Corrosion Fatigue of a Low-Pressure Steam Turbine Blade

A low-pressure (LP) steam turbine blade of the steam turbine last stage of a thermal power plant had failed after it had been only a few years in service. The failure comprised of surface damage that formed over the leading edge and a tangential crack that formed between the tie-rod hole and the damaged leading edge. The surface damage occurred on the leading edge was typical of pit-like defects of honeycombed texture. Type and factors that may have caused the failure of the LP steam turbine blade are discussed in this paper. The metallurgical assessment was conducted by preparing a number of specimens from the as-received failed LP steam turbine blade. Various laboratory examinations were performed including visual and macroscopic examination, chemical analysis, metallographic examination, hardness testing, and scanning electron microscopy equipped with energy-dispersive spectroscopy analysis. Results of the metallurgical assessment obtained showed that the failed LP steam turbine blade had been experiencing corrosion fatigue due to the combined effect of cavitation erosion and fatigue. Fatigue crack was initiated from the internal wall of the tie-rod hole where several pit-like defects, typical of cavitation erosion present. Formation of this cavitation erosion may be considered as physical corrosion. Similar cavitation erosion was also formed over the damaged leading edge. The fatigue crack was subsequently propagated in tangential direction toward the nearest damaged leading edge of the vane blade and eventually forming the final fracture thereon.