Morphological instability of the G-phase induced a different contribution to hardening of ferrite phase in a duplex stainless steel

Abstract

G-phase plays an indispensable role in the hardening of ferrite during thermal aging process of the duplex stainless steel. Although there are many studies on quantifying the contribution of spinodal decomposition and G-phase precipitation to the hardening of ferrite, details regarding the influence of G-phase morphology transformation on hardening are lacking. These knowledge gaps hinder a deep understanding of the correlation between G-phase and ferrite hardening, and hence hinder the exploration of new techniques for repairing performance degradation. In this study, after aging at 400 °C and 475 °C, driven by the minimization of interfacial energy, the morphology of G-phase changes from sphere to cube, and consequently the contribution of G-phase to hardening increases significantly. After annealing at 550 °C, the G-phase does not dissolve, but coarsens, resulting in a further increase in the contribution to hardening. The difference from annealing is that after pulsed electric field treatment at 460 °C, the unstable spherical G-phase precipitates are almost completely dissolved, while the stable cubic G-phase is spheroidized. The behaviors of dissolution and spheroidization are driven by the intervention of the electric free energy, and as a result, the hardening caused by G-phase is eliminated or relieved due to the dissolution and refinement of G-phase. It is conceivable that this work is a decisive step to completely repair the deteriorated performance of duplex stainless steel after long-term operation.