Origins of Negative Strain Rate Dependence of Stress Corrosion Cracking Initiation in Alloy 690, and Intergranular Crack
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Kuang[1] showed that the crack length per unit area increased significantly in solution-annealed (SA) and thermally treated (TT) Alloy 690, both of which were subjected to 20 pct cold rolling, termed herein SA- and TT-Alloy 690, respectively, when the strain rate decreased to 1 9 10 8/s from 5 9 10 8/s during slow strain rate tests (SSRTs) in 633 K (360 C) primary water. Here, TT-Alloy 690 was subjected to solution annealing at 1373 K (1100 C) for 1 hour, water quenching, thermal treatment at 973 K (700 C) for 17 hours, and then air cooling. Given these observations, he concluded that stress corrosion cracking (SCC) initiation was dominated by the time-dependent corrosion process, the details of which were not clarified. In fact, this negative strain rate effect on SCC initiation was observed not only in Alloy 600[2,3] but also in unirradiated 304 stainless steel in high-temperature YOUNG SUK KIM and SUNG SOO KIM, Principal Researchers, are with the Korea Atomic Energy Research Institute, Daeduk-daero 989-111, Yuseong, Daejeon 34057, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted December 27, 2015. Article published online July 12, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
water,[4] and irradiated 304 stainless steel in Ar at 588 K (315 C)[5] where no corrosion occurred. Even in ferritic steels including carbon steels and 4340 steel, intergranular cracks were observed only at a slower strain rate even in pure hydrogen gas without corrosion.[6] These observations indicate that the negative strain rate dependence of SCC initiation is a generic phenomenon occurring in metals and has no relevance to the environment or the environment-related process such as corrosion. Considering these facts, we claim that Kuang’s conclusion is unreasonable. Evidence that SCC of nickel alloys is independent of grain boundary corrosion is provided by pevious observations[7–9] that intergranular cracking of nickel alloys occurs during constant extension rate tests at 3 9 10 7/s even in 633 K (360 C) argon without grain boundary corrosion and is unaffected by chromium depletion. The most definitive evidence is the authors’ observation[10] that intergranular and transgranular cracks are formed in the inner and outer regions of an Alloy 600 specimen, respectively, during SCC tests in 633 K (360 C) water with 8 ppm of dissolved oxygen, and Angeliu’s observation[11] of a higher number of intergranular cracks in austenitic Ni-Cr-Fe alloys, preferentially in the inner regions of the fracture surface rather than on its perimeter. Considering that interior intergranular cracks were surrounded completely by ductile fracture formed on the perimeter of the fracture surface that prevented direct interaction with the environment,[10,11] it is definitively evident that intergranular stress corrosion cracking (IGSCC) or intergranular cracking of Alloy 600 has nothing to do with corrosion. Consequently, the negative strain rate dependence of SCC initiation is related not to the corrosion process but to something else such
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