Bruchmechanische Untersuchungen an Werkstoffen für Dampfkraftwerke mit Frischdampftemperaturen zwischen 550 und 650°C

The higher steam temperatures and pressures in advanced supercritical steam turbines require the use of steels with improved creep rupture strength. The tempered martensitic steel P92 is a creep resistant material used for the production of components for live steam with operating temperature 550-630°C. For the application of these steels is essentially necessary to know the impact of different test atmospheres and temperatures on the crack growth rate. In the present work, the influence of atmosphere (air, vacuum, simulated live steam), proof-temperature, hold time, thermal exposure on the crack growth has been studied at temperatures between 500 and 600°C. The influence of proof-temperature and atmosphere has been reflected on micrographs of fracture surface. SEM-micrographs of the principal crack and also optical micrographs coming from the cross sections perpendicular to the plane of a crack propagation (secondary cracks) show changes in the character of a crack propagation dependent of the loading cycle and temperature. By fatigue crack growth tests has been enhancement of crack growth due to the aggressive atmosphere (air) noted. The fatigue crack growth rates in air are at least one order of magnitude faster than those in vacuum (inert atmosphere). The effects of air, vacuum, simulated live steam on creep crack growth of P92 was also studied. A hold time leads to a higher crack growth rate compared with pure creep crack growth tests. The results from crack growth tests demonstrated, that steam oxidation plays a important role among the three factor, creep crack growth, fatigue crack growth and oxidation, in crack growth of these steel in superheated steam. In this work was also effect of side-grooves on the local stress and crack growth rate experimentally examined