Abstract.Over the last few years, corrosion fatigue fractures initiating from corrosion pits have been a root cause of failures of rotating blades of the third stage of the low-pressure parts in several 200 MW turbines in ČEZ a.s. power stations. This contribution deals with the analysis of several of these failures. Metallurgical investigation of the blades showed that the cause of the failures was the initiation and growth of a fatigue crack from a corrosion pit. ČEZ, a.s. has developed, based on the knowledge obtained by EPRI (Electric Power Research Institute), a methodology which can, in real conditions during checks of turbines, reliably detect the parameters of corrosion pits and predict the possibility of development of fatigue damage from detected pits. The process of methodology and its uncertainties (influence of filling of pits with oxides, cyclic stress calculations, and the selection of the geometric factor Y) are summarized.

The paper describes the metallographic analysis of a metal part from the cupola construction in the Main Railway Station in Pilsen. The station in the Art Nouveau style was completed in 1907. The part was screwed to a vertical column and used as a pulley, probably for the transport of other parts. The cross-section consisted of visible bands. The microstructure was heterogeneous, formed by connected individual bands, ferritic structure predominated, but in some bands it was ferritic - pearlitic. The structure contained both minor inclusions and complex elongated inclusions consisting of grains and binder - their composition was determined by ED microanalysis. The analysis showed that it is a wrought (puddled) iron with a considerable amount of slag, which was produced with technology used in the 19th century. However, at the beginning of the 20th century, modern steel production methods were used for profiles and thus it is unclear whether this screwed part for manipulation represents the whole construction.

Two types of steels used for production of heavy forgings were selected for the experimental evaluation of the effect of long time dwell at elevated temperatures and cooling during heat treatment on their mechanical properties in order avoid the possibility of grain boundary embrittlement. Samples from evaluated steels 26NiCrMoV14-5 and 22CrNiMoWV8-8 were austenitized for 2 hours at temperature of 1200°C and oil quenched. Subsequently the annealing at temperatures (200 - 700)°C for 1 hour and 100 hours was applied. Selected mechanical properties, especially hardness and impact energy, were monitored. It was found that for steels 26NiCrMoV14‑5 and 22CrMoNiWV8-8 exist the temperature intervals (300 - 400)°C and (500 ‑ 600)°C respectively with the possible potential for toughness decreasing.

This contribution deals with the analysis of the failure of a rotating blade of the third wheel (L-1 stage) of the LP rotor of a 200 MW turbine at the power station in Počerady. Material analysis of the blade showed that the cause of the blade fracture was the initiation and growth of a fatigue crack from a corrosion pit. Failures of low-pressure blades have occurred repeatedly on machines of the same design in the power stations of the ČEZ group in recent years and by a similar mechanism. Therefore, based on the knowledge obtained by EPRI (Electric Power Research Institute), ČEZ, a.s. has developed a methodology which can, in real conditions during checks of turbines, reliably detect the parameters of corrosion pits and predict the possibility of development of fatigue damage from these pits. The work summarizes the methodology and the conditions of its use with an emphasis on the fields of its application.

Corrosion fatigue fractures initiating from corrosion pits are one of the most serious problem during service of rotating blades of the low-pressure parts of steam turbines. A methodology for fatigue failure prediction, originally based on the knowledge obtained by EPRI (Electric Power Research Institute), using corrosion pits parameters assessment and local stresses calculation was adapted to the conditions of ČEZ a.s. power stations. This contribution deals with the evaluation of the corrosion state of blades of three low pressure rotors after long service. Measurement was done in power stations equipped with turbines of power 200 MW and 110 MW respectively. Possibilities and uncertainties (influence of filling of pits with oxides, cyclic stress calculations, and the selection of the geometric factor Y) and their elimination are discussed.