Specific features of operational degradation of the structure of heat-resistant steel of the regulating valve rod of a thermal power plant steam turbine

Keywords

high-temperature oxidation, nitrided surface layer, steel, loading, structure, failure mechanism, steam turbine rod.

Cite as

Krechkovska H. V. Specific features of operational degradation of the structure of heat-resistant steel of the regulating valve rod of a thermal power plant steam turbine. Physicochemical Mechanics of Materials. 2026. 62(2), 040-046.

https://doi.org/10.15407/pcmm2026.02.040

Abstract

The causes for the failure of the control valve rod of a steam turbine made of 18Kh11MNFB steel (after 17 years of operation) were analyzed. It was found that failure started in several places along the rod perimeter, with subsequent fusion and spontaneous fracture. Cracking was recorded in the zone of the rod fillet transition. An X-ray diffraction spectral analysis indicated the presence of nitrogen in the surface layers. Increased microhardness was observed, with the highest value (9.3 GPa) near the rod surface, gradually decreasing to 3 GPa over approx 200 μm. A large number of cavities with oxidized surfaces were found within the nitrided layer, with their greater number near the outer surface of the rod, indicating the gradual spread of the internal oxidation process during operation. The fracture of the pore partitioning did not require high stresses and led to the formation of a network of surface cracks. The presence of large niobium carbides in the steel structure was also established, as well as the release of chromium carbides, which reduced its content in the matrix and, as a result, the ability of the steel to resist high-temperature fracture.

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