ANALYSIS OF GENERAL-ELECTRIC MODEL 7001 FIRST-STAGE NOZZLE CRACKING

Analyses of first-stage nozzle cracking in General Electric Model 7001 B and 7001E industrial gas turbines are presented. Empirical algorithm s are developed to predict the maximum extent of cracking that is visi ble on these nozzles as a function of engine cyclic history and the nu mber of fired hours. It ir; shown that the algorithms predict this cra cking to within a factor of two. Metallurgical analyses of nozzles sho w that crack growth follows the carbide-matrix interface, environmenta l 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 rest dat a and stress analyses from the literature, are used to understand the nature of nozzle cracking. The maximum extent of cracking coincides wi th locations of maximum thermal stresses as determined by finite eleme nt 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.