Effect of Corrosion on the Low-cycle Fatigue Strength of Steels used in Frequent Start-up Power Generation Steam Turbine

The practical importance of fatigue failure in steam turbine materials has directed many experimental research towards assessing the physical reason for material sensitivity to corrosion fatigue and providing design rules for engineers. Metallic materials used in steam turbine are exposed to cyclic loading at high temperature and steam environment, during their service life. In this study, an original fatigue testing setup was developed to investigate the effect of aqueous solutions and temperature on the fatigue strength on the martensitic stainless steel X19CrMoVNbN11-1 used for rotating blades and (ii) a cast G17CrMoV5-10 steel used for casing. Fatigue tests were carried out in two environments: (i) in air at 90 °C and (ii) in distilled water at 90 °C (pH = 7.2 and 02 = 3 ppm) at a loading frequency of 1 Hz. Internal and surface crack initiation are observed in air at 90 °C, whereas in purified water at 90 °C, the crack initiated in the surface at corrosion defects. The decrease observed in the corrosion fatigue strength of specimens was more important at high plastic strain level of that on similar specimens tested in air. Based on fractography analysis, fatigue crack initiation mechanisms in air and in distilled water were identified. Two different scenarios for fatigue damage depending on the microstructure are proposed and will be discussed in this paper.