| description abstract | A utility experienced cracking in multiple stage 2 buckets (S2Bs) in a large F-class power generation gas turbine after only 7348 h and 31 starts following a standard repair and rejuvenation heat treatment cycle. The blades were cast from a directionally solidified (DS) superalloy and the cracking was found in the shroud section of the blade in the Z-notch region. A detailed failure investigation was conducted to determine the failure mechanism and most likely contributing factors to root cause in terms of design, operation, fabrication, and metallurgy. Three blades, two with visible cracks and one without cracks, were subjected to nondestructive and destructive evaluation. Methods consisted of visual inspection, three-dimensional scanning, physical measurements, chemical analysis, optical metallography, LED surface topological evaluation, and scanning electron microscopy. The failure mechanism was determined to be due to creep with damage propagating along the grain boundaries in a high stress concentration area. Evaluation of precipitate structure and size also confirmed this area exhibited the highest temperature during operations. High temperature creep testing was also conducted in material from the two different casting houses to confirm metallurgical risk did not have a significant impact on creep performance. An evaluation of the operational profile of an entire fleet of GE 7FA units showed that the failed 7F.03 S2Bs were in the highest operating temperature base loaded units. The findings from this work will help better define key factors that influence the high temperature performance of nickel-base superalloys used in the hot section of industrial power generating turbines. | |
