The Effects of Metallurgical Factors on PWSCC Crack Growth Rates in TT Alloy 690 in Simulated PWR Primary Water
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THE predecessor of TT Alloy 690 used extensively in the primary coolant circuits of PWRs, mill-annealed (MA) Alloy 600, was originally developed in the 1950s as an alternative material to austenitic stainless steel for steam generator (SG) tubing based because of its high resistance to stress corrosion cracking (SCC) in chloride solutions.[1] However, as early as 1959, Coriou[2–5] reported the SCC susceptibility of some nickel-rich, iron-nickel-chromium alloys in high temperature, highpurity water. Since then, many studies[6–8] have detected SCC in Alloy 600 and its compatible weld metals in PWR primary water environments. This particular type of (intergranular) SCC has habitually been called TOSHIO YONEZAWA, Professor, and MASASHI WATANABE, Associate Professor, are with the FRI, New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Contact e-mail: [email protected]. tohoku.ac.jp ATSUSHI HASHIMOTO, Research Engineer, is with the Kobe Material Testing Laboratory Co., Ltd., 47-13, Niijima, Harima-cho, Kako-Gun 675-0155, Japan. Manuscript submitted September 9, 2014. Article published online March 31, 2015 2768—VOLUME 46A, JUNE 2015
primary water SCC (PWSCC). In the 1970s, thermally treated (TT) Alloy 600 was developed to improve the PWSCC resistance of MA Alloy 600. However, PWSCC was also eventually detected in TT Alloy 600, initially in highly strained mechanical plugs for SG tubes. Yonezawa et al.[9] participated in the original development of high temperature MA TT Alloy 690 as an alternative material to MA and TT Alloy 600 for SG tubing. This was based on its high resistance to SCC in simulated PWR primary water and in highly caustic environments, etc.[9,10] Thin section, small diameter tubing such as SG tubes is fabricated by a cold finishing process, whereas relatively thick and large diameter pipes or forged materials, such as control rod drive mechanism (CRDM) nozzles, etc., are fabricated by a hot finishing process.[11] TT Alloy 690 has since been used not only for cold-finished materials such as SG tubes but also for hotfinished materials for several components of the pressure boundaries of PWR primary coolant systems, such as CRDM adapter nozzles, reactor vessel nozzles, etc., due to their excellent resistance to PWSCC.[12] Up until the time of this writing, no PWSCC has been reported in any TT Alloy 690 component or its welded joints in PWR primary circuits (except SG tubes) after METALLURGICAL AND MATERIALS TRANSACTIONS A
more than 15 years of operational experience worldwide. However, in recent years, Arioka[13] and Paraventi and Moshier[14] have reported PWSCC growth rates (PWSCCGRs) in highly cold-worked TT Alloy 690 up to about 1/5 to 1/10 of those observed in Alloy 600, which are not negligible. These reports have had a considerable impact on PWR fabricators, owners, and regulatory authorities and have been followed up by several other investigations[15–21] that have expanded greatly the range of observations and even observe
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