Development of steel tubes for steam generator of fast breeder reactor

When austenitic steel and ferrite steel coexist in liquid sodium, the elements in the steels leach out and move through the liquid sodium. Especially when the decarburization occurs in ferritic steel, the lowering of high temperature strength becomes a design problem. The high-temperature strength, weldability, and decarburization behavior of 2 1/4 Cr-1 Mo steel with a wide variety of heat treatments and some carbon-stabilized 2 1/4 Mo steels in liquid sodium were examined for the purpose of material selection for the steam generator tubes of liquid-metal fast breeder reactors (LMFBR). The decarburization of 2 1/4 Cr-1 Mo steel in sodium depends on the matrix and the precipitates, and can be suppressed when the steel is appropriately normalized and tempered. The 2 1/4 Cr-1 Mo-Nb-Ni steel is superior to other stabilized steels, since the small addition of nickel assures sufficient strength, even if the normalizing temperature is as low as 950 degree C. Stabilized steels should be welded carefully to obtain fully penetrating beads. Preheating above 200 degree C and postheating above 720 degree C are required for the stabilization. As comparing all the investigated properties, the materials recommended for LMFBR steam generator tubes are normalized and tempered 2 1/4 Cr-1 Mo steel and, when the decarburization is not allowable, 2 1/4 Cr-1 Mo-Nb-Ni steel, with the condition that sound-welded joints can be available. (Kako, I.)