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The subject of the research was a material in the form of a ϕ38×4 pipe from HCM12 steel used for steam superheater coils in power boilers with supercritical and ultra-supercritical steam parameters. This article presents results of investigations of mechanical properties and microstructure of material of the examined tube in the as-received condition and after 100,000 h service at 585°C under creep conditions. Creep tests were carried out for the material after 100,000 h service to determine the disposable residual life. The impact of changes in mechanical properties and degradation of the microstructure on the loss of creep resistance compared to the as-received condition was assessed. The investigations have revealed that the examined steel after 100,000 h service meets the requirements for this steel in the as-received condition with regard to strength properties at room and elevated temperature. The creep tests have revealed a significant loss of creep resistance, which was estimated at around 60-70% compared to the as-received condition.