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 degreesC for 3 min) followed by continuous cooling at four respective cooling rates (1, 0.5, 0.25 and 0.1 degreesC 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 sigma phase transformation. The sigma precipitate could be detected when the specimens were solution-treated at 1020 and 1080 degreesC and cooled at the rate of 0.25 degreesC s(-1); substantial amounts of sigma phase formed when the specimens were cooled at 0.1 degreesC s(-1). The precipitation of sigma phase was considerably retarded as the solution temperature increased from 1080 to 1200 degreesC. The precipitation behaviors of sigma phase and M23C6 carbide were characterized by transmission electron microscopy. The results indicated that the Cr- and Mo-rich sigma phase preferentially nucleated at the pre-formed M23C6 carbide particles, which were located at delta/gamma interface or within delta ferrite grain. The selected area diffraction patterns displayed the complicated crystallography of a phase, and revealed the orientation relationships between the interfacial a precipitate and adjacent matrix phases. (C) 2001 Elsevier Science B.V. All rights reserved.