Behavior of grain boundary chemistry and precipitates upon thermal treatment of controlled purity alloy 690
- PDF / 1,998,303 Bytes
- 11 Pages / 594 x 774 pts Page_size
- 97 Downloads / 181 Views
I.
INTRODUCTION
IN the late 1970s, an alternate alloy was sought to replace nickel-base alloy 600 in nuclear steam generators. This was motivated by the susceptibility of this alloy to intergranular stress corrosion cracking (IGSCC) in water environments, t~1 The most promising replacement material has proved to be nickel-base alloy 690, which has been found to be immune to IGSCC in various water environments.t21 No clear explanation exists for the improved resistance of alloy 690 to IGSCC in caustic 13~and deaerated neutral environments. I41 Since IGSCC is a grain boundary phenomenon, characterization of the grain boundary chemistry is an important element in determining the IGSCC susceptibility. However, unlike alloy 600, alloy 690 has not been well characterized with respect to its grain boundary chemistry and precipitate nature. Two important characteristics of the grain boundary are the chromium depletion zone width and depth produced upon the precipitation of Cr-rich carbides and the composition, structure, and distribution of the carbides. In alloy 600, the Cr depletion zone has been well documented and found to be a factor in the IGSCC suscepTHOMAS M. ANGELIU, Graduate Research Assistant, Deparlment of Materials Science and Engineering, and GARY S. WAS, Associate Professor, Department of Nuclear Engineering and Department of Materials Science and Engineering, are with the University of Michigan, Ann Arbor, MI 48109. Manuscript submitted October 4, 1989.
METALLURGICAL TRANSACTIONS A
tibility in sodium tetrathionate151 and aerated water environments. [61The composition and structure of the grain boundary carbides will affect the width and depth of the Cr depletion zone.t7] The distribution of the grain boundary carbides may induce a mechanical effect where fewer carbides cause a localization of stress and thus increase the crack susceptibility,tSj The grain boundary chemistry and microstructure of alloy 600 have been well characterized, t9't~ However, only one paper to date has been published on Cr depletion in alloy 690. tll~ This paper describes the development of the Cr depletion zone and the precipitation of carbides in a controlled purity alloy 690-type material. Chromium depletion was modeled using a modified, integrated thermodynamic and kinetic model established by Was It2~ for Ni-16Cr-9Fe alloy (typical of alloy 600). The expanded model computes the redistribution of chromium near the grain boundary for a Ni-30Cr-9Fe alloy (typical of alloy 690) during thermal treatment. The expansion involves accounting for the formation of Cr23C6, rather than Cr7C3, as the dominant grain boundary carbide and applying the appropriate carbon solubility and chromium diffusion data for alloy 690. The chromium depletion zone is computed as a function of alloy composition, grain size, and time at temperature. The results are compared with experimentally obtained Cr depletion profiles measured by energy dispersive X-ray analysis via scanning transmission electron microscopy (EDAX-STEM). The precipitates are characteri
Data Loading...
