (PWR) primary water under static load condition. Three important patterns were observed. First, intergranular stress corrosion cracking (IGSCC) was observed in CW TT 690 in PWR primary water in the range between 320 degrees C and 360 degrees C; this rate of SCC growth was slower than in CW mill-annealed Alloy 600 (UNS N06600, CW MA 600). No significant IGSCC was observed in CW ST 690 after 5,109 h in hydrogenated PWR primary water at 360 degrees C. This is opposite of the behavior reported in the literature for high-temperature caustic solutions. Second, to assess the role of creep, rates of creep crack growth were measured in air, argon, and hydrogen gas environments using 20% CW TT 690 and 20% CW MA 600 in the range between 360 degrees C and 460 degrees C; intergranular creep cracking (IG creep cracking) was observed in both materials, even in air. Similar temperature dependence for IGSCC and IG creep crack growth was observed in 20% CW TT 690. The similar fracture morphology and temperature dependence suggests that creep is important in the growth of IGSCC for CW TT 690 in high-temperature water. Third, cavities and pores were observed at grain boundaries near tips of IGSCC and IG creep cracks, although the sizes of the cavities for IGSCC were smaller than that for creep cracks. Also, the population and size of cavities seem to decrease by decreasing test temperature. These results suggest that the difference in the size and population of cavities might be related with the difference in crack growth rate. The cavities seem to be formed by the collapse of vacancies at grain boundaries or interfaces as the crack initiates. This result suggests that condensation of vacancies in highly stressed fields occurs in high-temperature water and gas environments coincident with crack advance. As a model for IGSCC in CW materials in high-temperature water, the formation of crack embryos from the collapse of vacancies seems to be the best interpretation of the present data.