Effect of Mn contents on a precipitation and mechanical properties of ultra-low nickel 22-23% Cr duplex stainless steel aged at 850 °C

The effects of Mn contents (8.4-16.8 wt%) on the microstructure and mechanical properties of ultra -low nickel type duplex stainless steels (DSS) were investigated by isothermal aging at 850 degrees C. The results show that as the Mn content increased, the two-phase equilibrium temperature of tested DSS decreased and the nose temperature increased. In low Mo content system, the a -phase precipitation rate was slow with comparative low Mn content addition, whereas more Mn addition from 10.95 to 16.79% significantly promoted a -phase precipitation rate because higher Mn and Cr contents participate in its formation, resulting in a two order of magnitude increase in reaction constant K. In compared with solution treated samples, higher impact energy of 283.6 J was obtained for tested steel with 8.39% Mn addition after aging for 54 h, which is attributed to the reduction of internal stress and dislocation density, and the refined subgrains formation of 6 phase during impact deformation. The transformation -induced plasticity (TRIP) and twin -induced plasticity (TWIP) for 8.39% Mn DSS occurred during tensile deformation by prolonging aging time to 100 h, which effectively improved plasticity and maintained high tensile strength. With the increase of Mn content in tested DSS, the precipitation rate and the amount of a -phase precipitation also increased, which led to the deterioration of mechanical properties at earlier aging time.