Analysis of General Electric Model 7001 First-Stage Nozzle Cracking

Analyses of first-stage nozzle cracking in General Electric Model 7001B and 7001E industrial gas turbines are presented. Empirical algorithms are developed to predict the maximum extent of cracking that is visible on these nozzles as a function of engine cyclic history and the number of fired hours. It is shown that the algorithms predict this cracking to within a factor of two. Metallurgical analyses of nozzles show that crack growth follows the carbide-matrix interface, environmental attack occurs at the crack tip, and that the microstructure changes by increasing the amount of carbide precipitation, which increases the hardness. These metallurgical results, along with mechanical test data and stress analyses from the literature, are used to understand the nature of nozzle cracking. The maximum extent of cracking coincides with locations of maximum thermal stresses as determined by finite element analyses of similar nozzle designs. This location is at the airfoil-shroud junction on the middle vanes of multivane castings. The use of these algorithms as a predictive maintenance tool and the ability to inspect nozzles visually in the engine also are discussed.