Correlation of ferrite-phase reconfiguration and mechanical properties in thermally aged duplex stainless steel

The thermal aging behavior and subsequent recovery treatment of duplex stainless steel are investigated by mechanical properties testing and microstructure characterization. After thermal aging at 475 °C for 500 h, the ferrite phase undergoes severe spinodal decomposition and precipitation, which results in a significant increase in yield strength, and a substantial reduction in ductility. Combining the nano-indentation, geometric phase analysis, and tensile testing, it can be confirmed that spinodal decomposition is the main contributor to the hardening of the ferrite phase. The hardening of the ferrite phase is induced by the semi-coherent interface between α´-phase and α-phase and higher shear modulus of Cr-enriched domains. The increased yield stress calculated by the simplified equation is in good agreement with the experimental value. When the aged samples were treated by high-temperature annealing and pulsed electric field, the hardening is significantly eliminated, and the ductility is correspondingly restored. Further, the essence of the difference in required temperature to restore deteriorated performance is the change of system free energy caused by temperature and pulsed electric field.