Transition pieces of Hastelloy X in gas turbine combustor were suffered significant creep deflection due to material degradation during the operations. Specimens of Hastelloy X were aged up to 10000h in the temperature range of 750900°C. By using these prior-aged specimens, metallurgical observations and short term creep tests at 850°C under the stresses of 78.5 and 49 MPa were carried out to evaluate the degree of degradation, and precipitation behaviors were quantitatively analyzed by using the image analyzer. The density of precipitates, the grain boundary coverage ratio, and volume fraction of precipitates unexpectedly become larger with increasing the aging time. Increases of tensile strength, 0.2% proof stress, and hardness at room temperature were interpreted by the increase in the density of precipitates. But the precipitates, which commonly exert as strengtheners, exerted inversely to the creep resistance. This phenomenon was figured out due to the precipitation of Mo-rich, μ-phase in the grains, which reduced the Mo concentration in the matrix and reduced the solid-solution strengthening effect.
