The effect of microstructure on creep resistance of the low carbon chromium steel X20CrMoV121 after 100‐hours of static‐load test at a temperature of 580 °C and constant stress of 170 MPa was investigated. The specimens for the experiments were extracted from steam pipes of a steam power plant and heat treated. The effect of isothermal annealing on the microstructure and hardness as well as the kinetics of the precipitation of the carbide particles were determined.

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Microstructure of the steel X20CrMoV12.1, used for thermal power plants, after tempering at approximately 500 • C consists of tempered martensite with carbide precipitates.The evolution of the chemical and phase composition of carbide precipitates in X20CrMoV12.1 steel was studied in the as-received state after 56 000 h at 470-530 • C under load and in heat-treated state (1-1344 h at 800 • C) using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) technique.The precipitates found in service-loaded state as well as in heat-treated state were of M 23 C 6 type. In all samples two morphologically different types of carbides were established.

The steel X20CrMoV12.1 has been widely used for the steam pipes in power plants since 1960s. It exhibits a good combination of high-temperature strength, toughness and creep strength. The microstructure consists of high-tempered martensite with finely dispersed carbide precipitates along the boundaries of the ex-austenitic grains and the ferritic sub-boundaries. The carbides change their chemical and phase composition as well as their size with time and temperature until equilibrium is reached. However, this process lasts for thousands of hours. For this reason, we simulated these conditions in the laboratory using higher temperatures in order to accelerate the process. After different annealing times the carbide precipitates and their orientational relationships with the neighbouring grains were examined by electron-backscatter diffraction (EBSD). After 24 h of annealing all the carbides were M 23 C 6 and were in an orientational relationship with the matrix. The volume fraction of carbides is independent of annealing time and temperature.