Effect of solution annealing temperature on precipitation in 2205 duplex stainless steel

In the present study, effect of solution annealing temperature (1050 °C and 1100 °C) and isothermal ageing (700 °C: 15 min to 6 h) on the microstructural changes in 2205 duplex stainless steel has been investigated systematically. Scanning electron microscopy and X-ray diffraction were adopted to follow the microstructural evolution, while an energy dispersive spectrometer attached to scanning electron microscope was used to obtain localised chemical information of various phases. The ferritic matrix of the two phase 2205 duplex stainless steel (~ 45% ferrite and ~ 55% austenite) undergoes a series of metallurgical transformations during ageing—formation of secondary austenite (γ2) and precipitation of Cr and Mo rich intermetallic (chi-χ and sigma-σ) phases. For solution annealing at 1050 °C, significant amount of carbides were observed in the ferrite grains after 1 h of ageing at 700 °C. χ Phase precipitated after the precipitation of carbides—preferentially at the ferrite–ferrite and also at the ferrite–austenite boundaries. σ Phase was not observed in significant quantity even after 6 h of ageing. The sequence of precipitation in samples solution annealed at 1050 °C was found to be carbides → χ → σ. On the contrary, for samples solution annealed at 1100 °C, the precipitation of χ phase was negligible. χ Phase precipitated before σ phase, preferentially along the ferrite–ferrite grain boundaries and was later consumed in the σ phase precipitation. The σ phase precipitated via the eutectoid transformation of ferrite to yield secondary austenite γ2 and σ phase in the ferrite and along the ferrite–austenite grain boundaries. An increase in the volume fraction of γ2 and σ phase with simultaneous decrease in the ferrite was evidenced with ageing.