<i>In-situ</i> Observation of <i>δ</i>↔<i>γ</i> Phase Transformations in Duplex Stainless Steel Containing Different Nitrogen Contents

Abstract

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.

Phase transformation and microstructure evolution of pearlite heat-resistant steel during heating

“…At the early stage of the growth of the δ-ferrite platelet, the migration rate of the δ/γ IBs is mainly controlled by δ/γ IB types due to the short-range diffusions of all elements that occur near the thin δ/γ IBs. δ/γ IBs at the tip of δ-ferrite platelets are incoherent with high mobility, although the δ/γ IBs along the thickness direction are semi-coherent with low mobility [26,27]. As a result, the migration rate of the δ/γ IBs along the longitudinal direction is far greater than along the transverse direction.…”

Formation of delta-ferrite and its effect on impact toughness of GTAW weld metals in a 12 wt-% Cr steel

“…In the previous studies, more attention was paid to the solid-state phase transition of δ-phase in the steels, and the formation of the δ-phase in the solidification stage was less studied [10,11]. Generally, the process of the δ-phase formation in the F/M steel is more complicated than that in the austenitic heat-resistant steel.…”

In Situ Observation and Phase-Field Simulation Framework of Duplex Stainless-Steel Slab during Solidification

“…High-temperature confocal microscopy (HTCM) was used to observe the solidification process of S32101 DSS in the present research, and this technique can provide a so-called in situ observation. Zhao and Sun [ 4 ] applied laser scanning confocal microscopy (LSCM) in their research on DSS. They observed and discussed the effect of different N contents on the solid-state δ γ phase transformations and concluded that N could hinder the migration of δ/γ interface boundaries (IBs).…”