Effects of heating and cooling rates on δ↔γ phase transformations in duplex stainless steel by in situ observation

Ultra high temperature confocal laser scanning microscope is applied to observe the characteristics of the γ↔δ phase transformations at different heating and cooling rates on the surface of duplex stainless steels. It is found that the migration of the δ/γ IB is always the main form of the phase transformation and leads to the continuous decline and final disappearance of the retained γ-phase during the γ→δ phase transformation at different heating rates. Heating rate does not radically later the mode of transformation under the current composition system. Interestingly, the δ-phase inside the γ-phase acts as the nucleation cores and grows more rapidly at the slow heating rate than at the rapid heating rate. During the δ→γ phase transformation, the γ-cells prefer to precipitate along the δ/δ grain boundaries with a flaky pattern, and their fronts are jagged in shape at the slow cooling rate, but in needle-like feature at the rapid cooling rate. The γ-cells also nucleate inside the δ-ferrite grains with a flaky pattern at the slow cooling rate and with a needle-like pattern at the rapid cooling rate. Furthermore, at different cooling rates, the growth speed in the longitudinal direction is always much faster than that in the lateral direction. More importantly, the effects of cooling rates on the precipitating morphologies of γ-phase are clarified based on the experimental results and the diffusion controlled growth theory.