Steel is nowadays an indispensable material and because of its versatile physical and mechanical properties it is practically ubiquitous. There are a few thousand different varieties of steels, a great part of which is used also in power plants. In thermal power stations most components operate in difficult conditions, at high temperatures and high pressures. Consequently there is a need for materials with better mechanical properties at high temperatures, high corrosion resistance, creep-induced strain resistance and thermal cyclic fatigue. X20CrMoV12-1 steel is one of the materials suitable for such purposes. Microstructure can be changed when components operate at temperatures up to 550 °C for long periods. Understanding and monitoring of such changes is crucial for estimating fitness of such components. In this graduation thesis a tube from steam superheater from Šoštanj thermal power plant is analysed. It clocked 235.900 hours of service at steam temperature of 500 °C and 180 bar of pressure. A comparison was made between this and a new tube. Surface analysis, deposit analysis, microstructure characterisation, tensile test and hardness test were conducted with both tubes and the effect of ageing on microstructure and mechanical properties of said steel was characterised.