2017 年 57 巻 9 号 p. 1637-1644
The characteristics of the γ↔δ phase transformations were observed on the surface of duplex stainless steels with different nitrogen concentrations by the Ultra High Temperature Confocal Laser Scanning Microscope (UHT-CSLM). The effects of the nitrogen content on the phase transformations are discussed based on the experimental results and thermodynamic calculations. It is found that the migration of the δ/γ IB is 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 in the high nitrogen steel (N1) and low nitrogen steel (N2). During the δ→γ phase transformation, the γ-cells prefer to precipitate along the δ/δ GBs and develop into the δ-ferrite matrix with finger-like or sword-like patterns. Then the γ-cells also nucleate in the δ-grains with a sword-like pattern and the growing speed in the longitudinal direction is much faster than that in the lateral direction. More importantly, the high nitrogen content can hinder the migration of δ/γ IBs during the γ→δ transformation and also promote the nucleation and growth of γ-phase during the δ→γ transformation by increasing both the starting and finishing temperatures of the phase transformation. Interestingly, the original δ/δ GB which is covered by the precipitation of γ-phase during the δ→γ transformation will reappear first at the same position by the moving of δ/γ IBs during the γ→δ transformation, but it is unstable and migrates fast with further heating.
