An assessment of carburization-decarburizatton behaviour of Fe-9Cr-Mo steels in a sodium environment

A critical assessment is made of the carburization-decarburization kinetics of Fe-9Cr-Mo steels exposed to a sodium environment, using the available information on carbide phase morphology, chromium activity in a ferrite matrix, chromium carbide activity in mixed carbides, carbon solubility in Cr-Mo ferritic steels, and activity-concentration relationships based on α-phase/M₆C or M₂₃C₆carbide equilibrium. Experimental data are presented on the decarburization of Fe-5Cr-Mo and Fe-9Cr-Mo steels at 973 K in a sodium environment to ascertain the long-term behaviour of these steels. The analysis shows that the decarburization of ferritic steels is largely dependent on the chemical reaction at the carbide/α interface and that at carbon activities <0.04, the rate is predominantly determined by the dissolution of (Fe, Mo)₆C carbides.