Microstructure and mechanical properties of 12 % Cr ferritic-martensitic steel after aging at 450°C during 5000 hours

The microstructure, tensile mechanical properties and microhardness of heat-resistant 12 % chromium ferritic-martensitic steel 16Cr12MnWSiVNbB (type EP-823) after its long-term (for 5000 h) aging at 450°C are studied. It is shown that the grain-subgrain structure of the steel does not undergo qualitative changes compared to the as-received (AR) state. The average grain size of the prior austenite is approximately equal to 18.5 µm. Both in the AR and aged states, ferrite grains and subgrains are located inside prior austenite grains, the boundaries of which are decorated with nonequiaxed particles of M23C6 carbides. The main quantitative differences between the aged and AR states consist in a lower scalar dislocation density (up to ρ ≈ (2 – 5) ×1010 cm−2) and the increased average size (up to 257 nm) and volume fraction (up to 5 wt.%) of M23C6 carbides. No new phases are formed after aging of the steel. Good thermal stability of the microstructure at 450°C maintains its strength properties at almost the same level. A slight (by about 60 MPa at T = 20°C and by 10 MPa at 650°C) decrease, relative to AR, in the yield strength of the steel after aging is due to a decrease in the efficiency of the substructural and solid-solution strengthening mechanisms. The increase in the microhardness values of the steel as a result of aging is apparently associated with an additional precipitation of the second-phase particles and, accordingly, an increase in the efficiency of dispersion strengthening. No effect of ageing on the mechanism of steel fracture after tensile testing was detected. At the temperatures studied, it fractured by the mechanism of ductile dimple transcrystalline fracture.