Comparison of various chromium--molybdenum low-alloy steels for liquid-metal fast breeder reactor steam generators

High-temperature strength, weldability, and decarburization behavior in liquid sodium of 21/4 Cr--1 Mo steel with a wide variety of heat treatments and some carbon stabilized 21/4 Cr--1 Mo steels are examined for the purpose of material selection of LMFBR steam generator tubes. Decarburization of 21/4 Cr--Mo steel in sodium can be suppressed when it is appropriately normalized and tempered instead of annealed. The 21/4 Cr--1 Mo--Db--Ni steel is considered to be preferable to other stabilized steels, since a small addition of nickel assures sufficient strength, even if the normalizing temperature is as low as 9500C. Stabilized steels should be welded carefully to obtain a fully penetrated bead. Preheating above 2000C and postheating above 7200C are required for stabilization. Compared with all the investigated properties, the materials recommended for LMFBR steam generator tubes are normalized-tempered 21/4 Cr--1 Mo steel and, if decarburization is not allowable, 21/4 Cr--1 Mo--Nb--Ni steel, with the condition that a sound-welded joint can be available