The Influence of Composition on the Solidification Path and Microstructure of HP-Nb Alloys

Austenitic stainless steels such as 25Cr-35Ni-Nb, HP-Nb alloys are commonly used in high-temperature applications above 850 °C. The HP-Nb alloys are used in the as-cast condition and exhibit good creep properties due to chromium and niobium eutectic carbide strengthening. The size, morphology and distribution of these carbides are related to the nominal composition and solidification conditions, which result in differences in mechanical properties. Although previous work has provided qualitative relations between composition and microstructure, there is insufficient quantitative information regarding the influence of composition on the solidification behavior and resultant microstructure. Therefore, the objective of this study is to improve the understanding of the solidification of HP-Nb alloys through the development of a quantitative model. A systematic matrix of 12 alloys was characterized via quantitative image analysis, differential thermal analysis and electron microprobe analysis. A quantitative solute redistribution model was developed using solidification equations and an experimentally derived γ -C-Cr-Nb, pseudo-quaternary liquidus projection. The quantitative model was validated for three experimental alloys and two industrial alloys, with good agreement between the measured and predicted percent total eutectic. Experimental results indicate that the volume fraction of chromium carbides are primarily controlled by carbon additions and niobium carbides are primarily controlled by niobium additions. The results of this work show the novel development and application of a pseudo-quaternary liquidus projection and improves the understanding of the relationship between composition and the solidification of austenitic stainless steels.