Abstract
The present study investigated the microstructure of superalloy gas turbine damping bolt. These damping bolts were selected from new and service-exposed ones made of nickel-based Nimonic 90, which had been served for a long time. The microstructure of new sample had an austenitic matrix with many twins. These twins were located in the intergranular and intragranular regions of the primary carbides. Fine gamma-prime precipitates also could be found in the field. However, due to the high amount of service time, the arranged structure turned into a complex one in which the Sigma and Mayo phases with the Widmanstätten pattern appeared near the grain boundaries. Therefore, secondary continuous carbides formed near the grain boundary, and γ′ precipitates grew, which increased the hardness of the samples by about 30 Vickers. The mentioned microstructural changes reduced the creep rupture to 140 MPa at 870 °C in 11 h, while the sample had a creep life of about 21 h. This indicates that the resulting microstructural changes reduced brittleness and creep life. The soft intergranular failure mechanism in the new damping bolt became an intergranular mechanism. Moreover, the effects of intragranular failure were also abundant.
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