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

A commercial 2205 duplex stainless steel with three different solution trea tments (at 1020. 1080 and 1200 degreesC for 3 min) followed by continuous c ooling 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 el ectron microscopy, electron probe microanalysis, X-ray diffraction and diff erential thermal analysis. It is found that the lower solution treatment te mperature with the lower cooling rate significantly enhanced the sigma phas e transformation. The sigma precipitate could be detected when the specimen s 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 sp ecimens 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 carb ide were characterized by transmission electron microscopy. The results ind icated that the Cr- and Mo-rich sigma phase preferentially nucleated at the pre-formed M23C6 carbide particles, which were located at delta/gamma inte rface or within delta ferrite grain. The selected area diffraction patterns displayed the complicated crystallography of a phase, and revealed the ori entation relationships between the interfacial a precipitate and adjacent m atrix phases. (C) 2001 Elsevier Science B.V. All rights reserved.