Effect of Graphite on Residual Lifetime of Superheater Tubes Made of Steel 16Mo3 after Creep Exposure

Carbon and low-alloys steel components are used in various stages of steam power plants which are operating at elevated temperatures (up to 530°C). Long-term exposure of low-alloy steel components at elevated temperatures inevitably result in some kind of structural degradation; for example, creep cavitation, carbide coarsening and/or spheroidization, and, less commonly, also graphitization. Graphitization can be detected mainly in carbon steels, but also in 16Mo3 steel where the ferritic matrix is strengthened by Mo in the solid solution. The absence of strong carbide forming elements (typically Cr, V, Nb) is the principal reason why these steels are susceptible to cementite decomposition and graphitization much more than the other creep resistant steels. The paper describes microstructure and material properties of superheater 16Mo3 tube, in which graphite was found after 70,000 hours of exposure at 445 °C. The actual material properties were compared to another tube of the same material delivered in the as-received state. The comparison was based on evaluation and testing of mechanical strength, fracture properties and microstructure. Creep resistance of the steel was studied using small punch creep tests (SPCT).