Mechanisms of Sensitization and Stablization of Incoloy Nickel-lron-Chromium Alloy 825

Abstract

Incoloy(1) nickel-iron-chromium Alloy 825 can become sensitive to intergranular attack by exposure to temperatures in the 1200–1400 F (649–760 C) range. Sensitivity to intergranular attack (as measured by the Huey test) can be correlated to the presence of a chromium-depleted area adjacent to the grain boundary. This area is caused by the cellular precipitation of chromium-rich M23C6 during sensitizing treatments. The chromium-depleted region is between the M23C6 cells, but because the carbides grow by boundary migration the depletion is not in the plane of the grain boundary. Incoloy Alloy 825 can be stablized against intergranular attack by eliminating the chromium-depleted area. The degree of stabiization is related to the amount of carbon remaining in solution after the stablizing treatment. The titanium content of the alloy helps to effect stabtization. Because it has a greater affinity for carbon than does chromium, the titanium reacts to form stable carbides and thus reduces chromium depletion. Maximum stabtization of Incoloy Alloy 825 cannot be obtained if titanium carbide is used as the sole stablizing mechanism. The principal mechanism of stablization for this alloy is that of precipitating the M23C5 at a temperature where the diffusion of chromium is sufficiently rapid to prevent chromium depletion. This occurs at temperatures in the 1700–1800 F (927–982 C) range.