Evolution of Microstructure and Mechanical Characteristics in Rejuvenation of a Hot Gas Pass Component of a Gas Turbine by Heat Treatment and HIP

Abstract

The evolution of microstructure and mechanical characteristics during the rejuvenation of a hot gas pass component of a gas turbine was investigated with an actual service-exposed bucket in a power plant. Heat treatment and hot isostatic pressing processes were conducted to rejuvenate a hot gas pass component of a F class gas turbine which had been operated for 101.15% of its expected lifetime. In the three step heat treatment, the specimens were exposed to 1210 degrees C, 1120 degrees C, and 845 degrees C for periods of 2 h, 2 h, and 24 h, respectively, with rapid cooling. Then, the specimens were exposed to conditions of 1200 degrees C and 100 MPa for 4 h to enhance their integrity. Analysis of the microstructure determined that after heat treatment the average size of gamma' decreased around 60 mu m, while the area fraction of gamma' increased around 20% compared to before heat treatment. With respect to mechanical characteristics, the samples' stress-rupture time, yield strength, and ultimate strength improved with heat treatment and hot isostatic pressing, while hardness did not show any meaningful variation. These phenomenological results suggest that heat treatment and hot isostatic pressing play a critical role in the rejuvenation of a hot gas pass component, and can provide an operating and maintenance strategy to enhance the economic feasibility of a combined cycle power plant.