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.
Highlights
ALLOY 690 was developed about 50 years ago by the International Nickel Company as a corrosion resistant Nickel based 30Cr–9Fe alloy for many demanding high temperature environments.[1]
The number, shape and size of the cavities at or near grain boundary (GB) carbides in specimens of the cold worked commercial TT Alloy 690 were observed before and after long-term heating in air as well as before and after long-term exposure in simulated Pressurized Water Reactors (PWRs) primary water at 633 K
In order to better understand the mechanism of cavity formation for heavily cold rolled TT Alloy 690, the relationship between the number of cavities at or near GBs and carbides as a function of GB orientation to the rolling direction (RD) had to be determined for each specimen
Summary
ALLOY 690 was developed about 50 years ago by the International Nickel Company as a corrosion resistant Nickel based 30Cr–9Fe alloy for many demanding high temperature environments.[1]. Corporation).[2,3] This development of TT Alloy 690 depends on controlling combinations between the fully SHT and TT conditions as a function of carbon content that are selected to generate an optimized microstructure, as revealed by transmission electron microscopy (TEM).[4,5] This TT Alloy 690 has excellent stress corrosion cracking (SCC) resistance in concentrated secondary water SG crevice environments and in PWR primary water environments It has been widely used as the alternative material to MA Alloy 600 for PWR SG tubes, and for various PWR primary circuit pressure boundary components exposed to the primary water environment. TT Alloy 690 has been used for more than 30 years in PWR primary water environments without any known SCC indications anywhere in the world.[6]
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