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Precursor Evolution and SCC Initiation of Cold-Worked Alloy 690 in Simulated PWR Primary Water

intergranular, stress corrosion cracking, crack initiation, UNS N06690, cold work, creep cavities, grain boundary microstructure
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Stress corrosion crack initiation of two thermally-treated, cold-worked UNS N06690 materials was investigated in 360 ºC simulated primary water using constant load tensile tests and blunt notch compact tension tests equipped with direct current potential drop for in-situ detection of cracking.

Product Number: 51317--9475-SG
ISBN: 9475 2017 CP
Author: Ziqing Zhai
Publication Date: 2017
Industries: Materials Selection and Design , Science of Corrosion
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Stress corrosion crack (SCC) initiation of two thermally-treated alloy 690 CRDM tubing heats in 21% and 31% cold-worked (CW) conditions have been investigated using constant load tensile tests (CLT) and blunt notch compact tension tests (BNT) in 360oC simulated primary water. All specimens were constantly monitored by direct current potential drop (DCPD) to detect the onset of cracking. SCC initiation was not detected by DCPD for the CLT specimens loaded at their yield stress for ~9220h however intergranular (IG) precursor damage and isolated surface cracks were observed on two of the 31%CW specimens. Some evidence for IG precursors was also discovered on the 21%CW CLT specimens without any significant crack nucleation. On the other hand crack initiation was detected by DCPD after ~11700h in the BNT specimens from the same two 31%CW CRDM materials loaded at moderate stress intensity. Blunt notch testing included a few slow loading ramps during the first 4000h before maintaining constant load for the remainder of the test. The BNT DCPD response for the 31%CW specimens resulted from significant IG crack nucleation and growth. On the contrary the 21%CW BNT specimens only exhibited isolated small IG surface cracks and showed no apparent DCPD change throughout the test. Interestingly post-test cross-section examination revealed many grain boundary nano-cavities in the bulk of all the specimens in both the CLT and BNT tests. The size and density of these cavities decrease as the cold-work level drops from 31% to 21%. Cavities were also found ahead of most IGSCC crack tips suggesting that the cavities are directly associated with SCC nucleation and possible growth processes. While the detailed characterizations performed on these grain boundary cavities are presented in a companion paper this current paper provides an overview of the test approach and quantitative assessments on cavity formation with regard to cold work level applied stress and exposure time. The role of these grain boundary cavities as precursors to IGSCC initiation will be discussed.

 

Key words: intergranular, stress corrosion cracking, crack initiation, UNS N06690, cold work, creep cavities, grain boundary microstructure

Stress corrosion crack (SCC) initiation of two thermally-treated alloy 690 CRDM tubing heats in 21% and 31% cold-worked (CW) conditions have been investigated using constant load tensile tests (CLT) and blunt notch compact tension tests (BNT) in 360oC simulated primary water. All specimens were constantly monitored by direct current potential drop (DCPD) to detect the onset of cracking. SCC initiation was not detected by DCPD for the CLT specimens loaded at their yield stress for ~9220h however intergranular (IG) precursor damage and isolated surface cracks were observed on two of the 31%CW specimens. Some evidence for IG precursors was also discovered on the 21%CW CLT specimens without any significant crack nucleation. On the other hand crack initiation was detected by DCPD after ~11700h in the BNT specimens from the same two 31%CW CRDM materials loaded at moderate stress intensity. Blunt notch testing included a few slow loading ramps during the first 4000h before maintaining constant load for the remainder of the test. The BNT DCPD response for the 31%CW specimens resulted from significant IG crack nucleation and growth. On the contrary the 21%CW BNT specimens only exhibited isolated small IG surface cracks and showed no apparent DCPD change throughout the test. Interestingly post-test cross-section examination revealed many grain boundary nano-cavities in the bulk of all the specimens in both the CLT and BNT tests. The size and density of these cavities decrease as the cold-work level drops from 31% to 21%. Cavities were also found ahead of most IGSCC crack tips suggesting that the cavities are directly associated with SCC nucleation and possible growth processes. While the detailed characterizations performed on these grain boundary cavities are presented in a companion paper this current paper provides an overview of the test approach and quantitative assessments on cavity formation with regard to cold work level applied stress and exposure time. The role of these grain boundary cavities as precursors to IGSCC initiation will be discussed.

 

Key words: intergranular, stress corrosion cracking, crack initiation, UNS N06690, cold work, creep cavities, grain boundary microstructure

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