Ronald N. Salzman

Corrosion-Fatigue Prediction Methodology for 12% Cr Steam Turbine Blades

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.Copyright

Turbine Blade Fatigue Crack Growth

This paper presents an approach that has not previously been applied to predict power plant component fatigue failure. While many computer tools and procedures do exist for life prediction, recent experience shows that those methods often lack needed precision. In addition detailed knowledge of the material fracture mechanics properties and of the component load history, including the sequence of events. For complex load applications the sequence of events has been shown to be significant. For these applications Linear Damage Models (Minors Rule) are inadequate. A case where the accuracy of existing methods was found to be insufficient involved a steam power plant. Fatigue cracks were observed in the blade root area of 10% of the L-0 row turbine blades. The cracked blades were removed and replaced with new blades. The question to be answered was how safe are the reinstalled blades that had no visible damage. A successful solution was obtained by integrating NASA technology with STI experience in the analysis of steam and gas turbine blades. This led to the development of a proprietary, advanced life prediction computer code that: • incorporates contributions from both HCF and LCF to calculate crack growth, • treats the actual sequence in which all HCF and LCF loading has been applied, and • initiates the crack growth process from the microscopic inclusions and flaws. Complete analysis included comprehensive material tests, performed at a qualified material-testing laboratory. Test specimens were created from actual components, which had been subjected to over 30 years of service. Realizing that fatigue testing data variations can be relatively large, results from Life Cycle showed solid agreement with both short-term (105 to 106 cycles) material tests and with long-term (∼160×109 cycles) operation under high strain steady load and low strain cyclic load conditions.© 2004 ASME

Application Of A Crack-Closure Model For Turbine Life Assessment.

This paper describes the application of a fracture mechanicsbased procedure for life estimation that allows complex load cycle effects to be included. The procedure is based on technology developed by the aerospace industry, which includes LCF and HCF loading and incorporates local plastic strain effects at the crack tip. The procedure assumes that starter cracks are initiated at inherent defects (voids or inclusions) found in the material. Small specimen test data were obtained to calibrate the fracture mechanics characteristics and the early crack development for the blade material. There was a close correlation between the calculated and measured fatigue life with this model. The procedure was then applied to a steam turbine blade row with documented cracks developed after long-term service (Rieger, et al., 2002). By using known history of the service loads and the fracture properties from the specimen tests, it was possible to develop life predictions for the remaining blades in the row. To date the blades have operated in accordance with the predictions and no failures have occurred since the unit has returned to service.