A cold-rolled 25Cr-7Ni-3Mo-0.2N duplex stainless steel(DSS)has been aged in two steps.Firstly,the aging treatment at interval of 50℃ in a temperature range from 900 to 1050℃ was carried out in order to obtain fine grains.Secondly,another aging treatment at 850 ℃ was performed to reveal theσ-phase precipitation behavior.A detailed microstructure evolution during those two aging steps was observed by the optical microscope(OM),the scanning electron microscope(SEM),the electron backscatter difraction(EBSD)and the transmission electron microscope(TEM).The results revealed that the micro-duplex structure with grain size of lower than 10μm appeared after the first aging step.However,their grain size was rapidly increased with increasing aging temperature.Meanwhile,theδ→γand/orδ→γ+σtransformations took place in association with the occurrence of the extensive recovery or a little recrystallization inδ-grains.During the second aging treatment,σ-phase mainly nucleated atδ/γinterfaces and further grew along those interfaces into various morphologies(e.g.,butterfly and granule).A novel precipitation behavior was found in this study that theγ-grain boundaries bulged not only into theδ-grains as usual,but abnormally into theσ-phase precipitates without the prior precipitation of the isolated secondary austeniteγ2or another phases.
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