Creep rupture tests on six candidate alloys for intermediate heat exchangers of high-temperature gas-cooled reactors were carried out at 1173 to 1323 K in helium with small amounts of H//2, CH//4, CO, and CO//2, and at 1173 K in H//2 plus 15% CO plus 5% CO//2. The creep rupture strengths of each alloy were scarcely different at 1173 K in both environments. At higher temperatures in helium environments, the degradation of the creep rupture strengths appeared in carbide-strengthened alloys because of decarburization. The alloy, which mainly uses alpha -W as a strengthener, showed stable creep rupture strength up to 1323 K in spite of severe decarburization.
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