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Steel 16Mo3 (P1 according to ASTM) represents still a popular material used in waterwalls, pipelines and other components of steam power plant boilers operating at temperature up to 530 °C. However, the absence of other alloying elements (especially chromium) reduces the long-term structural stability of this steel especially at temperatures higher than 450 °C. The paper describes the results of an analysis of a pipeline bend that burst after nearly 180,000 hours of operation. The complex analysis of material properties and microstructure revealed the extensive graphitization, although mechanical strength, plasticity as well as notch toughness were affected only negligibly. Particles of graphite were randomly distributed in the microstructure and heavy cavitation damage was detected namely close to the fracture surface. Analysis of operation data then showed that the temperature in this part of the steam pipeline was higher due to the changes in the fuel mix and exceeded the maximum 470 °C recommended for the steel 16Mo3 just in order to avoid the danger of massive graphitization

Keywords: 16Mo3 steel, steam pipeline, creep failure, graphitization, material properties

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