THE EFFECT OF SELECTIVE OXIDATION OF CHROMIUM ON THE CREEP STRENGTH OF ALLOY-617

In order to investigate the effect on creep strength of the selective oxidation of chromium which causes the formation of a carbide-free sub surface zone, specimens of Ni22Cr12Co9MolAl (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 t ests carried out at 900-degrees-C, specimens with extensive chromium-d epleted and carbide-free subsurface zones exhibited higher creep stren gth than specimens thermally aged for the same durations, but having n o carbide-free zone. The carbon released by dissolution of the carbide s 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 cree p strength of the subsurface zone.