9 Environment Assisted Cracking of Steam Turbine Blade Steels – A Consistent Rationalization Based on Hydrogen Assisted Cracking

The performance of two steam turbine blade steels, PH13-8, an advanced high strength steel, and FV566, a medium strength steel typical of conventional turbine blades, has been evaluated under low frequency corrosion fatigue and stress corrosion cracking conditions. The primary environments adopted were aerated and deaerated 300 ppb Cl^- and 300 ppb SO₄^2- but with additional stress corrosion tests in aerated 35 ppm Cl^-. The test temperature was 90 °C. Measurements were made also in the short crack regime for FV566 steel. A consistent explanation for the complex dependence of the threshold for cracking and of crack growth rates on alloy composition, strength level, solution conductivity and crack size is proposed based on a hydrogen assisted failure mechanism.