Carbon and nitrogen transfer in Fe-9 Cr-Mo ferritic steels exposed to a sodium environment

Abstract

Kinetics of carburization/decarburization of five commercial and two high-purity Fe-9 Cr-1 to 2.5 Mo feiritic steels have been studied in a sodium environment at temperatures between 773 and 973 K. Carbon concentration-distance profiles were obtained as a function of sodium-exposure time, temperature, and carbon in sodium and the carburization/ decarburization rate constants were evaluated. The results show that the Fe-9 Cr-Mo steels are more resistant to carbon transfer than the low-alloy Fe- 2 1 4 Cr-1 Mo steel. The conditions of temperature and carbon concentration in sodium for carburization or decarburization of Fe—9 Cr—Mo steels are quite similar to those for stainless steels. However, the extent of carbon transfer in Fe-9 Cr—Mo steels is lower than that of the stainless steels. The composition and carbide structure of the steel had a significant effect on the carburization/decarburization behavior. Fe-9 Cr-Mo steels that decarburize to very low carbon concentrations either contain M 2X phase or have M 6C as the only stable carbide.