1983 Volume 69 Issue 8 Pages 1006-1013
Corrosion tests of Inconel 617 were carried out in a temperature range from 900°C to 1040°C in helium environment with small amounts of impurity gases, which was the simulated primary coolant of High Temperature Gas cooled Reactor (HTGR), and the carburization, decarburization, and oxidation were investigated in terms of temperature.
(1) The alloy was carburized in the temperature range from 900°C to 970°C, but was decarburized above 990°C. The transition temperature from carburization to decarburization was estimated to be 975°C. In the temperature range, where carburization occurred, the largest carbon pickup was observed at 940°C, however, in the temperature range of decarburization, the degree of decarburization increased with the increase of temperature.
(2) In the temperature range of carburization, the carburized layer was observed at near-surface region, and Cr-Mo carbides precipitated along the grain boundaries and twin boundaries. In the temperature range of decarburization, the carbides along the grain boundaries disappeared, and this led to the formation of altered layer, the depth of the layer reaching to about 2000μ after 500h corrosion at 1040°C.
(3) In contrast to the results that the oxide films formed at high temperature in air were dense and protective, the films formed in the helium environment were very porous and less protective. Further, the chromium content in the oxide films along the grain boundaries beneath the surface decreased with the increase of test temperature. This could lead to make the films much less protective.