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51317--9717-Characterization of SCC Initiation Precursors in Cold-Worked Alloy 690

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To evaluate the stress corrosion cracking initiation response of highly cold-worked UNS N06690, constant load tensile and blunt-notch compact tension testing were performed in 360°C simulated pressurized water reactor primary water.

Product Number: 51317--9717-SG
ISBN: 9717 2017 CP
Author: Karen Kruska
Publication Date: 2017
Industries: Materials Selection and Design , Science of Corrosion
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Due to its superior resistance to corrosion and stress corrosion cracking (SCC) high Cr Ni-base Alloy 690 is now commonly used in pressurized water reactors (PWRs). Even though highly cold-worked (CW) Alloy 690 has been shown to be susceptible to SCC crack growth in PWR primary water environments an open question remains whether SCC initiation was possible for these materials under constant load test conditions. Testing has been performed on a series of CW alloy 690 CRDM tubing specimens at constant load for up to 15000 hours in 360°C simulated PWR primary water. A companion paper will discuss the overall testing approach and describe results on different alloy 690 heats and cold work levels. The focus of the current paper is to illustrate the use of focused ion beam (FIB) scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for the high-resolution investigation of precursor damage and intergranular (IG) SCC crack nucleation in these specimens. Three-dimensional (3D) FIB/SEM imaging has been conducted on a series of IG precursors such as grain boundary (GB) oxides small cavities and even shallow cracks observed at the specimen surface. Contrast variations and EDS mapping were used to distinguish oxides carbides and cavities from the matrix material. Nanometer-sized cavities were observed associated with GB carbides in the highly CW specimens. Shallow IG cracks were present in the 30%CW specimens and exhibited oxidized crack flanks and a higher density of cavities ahead of the oxide front in all cases. The shape and distribution of carbides and cavities in the plain of the cracked GBs was analyzed in 3D to gain a mechanistic understanding of the processes that may be leading to SCC initiation in highly CW alloy 690.

Key words: 3D FIB/SEM, Ni-base alloys, SCC initiation, cold-work, UNS N6690, (Alloy 690)

Due to its superior resistance to corrosion and stress corrosion cracking (SCC) high Cr Ni-base Alloy 690 is now commonly used in pressurized water reactors (PWRs). Even though highly cold-worked (CW) Alloy 690 has been shown to be susceptible to SCC crack growth in PWR primary water environments an open question remains whether SCC initiation was possible for these materials under constant load test conditions. Testing has been performed on a series of CW alloy 690 CRDM tubing specimens at constant load for up to 15000 hours in 360°C simulated PWR primary water. A companion paper will discuss the overall testing approach and describe results on different alloy 690 heats and cold work levels. The focus of the current paper is to illustrate the use of focused ion beam (FIB) scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for the high-resolution investigation of precursor damage and intergranular (IG) SCC crack nucleation in these specimens. Three-dimensional (3D) FIB/SEM imaging has been conducted on a series of IG precursors such as grain boundary (GB) oxides small cavities and even shallow cracks observed at the specimen surface. Contrast variations and EDS mapping were used to distinguish oxides carbides and cavities from the matrix material. Nanometer-sized cavities were observed associated with GB carbides in the highly CW specimens. Shallow IG cracks were present in the 30%CW specimens and exhibited oxidized crack flanks and a higher density of cavities ahead of the oxide front in all cases. The shape and distribution of carbides and cavities in the plain of the cracked GBs was analyzed in 3D to gain a mechanistic understanding of the processes that may be leading to SCC initiation in highly CW alloy 690.

Key words: 3D FIB/SEM, Ni-base alloys, SCC initiation, cold-work, UNS N6690, (Alloy 690)

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