Abstract <p>The effect of long-term (1000-5000&#xa0;h) aging at 580&#xa0;°C on the microstructure and mechanical properties of 12% Cr ferritic–martensitic steel 16Cr12MnWSiVNbB (EP-823-Sh type) is studied. Steel aging results in the following structural changes in comparison with the initial state: lower dislocation density and fewer crystal lattice microdistortions due to polygonization and larger number density and volume fraction of the second-phase particles. After 5000&#xa0;h of aging, finely dispersed precipitates of the Laves phase (Fe, Cr)₂(Mo, W) heterogeneously nucleate at the grain boundaries and the M₂₃C₆ carbides. Aging causes depletion of the matrix in Mo, which diffuses into the M₂₃C₆ carbides (after 1000&#xa0;h) and Laves phase particles (after 5000&#xa0;h). It is accompanied by the steel crystal lattice parameter decrease. The yield stress and strength of steel at room temperature are reduced (by ≈ 1.2 times), compared to those in the initial state, due to a decreased solid-solution and dislocation strengthening efficiency. The precipitation of Laves phase after 5000&#xa0;h provides additional dispersion strengthening, increasing the yield strength, compared to that after 1000&#xa0;h of aging. At the elevated test temperatures (580 and 650&#xa0;°C) the steel strength properties remain unchanged. A fractographic analysis shows predominantly ductile dimple transcrystalline fracture at the temperatures within 20-650&#xa0;°C. After 5000&#xa0;h of aging, there are areas of intercrystalline fracture, indicating a local embrittlement of the boundaries due to the Laves phase precipitates.</p> Graphical Abstract <p></p>