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.

The aim of the performed tests was to analyse the precipitation processes in the 23Cr-45Ni-6W-Nb-Ti-B (HR6W) alloy based on nickel. The examined material was subjected to the process of long-term isothermal annealing at 700 • C, for various time periods of up to 10000 h. The scope of the tests included: structural tests using scanning electron microscopy, the analysis of the precipitation processes with the use of X-ray phase analysis and electron diffraction with transmission electron microscopy. The tests showed that only MX-type precipitates occurred in the analysed alloy in the as-received condition, whereas the long-term annealing contributed to the precipitation of secondary phases of M23C6 carbides and the Laves phase.

The article presents the results of research on the microstructure and selected mechanical properties of HR6W nickel-base alloy. The test alloy was subjected to isothermal ageing at 700°C and for up to 10000h. The tests of the HR6W microstructure were performed using the scanning electron microscopy (SEM) and the transmission electron microscopy (TEM). The performed microstructural tests of the HR6W alloy showed that in the as-received condition it was characterised by the structure of nickel austenite with numerous primary precipitates of NbC and TiN. Ageing of the investigated alloy contributed to the precipitation of numerous particles of varying morphologies inside the grains and at the grain boundaries, as well as at the boundaries of twins - they were the secondary precipitates of M23C6 and Laves phase. The number of the particles precipitated at the boundaries was so large that they formed the so-called continuous grid of precipitates. Inside the grains, the presence of compound complexes of precipitates was observed. These complexes consisted of the TiN particles, as well as the M23C6 carbides and Laves phase nucleating on them. The tests of the mechanical properties of HR6W alloy showed that in the as-received condition the alloy showed high plastic properties, with relatively low strength properties - in particular, the yield strength. Ageing of the HR6W alloy, as a result of precipitation of numerous particles in the matrix, through the strengthening with the precipitation mechanism, resulted in a considerable growth of the strength properties - inter alia the yield strength by over 60%, with the reduction of the plastic properties - elongation decreased by around 40%. Similar growth in the test alloy was observed for hardness.

Purpose: The purpose of the research was to determine and analyse the changes in the microstructure and mechanical properties of the Sanicro 25 steel in the as-received condition and after ageing at 600, 650 and 700°C for up to 10,000 hours. Design/methodology/approach: The scope of the investigations included: microstructural investigation – SEM microscopy, analysis of precipitation performed using TEM microscopy, investigation of mechanical properties, Vickers hardness measurement. Findings: In the as-received condition, the Sanicro 25 steel was characterised by austenitic microstructure with annealing twins and numerous primary precipitates. The analysis of Sanicro 25 steel microstructure after ageing at 600 and 700°C for up to 10,000 hours revealed significant changes in the microstructure consisting mainly in a tendency to create unfavourable morphology of secondary precipitates – M23C6 carbides that form continuous carbide systems along the grain boundaries. The observations have shown that during long-term ageing the secondary carbides were also precipitated inside the grains and at the interface of three grain boundaries – σ phase. Research limitations/implications: The analysis of the microstructure of the examined steel using SEM and TEM was performed to determine the influence of ageing on the processes of changes in the precipitate morphology. Practical implications: The results obtained based on the performed research constitute a building block for the degradation characteristics of the microstructure and mechanical properties of the 23/25-type austenitic steels. Originality/value: The results of the investigation and analysis of the metallographic and mechanical properties of the Sanicro25 austenitic steel in as-received condition and after ageing are presented.

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