Metallurgical Investigation of Cracking in an Industrial Gas Turbine Blade

A utility experienced cracking in multiple row 2 blades in a large power generation gas turbine after only ~7,000 hours and ~30 starts following a standard repair and rejuvenation cycle. The blades were made 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 possible/likely contributing factors to root cause in terms of design, operation, and metallurgy. Three blades, two with visible cracks and one without cracks, were subjected to non-destructive and destructive evaluation including visual inspection, 3-D scanning, physical measurements, process compensated resonant testing, chemical analysis, optical metallography, LED surface topological evaluation, and scanning electron microscopy including SE, BSE, EDS, and EBSD. The failure mechanism was consistent with creep but no evidence of localized overheating was observed.

This presentation will present the approach and findings from the failure investigation which identified metallurgical risk, due to cast-to-cast compositional differences, as a likely contributing factor in the propensity for cracking in the utilities overall population of blades. The results are compared with other information available in the industry to make informed recommendations to the end user for future actions.