The effects of heat treatment on the chromium depletion, precipitate evolution, and corrosion resistance of INCONEL allo
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I.
INTRODUCTION
I N C O N E L * Alloy 690 is an austenitic nickel-based alloy which has been proposed as a substitute material for INCONEL Alloy 600 in steam generator ( S / G ) tubing in P W R ' s because of its much higher chromium concentration (28 to 31 wt pct v s 15 to 17 wt pct). It is well-known that I N C O N E L 600 suffers IGA and SCC failure if it is used in an aggressive environment. Since 1970, broken tubes in S / G ' s have become a vital defect for PWR's. From the statistics presented by Tatone, t~l it is found that SCC is becoming the most serious cause for the failure of PWR steam generator tubes. Therefore, it is essential to find a material which has good resistance to SCC. There are some data indicating that thermally-treated INCONEL 690 is a better corrosion-resistant tubing material than other alloys, such as 304SS, 316SS, INCOLOY* 800, I N C O N E L 600, e t c . [2"31 This alloy is *INCONEL and INCOLOY are trademarks of Inco Alloys International, Inc., Huntington, WV.
immune to SCC in primary water, [4,5,61 all-volatile treated water (AVT) environment, I71 chloride solution, r5,81 and sulfate solution. 181 Although the corrosion behavior of INCONEL 690 has been reported frequently, there is still a lack of a systematic study of the microstructural evolution and the chromium depletion near grain boundaries during heat treatment and the effect of that on the IGA and SCC resistances of I N C O N E L 690. This study was aimed to investigate the effect of heat treatment on the chromium depletion and precipitate evolution near grain boundaries of INCONEL 690 and
J.J. KAI, Associate Professor, G.P. YU, Professor, C.H. TSAI, Professor and Chairman, M.N. LIU, Graduate Student, and S.C. YAO, Graduate Student, are with the Department of Nuclear Engineering, National Tsing Hua University, 101, Section II, Kuang-Fu Road, Hsinchu, Taiwan, Republic of China. Manuscript submitted November 17, 1988.
to study the SCC and IGA behaviors of this alloy. The results of this study address the feasibility of substituting INCONEL 600 with a higher chromium alloy, INCONEL 690, to be used as S / G tubing material. II.
EXPERIMENTAL PROCEDURES
The plate-type INCONEL Alloy 690 used in this study was supplied by Huntington Alloys, Inc., Huntington, WV. It was received in a mill-annealed (MA) condition, and the chemical composition is given in Table I. Three types of specimens were prepared from the plate for various testing purposes. Specimens of 3-mm-diameter discs were used for scanning transmission electron microscope/ energy dispersive spectrometer ( S T E M / E D S ) examinations. Samples of 30 m m x 10 m m x 3 m m size were used for the IGA Huey test. The dimensions of the specimens used in the CERT were the same as those of the tensile test specimens which were described in Reference 12. The specimens were fast solution-annealed (SA) at 1150 ~ for 1 hour followed by water quenching (WQ) to dissolve all the carbides in the matrix. These specimens were then isothermally treated for various periods of time, respe
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