A number of stress corrosion cracking (SCC) failures of duplex stainless steel (DSS) equipments exposed to the sulfide containing caustic solutions has been reported in recent years. SCC susceptibility of the DSS may be due to changes in the microstructure of the steel brought about by various metal fabrication processes. During metal fabrication, localized areas of DSSs may be exposed to different cooling rates or aging temperatures, which may lead to changes in the microstructure. These changes in microstructure may in-turn affect the general and localized corrosion or SCC susceptibility of the affected area as compared to the rest of the metal. In this study the effect of different annealing and aging temperatures on the microstructure and SCC behavior of 2205 DSS in sulfide-containing caustic solution was evaluated. 2205 DSS samples were annealed at 1000ºC or 1150ºC and then aged at 475ºC, 600ºC or 800ºC. The heat-treated specimens were then tested for SCC susceptibility in white liquor. It was found that the changes in the microstructure of the steel due to various heat treatments had a significant effect on its SCC susceptibility in caustic environments.
The duplex microstructure of different ferritic-austenitic stainless steel grades is obtained by optimizing the chemical composition and the annealing temperatures of the steel to meet the mechanical properties and corrosion resistance requirements for various applications1. DSSs have very good corrosion resistance in a wide number of environments. The superior mechanical properties and corrosion resistance of DSS is attributed to the two-phase microstructure consisting of austenite and ferrite phases 2-4 and high chromium content. The reason for the increased strength is due to a number of factors including the solution hardening of the austenite phase due to the partitioning of carbon and nitrogen in the austenite; grain refinement due to the mixed structure; a fiber strengthening effect of the ferrite phase; and the dislocations generated during the thermal cycling of the steel 5. The dual phase is established by either hot working in a two-phase region or by producing a single-phase structure and then age hardening the alloy by precipitation of an additional phase or phases6. Annealing above the solvus temperature of austenite (g) results in a structure which is completely ferritic (a). The ferrite structure obtained by water quenching DSS samples from the solvus temperature is unstable with respect to austenite precipitation. Reheating into the a+g stability region results in the strengthening of the alloy due to the formation of austenite precipitates. Ferrite/austenite ratio in DSS plays an important role in mechanical and corrosion properties of the steel. Apart from the ratio of phases, the distribution of phases as well as partitioning of alloying elements in the two phases is also important. High and low temperature aging of duplex stainless steels may give rise to undesirable precipitates such as sigma, chi, chromium carbides, and nitrides or "475 embrittlement". In the temperature range of 600ºC to 300ºC, the ferrite phase undergoes a spinodal decomposition into Cr rich a' and Cr poor a phase7.
