Effect of Heat treatment cycles on Microstructure and Room Temperature Tensile Properties of Exposed Gas Turbine First Stage Nozzle made of Nickel Base Superalloy IN738LC

The degradation mechanisms are occurred in gas turbine nozzle imposed in the critical high stress and temperature. These mechanisms could lead to reduce the service life and failure. Rejuvenation heat treatment could recover some microstructural changes to the initial state and increase the service life. In this paper, the heat treatment influence on the microstructure of the first-stage gas turbine nozzle made of IN738LC was studied after 33,000 hr of operation in a 160 MW gas turbine. The microstructure was investigated by optical and scanning electron microscopes to show the changes of the precipitated phases size, distribution morphology. Precipitated phases are γ' and carbides in grain boundaries or inside grains. The results show the proper size and distribution of the γ' and carbides similar to the initial structure obtained by the combination of the full solution heat treatment and the standard heat treatment compared. Furthermore, the tensile test was performed to evaluate the performance of heat treated sample. The tensile results of full Solution heat treatment cycle confirm the microstructural observation and show the higher tensile properties (8% in 0.2%YS, 12% in UTS & 100% in El) Related to the Standard heat treated samples.