Frequency and hold time influence on crack growth behavior of a 9-12% Cr ferritic martensitic steel at temperatures from 300 °C to 600 °C in air

Due to an increase of renewable energies proportion, e.g. wind power and photovoltaics, which cannot supply energy constantly, modern power plants must be able to be operated flexibly in order to compensate the residual load. As a consequence of increasing alternating load, fatigue damage becomes more and more important, while creep damage caused by ever shorter holding times at high operating temperature decreases. In this study a turbine bypass valve, one of the most fatigue loaded power plant components, manufactured from widespread standard 12% Cr ferritic/martensitic steel X20 was investigated. Fatigue crack growth experiments showed that the crack growth rate increases slightly with decreasing frequency (20 Hz -> 5 Hz). In hold time tests (300 s -> 600 s, effective frequency 3.33 x 10(-3) Hz -> 8.33 x 10(-4)), larger crack propagation rates per cycle occur than in the fatigue crack growth experiments with 5 and 20 Hz. In comparison to pure cyclic loading maximum load holding time further required significantly higher Delta K values to start crack growth.