Features of the fracture of the control valve rod of a thermal power plant steam turbine

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Published: 17-08-2026
DOI: https://doi.org/10.33108/
Keywords:
steam turbine rod, steel, failure mechanism, nitrided surface layer , mechanical properties , loading

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Halyna Krechkovska
Karpenko Physico-Mechanical Institute of the NAS of Ukraine

Abstract

The reasons for the failure of the steam turbine control valve rod after 17 years of operation were analyzed. It was found that its fracture began in several places along the perimeter of the rod, followed by coalescence and subsequent spontaneous failure. Fractographic analysis showed that the crack propagated via a high-temperature fatigue fracture mechanism. Impact toughness, strength, and ductility properties determined on axial steel specimens of the control valve rod complied with regulatory requirements. However, the parameters of transverse specimens made from the rod head were lower. A decrease in all these characteristics was also observed in the high-temperature operating zone compared to the low-temperature zone, indicating more intense metal degradation. The control valve rod had 12091 operating hours and 206 shutdowns, which significantly impacted this process. Macrofractographic analysis showed that the rod fracture occurred because of multiple microcrack initiation in the nitrided layer along the perimeter in the zone of the fillet transition (stress concentrator) between the conical part and the head. Further growth of cracks into the depth of the section and their merging led to sudden fracture. Microfractographic studies confirmed the fatigue mechanism of crack initiation under low-amplitude cyclic loading. At the initial stage, characteristic fatigue grooves were observed, corresponding to the second section of the fatigue kinetic diagram. Their results showed that at a temperature of 540 °C, the crack growth rate was 5·10⁻⁸ m/cycle. The rod fracture occurred due to multi-cycle fatigue, and only at the final stage did signs of low-cycle fracture appear.

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