Structure, chemistry, and stress corrosion cracking of grain boundaries in alloys 600 and 690
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I.
INTRODUCTION
NICKEL-base alloys of the types alloy 600 and alloy 690 have been used extensively in nuclear reactor steam generators. Alloy 600, an alloy with the typical major element composition Ni-16Cr-9Fe used as steam generator tubing in reactors, has been found to be highly susceptible to environmentally induced intergranular stress corrosion cracking (IGSCC) on both the primary and secondary sides, while alloy 690, with a major element composition of typically Ni-30Cr-9Fe, has shown considerably better performance than alloy 600 in both primary water and caustic environments. Owing to the well-known susceptibility to IGSCC of alloy 600, considerable research efforts have been made to identify the mechanism by which cracking occurs. These investigations, reviewed by Was,m have shown that IGSCC is heavily dependent on the heat treatment received by the material. For instance, a heat treatment of mill-annealed (MA) alloy 600 in the temperature range 650 ~ to 750 ~ has been found to cause a drastic improvement of the resistance to IGSCC in pure high-temperature water. Although the exact mechanism of enhanced resistance to IGSCC is unclear and still under debate, there is general agreement that the chemistry and structure of grain boundaries are of crucial importance. In this connection, chromium depletion,t2 51 segregation of impurities to grain boundaries, I2,~81intergranular carbides and their mechanical effect on stress concentrations,t91 and grain boundary misorientationV01 are essential for the corrosion behavior of the material. This investigation was undertaken to examine the structure and chemistry of grain boundaries in alloys 600 and 690 in an attempt to correlate these to the intergranular stress corrosion behavior of the materials in high-purity wa-
ter with small hydrogen additions. Both MA and thermally treated (TT) variants of alloys 600 and 690 were considered. The choice of the materials was based on the crack initiation time obtained in the laboratory tests. In this way, materials having similar composition but possessing different susceptibility to IGSCC were selected for analysis. The techniques used were analytical transmission electron microscopy (ATEM), secondary ion mass spectroscopy (SIMS), and atom probe field ion microscopy (APFIM). II.
E X P E R I M E N T A L DETAILS
The corrosion testing was carried out on 181 specimens from 37 different tubes of alloy 600 and 690 materials. It was performed in autoclaves using reverse U-bend (RUB) specimens in high-purity water with additions of hydrogen (40 to 50 mL HJkg H20 ) at 365 ~ After this test, three alloy 600 variants (BL, BH, and IP) were selected for microstructural studies. The crack initiation time of the most resistant IP variant was two and 20 times longer than that of BH and BI, respectively. Since the amount of boron in both BL and BH conditions was high, the microstructure of one boron-poor variant (E) was additionally investigated. The information about heat treatment and crack initiation time for this material is, however
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