Structure property correlation study of a service exposed first stage turbine blade in a power plant

Abstract

This paper deals with assessment of high temperature force controlled fatigue and creep properties of as received and 31,325 engine operating hours service exposed first row gas turbine blade in a power plant. The mechanical properties were evaluated from various regions, namely root portion (bottom portion), middle region or the airfoil region and the top region of the blade. The root portion showed superior fatigue and creep properties compared to the aerofoil region of the blade where the temperature distribution is maximum and the top or tip region, which is considerably narrow and brittle. The microstructural assessment of as received service exposed gas turbine blade of first row of gas turbine have been interpreted, which is required for developing process and technology for rejuvenation of turbine blades. The mechanical properties of each region were correlated with its corresponding microstructural characteristics. The blade is made of nickel base superalloy Udimet-520. Significant coarsening of γ′ precipitates in hot zones, e.g., root and mid-sections, as compared to the base microstructure of below platform region was observed. The coarsening occurred in the aerofoil region and was maximum in the top region of the blade. The originally thin and nearly continuous distribution of grain boundary carbide precipitates became discontinuous with little increase in size in the root section whereas it changed to thick continuous precipitation enveloped by γ′ phase in mid-section. Grain boundary cavitations are not observed. In addition, the usual distribution of intragranular MC carbide precipitates in below platform section is nearly dissolved in mid-section while smaller size is observed in bottom or root section. But the blocky MC precipitates at grain boundaries or close to grain boundaries are degenerated into needle shaped sigma phase in the mid- and top sections. Formation of intragranular Cr-rich sigma phase is also observed in root, airfoil and top sections but their frequency of occurrence is more in mid- and top sections than the root section. The appearance of sigma phase also signals for the rejuvenation. All the grain boundaries in mid- and top section show enveloping of M23C6 grain boundary precipitates with γ′ phase, because of following transformation at grain boundaries: MC + γ M23C6 + γ′ (γ′ envelopes the M23C6). The remnant coating at root section is 100–120 μm. In mid-section it is about 10 μm with base metal attack down to 20 μm. In addition, the width of TCP phase formation zone in subsurface region of base metal is about 200 μm in mid-section while it is about 100 μm in root section. Compared to the root and below platform sections of the virgin material the service exposed blade revealed MC intragranular precipitation in below platform, dissolution of intragranular MC precipitates and coarsening of grain boundary precipitates in the root section respectively.