Effect of Long-Term Aging at 580 °C on Microstructure and Mechanical Properties of 12% Chromium Heat-Resistant Ferritic–Martensitic Steel

Abstract

The effect of long-term (1000-5000 h) aging at 580 °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 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 h) and Laves phase particles (after 5000 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 h provides additional dispersion strengthening, increasing the yield strength, compared to that after 1000 h of aging. At the elevated test temperatures (580 and 650 °C) the steel strength properties remain unchanged. A fractographic analysis shows predominantly ductile dimple transcrystalline fracture at the temperatures within 20-650 °C. After 5000 h of aging, there are areas of intercrystalline fracture, indicating a local embrittlement of the boundaries due to the Laves phase precipitates.