FRACTOGRAPHIC FEATURES OF FRACTURE OF A STEAM TURBINE CONTROL VALVE STEM

Abstract

Fractographic analysis of the fracture surface of a steam turbine control valve stem was performed, and its fracture mechanism was examined. Damage inside the nitrided layer and the base metal of a stem was investigated. Microanalysis revealed secondary cracks along the entire perimeter of a stem, a sign of weak adhesion between the base metal and the nitrided layer, and associated with internal oxidation. The classical failure mechanism by fatigue striation formation dominated practically the majority of a stem fracture, which is typical of high-cycle fatigue. Only at the final stage of stem failure were signs of low-cycle fatigue visualized.