in-situ Observation of δ⇆γ Phase Transformations in Duplex Stainless Steel Containing Different Nitrogen Contents

The characteristics of the delta <->gamma 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 delta/gamma IB is the main form of the phase transformation and leads to the continuous decline and final disappearance of the retained gamma-phase during the delta <->gamma phase transformation in the high nitrogen steel (N1) and low nitrogen steel (N2). During the delta ->gamma phase transformation, the gamma-cells prefer to precipitate along the delta/delta GBs and develop into the delta-ferrite matrix with finger-like or sword-like patterns. Then the gamma-cells also nucleate in the delta-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 delta/gamma lBs during the gamma ->delta Stransformation and also promote the nucleation and growth of gamma-phase during the delta ->gamma transformation by increasing both the starting and finishing temperatures of the phase transformation. Interestingly, the original delta/delta GB which is covered by the precipitation of gamma-phase during the delta ->gamma transformation will reappear first at the same position by the moving of delta/gamma lBs during the gamma ->delta transformation, but it is unstable and migrates fast with further heating.