The useful life of a steam turbine and the establishment of turbine outage schedules are often determined by corrosion fatigue to the low pressure (LP) blades in the phase transition zone (PTZ). Developing an effective corrosion damage prediction methodology is an important step to successfully reduce the number of unscheduled steam turbine outages.
Tests with dual certified 403/410 12% Cr martensitic steel were performed to quantify the influence of corrosion pits on the fatigue life during testing in environments that are comparable to operational conditions. Threshold stress intensity factors ΔKth and fatigue limits Δσ0 were determined in air and two aqueous solutions. Additionally, stress-life tests were performed with pre-pitted specimens in air and aqueous solutions.
The data for transition from a pit-to-a-crack have been correlated using the Kitagawa Diagram. This presentation of the data relates the steady stress, cyclic stress and pit width to the prediction of fatigue failure. Ultrasonic fatigue testing was an essential aspect of this program. This testing technique makes it possible to accumulate cycles at a rate of approximately 20 kHz. At this rate one billion (109) cycles are accumulated in less than 14 hours. One billion cycles has been used as the definition for non-progressive crack or specimen run-out life. All of the data for the survival and failure stress intensity factor was well represented by the El Haddad refinement to the Kitagawa Diagram.
Based on these test results a comprehensive methodology has been developed to quantify the risk of corrosion-fatigue failure at a pit.