Comparative evaluation of environment induced cracking of conventional and advanced steam turbine blade steels. Part 2: Corrosion fatigue

Corrosion fatigue crack propagation rates have been determined for two steam turbine blade steels, PH13-8, a candidate steel for advanced turbines and FV566, typical of current turbine blades. The testing was undertaken in simulated condensate environment, 300 ppb Cl- and 300 ppb SO42-, at 90 °C using trapezoidal loading with a rise time of 20 mins to simulate two-shifting (switching on and off load on a daily basis). Aerated and deaerated conditions were tested, the former being more representative of the retained aeration in the chamber as the system comes on load. In aerated solution at DK =20 MPa m1/2 the cyclic crack growth rate for the PH13-8 steel was about a factor of two lower than that for the FV566 steel but the reverse was true for deaerated solution. In both cases, the difference in growth rates diminished with increasing DK. Although the cyclic crack growth rate is high the number of cycles per annum is small and the direct impact on overall life may be modest, but not insignificant. The observed cracking behaviour is best explained by a hydrogen assisted cracking mechanism.