Effects of solution treatment and continuous cooling on σ-phase precipitation in a 2205 duplex stainless steel

A commercial 2205 duplex stainless steel with three different solution treatments (at 1020, 1080 and 1200°C for 3 min) followed by continuous cooling at four respective cooling rates (1, 0.5, 0.25 and 0.1°C s−1) has been investigated by means of optical metallography, transmission electron microscopy, electron probe microanalysis, X-ray diffraction and differential thermal analysis. It is found that the lower solution treatment temperature with the lower cooling rate significantly enhanced the σ phase transformation. The σ precipitate could be detected when the specimens were solution-treated at 1020 and 1080°C and cooled at the rate of 0.25°C s−1; substantial amounts of σ phase formed when the specimens were cooled at 0.1°C s−1. The precipitation of σ phase was considerably retarded as the solution temperature increased from 1080 to 1200°C. The precipitation behaviors of σ phase and M23C6 carbide were characterized by transmission electron microscopy. The results indicated that the Cr- and Mo-rich σ phase preferentially nucleated at the pre-formed M23C6 carbide particles, which were located at δ/γ interface or within δ ferrite grain. The selected area diffraction patterns displayed the complicated crystallography of σ phase, and revealed the orientation relationships between the interfacial σ precipitate and adjacent matrix phases.