Exploiting the non-equilibrium phase transformation in a 15Cr-2Ni-2Al-11Mn resource-saving duplex stainless steel

- Transformation of ferrite-to-austenite occurs at T < 1050 °C and austenite-to-ferrite occurs at higher temperature.
- Plastic deformation suppress ferrite-to-austenite transformation, while promotes the inverse one.
- Plastic deformation reduces not only the rate of phase transformation but also the equilibrium content of γ phase of DSS.
- Time-temperature-transformation diagram of ferrite-to-austenite transformation for 15Cr DSS were obtained.

Though most duplex stainless steels are quenched from high temperature, at which the equilibrium phase constitution is close to the desired magnitude, this work explored the non-equilibrium phase transformation in duplex stainless steel and showed that duplex stainless steels of equilibrated primary phase content can be obtained through different thermomechanical processes and exhibit disparate microstructures and mechanical property. Specifically, we studied the effects of annealing temperature, time, cooling method and plastic deformation based on the 15Cr-2Ni-2Al-11Mn DSS and established TTT diagrams of as-cast and cold-rolled duplex stainless steels using the Avrami equation, respectively. A simplified model of diffusion-mediated phase transformation was established to explain the stretched exponent 2.5 used in the Avrami equation. And it is found that plastic deformation not only changes the rate of phase transformation and the grain size of new phase but also changes the equilibrium phase constitution.

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