CHARACTERIZATION AND QUANTIFICATION OF DEFECTS IN ROTORS AND CASINGS OF STEAM-TURBINES

The loading situation of components in modern fossil-fired power plant s is characterised by higher cyclic service in comparison to older pla nts. This leads in combination with larger units to increased stresses in the relevant components. To evaluate the risk of failure due to na tural defects, life time approaches have been developed on the basis o f advanced methods of fracture mechanics. The assessment of natural de fects using fracture mechanics is, however, based on the following pre conditions: knowledge and data on the behaviour of such defects in ter ms of damage mechanism taking into account crack initiation and crack propagation under loading situations comparable to those in service, d ata on the real size and geometry of defects, data on the exact locati on of the defects. To create such a database an extensive joint resear ch programme was started in the early eighties by German turbine manuf acturers, steel makers and research institutes. The paper provides an overview of the results of these efforts. Special emphasis is placed o n describing the improvements in ultrasonic testing techniques. The re sults of feature tests are reported. They have been performed to obtai n information and data on the behaviour of natural defects under creep , creep fatigue and fatigue load. Subsequent metallographic investigat ions determine the real size and location of the defects but also the relevant damage mechanisms. Comparison of metallographic data and NDT data prove the reliability of ultrasonic testing. Thus an improved rat ing of the permissible ultrasonic indications in turbines and castings was obtained.