SCC INITIATION OF CW ALLOY TT690 AND ALLOY 600 IN PWR WATER

摘要

The purpose of this work is to understand quantitative processes which underlie the initiation of stress corrosion after long term exposures in high temperature water. Long term stress corrosion cracking (SCC) initiation tests up to about 30,000 h were performed using blunt notch compact tension (CT) type specimens and CT specimens with a shallow depth of pre-crack (～0.1 mm) of cold worked (CW) thermally treated alloy 690 (UNS N06690, TT 690), which were exposed in the primary coolant environment in pressurized water reactors (PWR) under static load condition at 320 and 360°C. Further, SCC initiation tests were also performed using blunt notch CT specimens of CW mill annealed Alloy 600 (UNS N06600, MA600) to consider the similarities and dissimilarities with Alloy TT690. Three important patterns were observed: first, intergranular (IG) cracking was observed in 20%CW TT690 from the shallow pre-crack at 360°C in PWR primary water after 20,653 h under static load conditions. Second, clear evidence of cavities were identified in 20%CW TT690 ahead of SCC tip at 360°C in PWR primary water after 20,653 h. 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. Third, SCC initiated in 20%CW MA600 at 360°C in PWR primary water after 1,522 h. The results suggested that the transition of SCC segments from initiation and steady growth seem to be occurred about 0.2 to 0.3 mm depth of intergranular cracking at high stress condition at 360°C in PWR water. As a model for the initiation of SCC after long term operations of CW TT690 in high temperature water, the combination of local corrosion and the formation of cavities from the collapse of vacancies seem to dominate the initiation of SCC after long times in high temperature water.