Compatibility of Fe-2¼ wt% Cr-1 wt% Mo Steel in a Sodium Environment

Kinetics of decarburization of Fe—2¼ wt% Cr— 1 wt% Mo steel in a sodium environment has been studied at temperatures between 480 and 650°C in the normalized and normalized-tempered conditions. Carbon concentration-distance profiles were obtained as a function of sodium exposure time and decarburization rate constants were evaluated. It was found that the heat treatment of the steel had no effect on the decarburization behavior at 650ºC; however, at lower temperatures, the normalized steel was found to decarburize significantly faster than the steel in the normalized-tempered condition. Microstructural examinations of specimens exposed at 650°C revealed that M_eC was the stable carbide, and the transformation of M₂₃C₆ to M₆C was accelerated by the decarburization process. In specimens exposed at 480°C, the stable carbides were found to be M₇C₃, Fe₃C, and M₂C. The results also showed that the steel would decarburize to a certain carbon level that corresponds to a stable carbide structure at each temperature, and any additional decarburization will be controlled by the dissolution rate of the carbide phases in the ferrite matrix.