Non-uniform phase separation in ferrite of a duplex stainless steel

Phase separation in ferrite of a duplex stainless steel during aging at 400 °C for up to 20,000 h has been found to be non-uniform in nature by using atom probe tomography. Aging-induced segregation of Ni takes place strongly at the austenite/ferrite interfaces accompanied by the formation of Ni depletion zones (NDZs) in the ferrite in the vicinity of the interfaces. Consequently, the phase separation in the NDZs is microscopically and kinetically different from that in the inner ferrite. The nucleation and growth of G-phase in the NDZs are suppressed, and the growth of α′-phase is decelerated. In contrast, the G-phase at the austenite/ferrite interfaces exhibits a delayed heterogeneous nucleation and thereafter accelerated growth. The phase-separation difference between the NDZ and the inner ferrite results in a gradient microstructure (from outer to inner) developing in each ferrite grain during the prolonged aging. The strong segregation of Ni and the high decoration of G-phase at the ferrite/austenite interface are responsible for degradation in impact toughness of the steel.

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