Temperature Estimation and Life Prediction of Turbine Blades Using Post-Service Oxidation Measurements

The depth of internal oxidation and nitridation from the surface of the 16 cooling holes in a first-stage turbine blade was measured by optical microscopy after 32,000 hours of service. Maximum depth of penetration was 15.5 mils (0.4 mm) at the trailing edge hole. An effort was made to predict hole surface metal temperatures based on these measurements using the Arrhenius relationship between time and temperature with depth of oxidation assumed to be parabolic with time. Reasonable correlations were obtained between finite element analysis results and temperature estimates based on the oxidation measurements. In the thickest part of the airfoil, where metal temperature is minimum, intergranular cracks up to 12.6 mils (0.32 mm) in depth were found at the surface of the cooling holes. Measurable oxidation attack was only one to two mils (0.025–0.050 mm). Based on an approximate elastic-relaxation-local inelastic stress analysis, it was calculated that inelastic local strains of over one percent occur at the points of cracking. No cracking was observed in the more heavily oxidized, lower stressed, hotter holes. However, cracking occurred in a trailing edge tip cooling hole when weld repair of the tip squealer was attempted, due to embrittlement and grain boundary oxidation from service exposure. Temperature estimates suitable for life assessment purposes using oxidation measurements appears to be a possible technique that should be further developed and validated.