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Purpose: The purpose of the investigations was to determine changes in the microstructureand mechanical properties of HR3C creep resisting austenitic steel after service.Design/methodology/approach: The investigations were performed on test specimens takenfrom a part of the steam superheater tube. The range of the investigations included: microstructuralinvestigations - light and SEM microscope; analysis of precipitates - carbide isolates; investigationsof mechanical properties - hardness measurement, static tensile test, impact test.Findings: The precipitation processes at the grain boundaries lead to increase inintergranular corrosion of the HR3C steel resulting in loss of grains in the structure. Theimpact strength testing on test specimens with reduced width may result in overestimationof crack resistance of the material after service.Research limitations/implications: The comprehensive analysis of precipitationprocesses requires TEM examinations. Finding the correlation between the impact strengthdetermined on standard vs. non-standard test specimens with reduced width.Practical implications: The obtained results of investigations are used in industrialpractice for diagnosis of pressure parts of power boilers. Test procedures developed based oncomprehensive materials testing conducted under laboratory conditions are used in upgradingand design of pressure parts of steam boilers. The results of investigations are also the elementof database of the materials characteristics of steels and alloys as well as welded joints made ofthem working under creep conditions developed by the Institute for Ferrous Metallurgy.Originality/value: The results and analysis of the investigations of microstructure andmechanical properties of HR3C steel after service under actual boiler conditions are presented.

High-chromium martensitic steels are one of the basic creep-resisting construction materials used for the modernization of old and the construction of new power units. During the service under creep conditions, the metastable microstructure of martensitic steels undergoes gradual degradation. The rate of degradation mostly depends on the operating temperature, but it is also affected by stresses. The changes in the microstructure of martensitic steels have an influence on the decrease in their mechanical properties, including creep resistance. The knowledge and description of the changes in the microstructure of steels working under creep conditions allow extending the time of safe operation of the elements of power systems. The paper presents and describes the main mechanisms of degradation of 9-12%Cr martensitic steels on the basis of the independent studies and literature data.

The paper presents the results of research on the microstructure of martensitic X10CrMoVNb9-1 (P91) and X13CrMoCo-VNbNB9-2-1 (PB2) steel subject to long-term ageing at the temperature of 620°C and holding times up to 30 000 hours. The microstructural tests of the examined steel types were performed using a scanning microscope Joel JSM -6610LV and a transmission electron microscope TITAN 80 -300. The stability of the microstructure of the investigated steels was analyzed using a quantitative analysis of an image, including measurements of the following: the density of dislocations inside martensite/subgrain laths, the width of martensite laths, and the mean diameter of precipitates. It has been concluded that during long-term ageing, the microaddition of boron in PB2 steel significantly influenced the slowing of the process of degradation of the martensitic steel microstructure, as a result of slowing the process of coagulation of M 23 C 6 carbides and Laves phase. It had a favorable effect on the stabilization of lath microstructure as a result of retardation of the processes of recovery and polygonization of the matrix.