Cracking in Gas Turbine Blades
Cracking in gas turbine blades was found to initiate from a mechanism of low-cycle fatigue (LCF). LCF is induced during thermal loading cycles in gas turbines. However, metallography of two cracked blades revealed a change in microstructure at as-cast surfaces for depths up to 0.41 mm (0.016 in.). Evaluation by SEM confirmed the difference in structure was associated with a lack of formation of coarse gamma prime structure in the matrix. Microhardness and miniature tensile test results indicated lower strength consistent with the absence of the coarse gamma prime constituent. The blade vendor found that the lot of hot isostatically pressed (HIP) blade castings had been exposed to an improper atmosphere during the HIP process, resulting in the weakened structure. Because subsequent failures were found in blades that did not come from the suspect HIP lot, the scope of the problem was considered generic, and the conclusion was that the primary failure mechanism was LCF. Material imperfections were a secondary deficiency that had the effect of causing the blades from the bad HIP lot to crack first.