Investigation of Carbide Precipitation Process and Chromium Depletion during Thermal Treatment of Alloy 690
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DUCTION
alloy 690* is an austenite nickel-based
*INCONEL is a trademark of Special Metals Corporation, Huntington, WV.
alloy that has been developed as a substitute material for alloy 600 in steam generator tubing in pressurized water reactors (PWRs) because of its much higher chromium concentration (28 to 31 vs 15 to 17 wt pct) and lower carbon content (±0.05 wt pct).[1] According to the study of the corrosion resistance of alloy 690,[2–15] it is found that stress corrosion cracking (SCC) is becoming the most serious cause for the failure of PWR steam generator tubes. Alloy 690 has excellent corrosion resistance ability that is seriously influenced by the carbide morphology and chromium concentration near the grain boundary (GB).[16–19] Sui and co-workers studied the SCC behavior
S.Y. JIAO, Doctoral Student, M.C. ZHANG, Associate Professor, L. ZHENG, Lecturer, and J.X. DONG, Professor, are with the Department of Material Science, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted April 23, 2009. Article published online November 6, 2009 26—VOLUME 41A, JANUARY 2010
of alloys 600 and 690 in the hydrogen/steam environment;[20] they found that the cracked alloy 600 and 690 materials contained very little GB precipitation, whereas the GBs in material that did not crack were covered by continuously distributed carbide. A similar result was found by De et al.,[21] who suggested that a high density of intergranular chromium carbide could improve the intergranular SCC (IGSCC) resistance in primary water and certain caustic environments; meanwhile Yun et al.[22] studied the effects of laser surface melting on the intergranular corrosion of the sensitized alloy 600 and found that the precipitation of GB chromium-rich carbides could enhance the intergranular corrosion resistance of this kind of alloy. Morever, Zagal investigated the hydrogen permeation behavior in alloy 690, because the severity of the IGSCC was significantly enhanced as hydrogen evolution (i.e., under cathodic conditions) when exposed to PWR environments,[23] and the testing results indicated that the presence of chromium-rich carbide led to a reduction in the effective hydrogen diffusivity and hindered the crack initiating or propagating. Bruemmer et al. suggested that the intergranular carbide promotes crack tip blunting,[24] decreases the crack tip stress state, and therefore increases the resistance to cracking. Therefore, it can be concluded that the precipitation of a high density of intergranular carbide by thermal treatments would improve the SCC resistance ability. During thermal treatment, chromium-rich carbide will be precipitated, METALLURGICAL AND MATERIALS TRANSACTIONS A
especially in GBs; the formation of this kind of carbide sucks chromium from GBs and the neighboring matrix, leaving a chromium-depleted zone. If the chromium concentration in the depleted zone is lower, then it will cause SCC.[25,26] Hence, it can be found that heat treatment for
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