Carbon Transfer Behavior of Materials for LMFBR's Steam-Generator in Flowing Sodium

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Using an AISI 304 stainless steel loop, the effect of exposure to flowing heated sodium was examined on materials considered suitable for LMFBR steam generators. The test specimens were exposed for 1, 000 hr at temperatures ranging of 550°425°C, descending in the direction of sodium flow. The oxygen concentration in the sodium was about 8 ppm. The materials tested were ferritic 2 1/41Cr-1Mo, 2 1/4 Cr-1Mo-Nb, high Cr-1Mo and austenitic AISI 316 stainless steel. The loss of weight through exposure to liquid sodium was found to depend upon the heat-treatment history in the case of the 2 1/4 Cr-1Mo steels. The principal cause of this loss of weight was confirmed to be decarburization, and dif-ferences in the heat-treatment history may possibly influence the decomposition behavior of carbides. Increasing the chromium content in Cr-1Mo steel specimens tended to modify the behavior of the specimen in the direction of increasing carbon concentration near the specimen surface during exposure to liquid sodium, altering the effect of exposure from decarburization to carburization at a chromium content somewhere between 3 and 5%. Thermal aging of annealed 2 1/4 Cr-1Mo steel was found to reduce its tensile strength at 500°C to a marked degree.
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