Microstructure evolution of a cold-rolled 25Cr-7Ni-3Mo-0.2N duplex stainless steel during two-step aging treatments

Abstract

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 °C in a temperature range from 900 to 1050 °C was carried out in order to obtain fine grains. Secondly, another aging treatment at 850 °C was performed to reveal the σ-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 diffraction (EBSD) and the transmission electron microscope (TEM). The results revealed that the micro-duplex structure with grain size of lower than 10 µm appeared after the first aging step. However, their grain size was rapidly increased with increasing aging temperature. Meanwhile, the δ → γ and/or δ → γ + σ transformations took place in association with the occurrence of the extensive recovery or a little recrystallization in δ-grains. During the second aging treatment, σ-phase mainly nucleated at δ/γ interfaces and further grew along those interfaces into various morphologies (e.g., butterfly and granule). A novel precipitation behavior was found in this study that the γ-grain boundaries bulged not only into the δ-grains as usual, but abnormally into the σ-phase precipitates without the prior precipitation of the isolated secondary austenite γ ₂ or another phases.