“…An engineering model was developed to predict initiation of IGSCC in nickel based Alloy 600 exposed to primary water [35]. This empirical model does not describe physical mechanisms, although it includes variables that most often have a physical meaning (temperature, stress, carbide precipitation).…”
Abstract:Even if temperature, pressure and chemistry of the cooling water are not very high and aggressive, materials used in PWRs (Pressurized Water Reactors) are exposed to different degradation mechanisms. One of the main goals of the research programs in this field is to develop physical model of the mechanisms down to the atomic scale. Such approach needs a clear description and understanding of the degradation mechanisms at the same small scale. This paper illustrates the benefits of different microscopies and of their last improvements up to the promising possibilities of monochromated and aberrations corrected TEM/STEM. A specific focus is placed on four different degradation mechanisms observed in austenitic stainless steel: irradiation ageing, corrosion fatigue, stress corrosion cracking and corrosion.
“…An engineering model was developed to predict initiation of IGSCC in nickel based Alloy 600 exposed to primary water [35]. This empirical model does not describe physical mechanisms, although it includes variables that most often have a physical meaning (temperature, stress, carbide precipitation).…”
Abstract:Even if temperature, pressure and chemistry of the cooling water are not very high and aggressive, materials used in PWRs (Pressurized Water Reactors) are exposed to different degradation mechanisms. One of the main goals of the research programs in this field is to develop physical model of the mechanisms down to the atomic scale. Such approach needs a clear description and understanding of the degradation mechanisms at the same small scale. This paper illustrates the benefits of different microscopies and of their last improvements up to the promising possibilities of monochromated and aberrations corrected TEM/STEM. A specific focus is placed on four different degradation mechanisms observed in austenitic stainless steel: irradiation ageing, corrosion fatigue, stress corrosion cracking and corrosion.
“…This result suggests that the bowed shape of the sample, associated with the presence of compressive stress on the peened surface, decreased the effective surface stress due to delayed straining and therefore retards crack initiation compared to the RS surface of the same specimen at low nominal stress levels. On the other hand, at equal strain levels, the effective stress on the SP side is expected to exceed that on the RS side due to deformation hardening caused by the peening process, which is detrimental with regard to EAC [18]. Figure 9 Stress–strain curves at the minimum cross section of the Alloy 182 FTT specimens with RS/SP surfaces.…”
Section: Resultsmentioning
confidence: 99%
“…In fact, when a material with a cold-worked surface is strained beyond yield, tensile stresses as high as 1000 MPa can be generated, e.g. in the cold-worked layer of Alloy 600 tubing [18]. The detrimental effect of compressive stress generated by SP has been reported on austenitic stainless steels exposed under dynamic deformation [34] as well.…”
Surface treatments of primary circuit components in light water reactors are regarded as possible ways to mitigate environmentally-assisted cracking (EAC). To date, it is not fully conclusive which surface condition is suitable to reduce the EAC initiation susceptibility. Constant extension rate tensile (CERT) tests were performed by several labs using flat tapered tensile specimens with different surface conditions (ground, industrial face milled, advanced face milled and shot peened), exposed to a boiling water reactor normal water chemistry environment at 288°C. Despite some scatter in the results, the CERT tests revealed that the EAC initiation susceptibility seems lowest for the advanced face milled surface and highest for the shot peened surface. However, it must be emphasised that the differences were moderate and that the surprising behaviour of the shot peened surface can be explained. The mechanical grinding of the surface did not significantly retard EAC initiation compared to industrial face milling.
“…To check the validity of such measurement for SCC applications, the strain rate at the crack tip is estimated from the supposed crack growth rate using the correlation established by Le Hong et al [25]. For a CGR of 1 mm/yr, a strain rate of 10 −7 s −1 is found.…”
Section: Application To the Plasticity-enhanced Diffusion Mechanismmentioning
Two mechanisms are studied to explain the asymmetrical chromium depletions observed ahead of SCC crack tips in nickel-base alloys: diffusion-induced grain boundary migration (DIGM) and plasticity-enhanced diffusion. On the one hand, DIGM is evidenced in a model Alloy 600 by focused ion beam (FIB) coupled with scanning electron microscopy (SEM) cross-section imaging and analytical transmission electron microscopy (TEM) after annealing at 500°C under vacuum and at 340°C after exposure to primary water. The occurrence of grain boundary migration depends on the grain boundary character and misorientation. On the other hand, the effect of plasticity on chromium diffusion in nickel single-crystals is investigated by performing diffusion tests during creep tests at 500 and 350°C. An enhancement of Cr diffusion is observed and a linear relationship between the diffusion coefficient and strain rate is evidenced. At last, in an attempt to discriminate the two mechanisms, an analytical modeling of the Cr-depleted areas observed at propagating SCC crack tips is proposed.
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