Abstract: The FV520B stainless steel subjected to solution treatment and intermediate treatment at 850 °C for 1 h was aged at 470 °C at various times. The hardness as a function of aging time showed two peaks. The microstructure and precipitated particles were examined using atom probe tomography and high-resolution transmission electron microscopy. The results showed that during the aging process, the Cu-rich phase, NiMn clusters, Cr-rich phase, and G phase were formed successively. In the early stage of aging, a large number of fine body-centered cubic-structured Cu-rich precipitates were formed, causing the hardness to reach a peak value at 1 h. Meanwhile, the NiMn clusters were formed at the Cu-rich precipitate positions. With increasing aging time, the Cu-rich precipitates grew larger with the structure transformed into the face-centered cubic structure, and the number density decreased, reducing the hardness. As the aging time increased, the Si atoms started to partition into NiMn clusters, forming NiMnSi clusters. Meanwhile, the Cr-rich phase was formed due to spinodal decomposition, causing the hardness to rise and reach the second peak value. Further increasing aging time caused the NiMnSi clusters to grow larger and form the G phase, and increased the size of the Cu-rich precipitates and reduced their number density, hence decreasing the hardness. The Cu-rich phase and G phase had the following orientation relationship with the matrix: [01$\overline{1}$]_Cu // [001]_M and (111)_Cu // (110)_M, [001]_G // [001]_M, (220)_G // (110)_M.

Key words: FV520B, Second phase, Precipitation strengthening, Atom probe tomograph