Effect of Cold Working and Long-Term Heating in Air on the Stress Corrosion Cracking Growth Rate in Commercial TT Alloy 690 Exposed to Simulated PWR Primary Water

Abstract

The authors have previously reported that the number of cavities at or near grain boundary (GB) carbides in commercial thermally treated (TT) Alloy 690 increases with increasing cold work reduction ratio and with heating temperature in air. In the present work after very long-term heating in air, the number of cavities at or near GB carbides in cold worked commercial TT Alloy 690 was observed to saturate, and the shape and size of the cavities changed. The shape and size of cavities and cracks were categorized, and a GB defect index number was defined as a function of their number, shape and size. Stress corrosion cracking growth rates in a commercial TT Alloy 690 with various levels of cold work exposed to simulated PWR primary water at 633 K (360 °C) have been measured and correlated with the defined GB defect index number. Cavities and cracks in the same materials before and after long-term heating in air have also been correlated with the defined GB defect index number. For the heavily cold worked (≥ 15 pct) commercial TT Alloy 690, a good correlation has been observed between the PWSCCGR and the GB defect index number. By contrast, for lightly cold worked (≤ 10 pct) commercial TT Alloy 690, the SCCGR in the simulated PWR primary water was very low and the GB defect index number was usually zero, regardless of cold working reduction ratio ≤ 10 pct. It is concluded that the mechanism of SCCGR for lightly cold worked TT Alloy 690 in PWR primary water is likely to be different from that for heavily cold worked TT Alloy 690.

On the role of intergranular nanocavities in long-term stress corrosion cracking of Alloy 690

“…As a result, intergranular carbides, despite their widely reported beneficial effects on the IGSCC of Alloy 600 [13,52] , could indirectly accelerate the formation of cavities in Alloy 690, contributing to the weakening of the GBs. Similar results were also reported by Kuang et al [40] and Yonezawa et al [53] .…”

“…In other words, it is necessary to show not only that the temperature dependency of crack growth rate and creep cavity formation are similar, but also that cavity formation and deformation have the same stress dependencies. However, the stress dependency was neither observed in three-point bent beam IGSCC tests nor in reserve U-bend specimen IGSCC tests [53] . Hence, Yonezawa et al [53] suggested that the creep damage might be not the mechanism of IGSCC of CW Alloy 690.…”

“…However, the stress dependency was neither observed in three-point bent beam IGSCC tests nor in reserve U-bend specimen IGSCC tests [53] . Hence, Yonezawa et al [53] suggested that the creep damage might be not the mechanism of IGSCC of CW Alloy 690.…”

“…While creep damage was accepted as a plausible mechanism of IGSCC of CW Alloy 690 [6] , in a recent study conducted by Yonezawa et al [53] , it was argued that the results in [6] were not sufficient to support the creep damage hypothesis. Yonezawa et al [53] believed that to confirm the creep damage hypothesis as the mechanism of IGSCC of CW Alloy 690, it is necessary to prove that not only is creep a necessary condition but also a sufficient condition.…”

“…While creep damage was accepted as a plausible mechanism of IGSCC of CW Alloy 690 [6] , in a recent study conducted by Yonezawa et al [53] , it was argued that the results in [6] were not sufficient to support the creep damage hypothesis. Yonezawa et al [53] believed that to confirm the creep damage hypothesis as the mechanism of IGSCC of CW Alloy 690, it is necessary to prove that not only is creep a necessary condition but also a sufficient condition. In other words, it is necessary to show not only that the temperature dependency of crack growth rate and creep cavity formation are similar, but also that cavity formation and deformation have the same stress dependencies.…”