In order to investigate the effect on creep strength of the selective oxidation of chromium which causes the formation of a carbide-fi-ee subsurface zone, specimens of Ni22Cr12Co9Mo1Al (Alloy 617) were subjected to heat treatments to simulate a long-term service exposure of a thin-walled heat exchanger tube operating at high temperatures. In creep tests carried out at 900°C, specimens with extensive chromium-depleted and carbide-free subsurface zones exhibited higher creep strength than specimens thermally aged for the same durations, but having no carbide-free zone. The carbon released by dissolution of the carbides in the chromium-depleted zone migrated to the centre of the specimen, producing enhanced carbide precipitation. This led to an increase in the creep strength of the specimen core which offset the loss in creep strength of the subsurface zone.
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