This paper studies the microstructure chage of Ni-base superalloy IN738LC in the temperature range of 750-900°C, and develops a new method of predicting the service temperature of gas turbine buckets operated for 20000hr. The specimens of IN738LC were aged in the temperature range up to 24000hr to achieve the microstructure change. The growth rate of γ' diameter in IN738LC was proportional to 1/3 order of aging time, and γ' density was inversely proportional to aging time Accuracy of the predicted temperature based on the γ' diameter and density increased with increasing aging time and temperature. The metal temperature of actual 20000hr serviced buckets estimated by the microstructure change agreed with the thermal analysis at the leading edge portion which is a most reliable point for the analysis. This shows the availability of the proposed method.
