The Precipitation Of σ Phase And Its Influence On Microstructure And Properties In2205Duplex Stainless Steel

The influence of heat treatment and thermal processing on the microstructure and properties of2205duplex stainless steel (DSS) is extremely significant. For example, the equilibrium between the two-phase in the welding heat affected zone of DSS can be easily destroyed, and carbides, nitrides and intermetallic phases such as a phase will precipitate during the process of welding. This will reduce the mechanical properties and corrosion resistance of the welding heat affected zone and among them the influence of a phase is the most serious. Therefore, it is absolutely necessary to study the precipitation of a phase in2205DSS and its influence on the microstructures and properties of the steel.In this paper, solution treatments at different temperatures and aging treatments at different temperatures for different times were carried out on2205DSS, and the influence of solution temperature on the microstructure and mechanical properties of2205DSS was studied. At same time, the behavior of a phase precipitation in2205DSS during ageing at different temperatures for different time was analyzed, and the influence of a phase on the mechanical properties and corrosion resistance of the steel was discussed. The obtained main results are as follows:1. The volume fraction of austenite in hot rolled2205DSS is only45%, while the volume fraction of austenite can exceed50%in the steel solutionized at950～1150℃for30min. The grain size of ferrite become coarse dramatically when solution treatment temperature is increased to1150℃, which reduces the tensile strength, the impact toughness and the ductility.2. The incubation period of a phase precipitation when aged at750～900℃after solutionized is less than30min, and the shortest when aged at850℃. The higher the ageing temperature the faster the a phase grows up. The amount and the size of precipitated a phase increase gradually with increasing the ageing time. The amount of precipitated a phase reached the saturation when aged at900℃for120min. 3. The precipitation of a phase occurs in two different ways:one is that the carbide M23C6and secondary austenite firstly precipitate on the ferrite-austenite phase boundaries in a way of lamellar eutectoid reaction, then a phase precipitates on the phase boundaries of the secondary austenite and the ferrite; the other is that ferrite transformed directly into secondary austenite and a phase in the way of eutectoid reaction.4. The precipitation of a phase has little effect on the yield strength and faintly increases the tensile strength, however seriously reduce the impact toughness and ductility. Meanwhile, the precipitation of a phase causes the self-corrosion potential negatively shifting, and the self-corrosion current increasing. As a result, the corrosion resistance was reduced.5. After aged, the segregation of Cr and Mo occur in the different phases to varying degrees. Cr and Mo are heavily enriched in a phase, while the secondary austenite is poor of Cr and Mo compared to primary austenite.