Evaluation of Gas Turbine Hot Section Blade Cracking under Oxidation, TMF, and Creep Conditions
The first-stage blades in a model 501D5 gas turbine had 16 cooling holes. After 32,000 h of service, the blades exhibited cracking at the cooling holes. The blade material was wrought Udimet 520 alloy, with nominal composition of 57Ni-19Cr-12Co-6Mo-1W-2Al-3Ti-0.05C-0.005B. The cooling holes' surface was not coated. Investigation supported the conclusions that the cracking at the cooling holes was due to grain-boundary oxidation and nitridation at the cooling hole surface, embrittlement and loss of local ductility of the base alloy, temperature gradient from the airfoil surface to the cooling holes, which led to relatively high thermal stresses at the holes located at the thicker sections of the airfoil, and stress concentration of 2.5 at the cooling hole and the presence of relatively high total strain (an inelastic strain of 1.2%) at the cooling hole surface. Recommendations include applying the specially designed methods given in this case study to estimate the metal temperature and stresses in order to predict the life of turbine blades under similar operating conditions.