Carburization of austenitic and ferritic alloys in carbon-saturated sodium at 916 K

Abstract

The liquid sodium coolant of fast breeder reactors is an effective carbon transfer medium. The relationship between sodium carbon activity, and the resultant carbide composition and microstructure of structural and cladding steels, is of particular importance. This initial investigation is concerned with the carburization of austenitic and ferritic steels exposed to carbon-saturated sodium. This condition could be approached in a reactor where adventitious ingress of carbonaceous material is possible. The extent and nature of carbide phases produced have been established by metallographic and X-ray techniques; the carbides (Fe,Cr)₇C₃ and (Fe,Cr)₂₃C₆ with a range of Fe/Cr ratios were identified in austenitic steels containing 18wt-% chromium, and the carbide (Fe,Cr)₃C was exclusively found in carburized ferritic steels containing 2¼ and 9wt-% chromium. A special feature of the work is the use of an electrochemical carbon cell for measuring the sodium carbon activity.