Fatigue Crack Growth Life Evaluation of the Turbine Blades in a Low-Pressure Steam Turbine

Abstract

The results of a fatigue crack growth life study of the service cracking in the dovetail notch roots of the turbine blades in the last stage of a low-pressure steam turbine are presented. After IS years or 91 518 service hours, the turbine was inspected for damage, and several cracked blades were found in the last stages of both spindles. Detailed stress analysis of the blade and rotor connection was accomplished by three-dimensional finite-element methods. Dynamic vibration analysis of the structural connection was also conducted. Environmental fatigue crack growth rate experiments were conducted using surface flawed specimens fabricated from the Type 403 stainless steel blades. The behavior in hotwell steam condensate showed acceleration over ambient laboratory air environment. Metallurgical and fractographic examination of several blade specimens showed that surface cracks emanated at the base of corrosion pits located at critically stressed locations in the dovetail notch roots. Fracture mechanics analysis showed that the applied ΔK level exceeded the threshold ΔKO level in hotwell steam condensate. Spectrum loading fatigue crack growth life analysis was conducted for the most critical location in the blade for a service spectrum consisting of hot start-up and shutdown cycles, small amplitude vibratory cycles due to steam flow, and an annual 20 percent overspeed. These results showed surface crack growth life to detectable size of 62 500 h and an inspection interval of 6750 h or 1 year of operation.