Detailed analysis of nucleation and subsequent δ to γ phase transformation of a high N super austenitic stainless steel
Super austenitic stainless steel (SASS) is widely used in extreme environments due to its excellent corrosion resistance and mechanical properties, and a feature of this grade is its relatively high nitrogen content. As valuable tools to study the solidification process and phase transformation, high temperature confocal microscopy (HTCM) and thermal analysis methods, such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC), have been widely reported in the investigation of SASS. However, the inherent limitations of these two techniques make it difficult to explain the complex solidification behaviors of SASS, which involves multiple phases. Here, a unique design of HTCM-DTA was employed to investigate the nucleation and subsequent phase transformation of SASS. This in-house build apparatus enables contemporaneous in-situ observation of the surface and thermal analysis of the bulk thermal events. Using the novel HTCM-DTA device, we systematically revealed the four solidification modes of SASS and analyzed them in detail through thermodynamic arguments and multiple sets of experimental data under different conditions: (a) bimodal solidification with solid-state phase transformation, (b) bimodal solidification with liquid-state phase transformation, (c) pseudo-bimodal solidification, and (d) unimodal solidification. By comparing with the findings of earlier studies, the limitation of spatiotemporal separation employing HTCM and thermal analysis methods was pointed out. The results suggest that both the shielding gas and the time of retaining in the liquid state affect the N content in the melt, leading to a change of the primary nucleation phase. The cooling rate not only affects the undercooling and microstructure but also the transformation stage of δ-ferrite to γ-austenite, and when the phase transformation occurs in the liquid state it is usually accompanied by recalescence. These insights contribute understanding required for adjusting the continuous casting process of high nitrogen alloys.
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University of Wollongong