Evolution of the EUROFER97 microstructure during thermal treatment up to 122,000 h

Detailed knowledge of the microstructural evolution of reduce activation ferritic-martensitic steel EUROFER97 after exposure at high temperatures is essential for determining its applications potential. For this proposal, EUROFER97 was annealed in the temperature range between 450 °C and 650 °C for up to 122,000 h (≈14 years) and subsequently analyzed using transmission electron microscopy (TEM) including high resolution TEM and two-dimensional energy dispersive X-ray (EDX) mapping. The study demonstrates the effects of thermal treatment on the size and composition of the precipitates and allows conclusions about their stability. Application of the extraction replication technique was used to analyze composition and morphology of four particle types present in the untreated EUROFER97: M23C6, VN, TaC and TiN with sufficient statistics. The rapid coarsening of the M23C6 precipitates was observed at 650 °C, while the MX particles were found to be more stable upon thermal treatment. It has been proved that new Laves (WFe2) and modified Z-phases (Cr(V,Ta)N) precipitates are formed in the temperature range from 500 °C to 600 °C. The detailed analysis allows the drawing a time–temperature formation diagram for these two phases, which could be valid for alloys with composition similar to EUROFER97.