Influence of Carbide-Promoting Elements on the Microstructure of High-Silicon Ductile Iron
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Copyright Ă“ 2020 The Author(s) https://doi.org/10.1007/s40962-020-00442-1
Abstract Because of its low cost, steel scrap is one of the most important raw materials for the production of ductile iron (DI). The amount of carbide-promoting elements in steel scrap, such as chromium, manganese, molybdenum, niobium and vanadium, is expected to increase in the future. Most of these elements have a negative impact on the microstructure and mechanical properties of DI. The solubility of carbide-promoting elements in solid solutionstrengthened DI materials, standardized in DIN EN 1563:2011, is modified by the high silicon content. For these new materials, the tolerance limits for carbide-promoting elements and their mutual influence must be known to ensure a sustainable production process. To investigate the individual and combined impact of carbide-promoting elements on the carbide content in high-silicon ductile iron EN-GJS-500-14, experimental investigations and
thermodynamic–kinetic microstructure simulations were carried out. Microstructure was characterized using metallographic analysis, and quantitative relations between chemical composition and microstructure were developed by means of regression analysis. Besides this quantitative analysis, it was found that the formation of grain boundary carbides can be detected via thermal analysis. Furthermore, experiments and simulations showed that vanadium promotes the formation of chunky graphite in high-silicon DI castings.
Introduction
in DI decrease the materials’ ductility and machinability. During the solidification of DI, carbide-promoting elements tend to segregate toward the remaining melt, which leads to carbide precipitation at the grain boundaries. On the contrary, silicon has a strong graphitizing potential and, as shown by Campomanes and Goller,1 thus counteracts the carbide-promoting impact of other elements. The new solid solution-strengthened ductile iron grades (SSDI) having silicon contents between 3.2 and 4.3 wt%, according to DIN EN 1563,2 exhibit a unique combination of strength and ductility. The increased Si content leads to a modified solubility of carbide-promoting elements in austenite. Some carbide-promoting elements can be tolerated in higher amounts compared to conventional DI.3
Steel scrap is an essential raw material for the production of ductile iron. Modern steels contain an increasing degree of alloying elements to achieve higher strengths. Most of these elements promote the formation of carbides in DI. The trend toward the use of high-strength steels affects the long-term composition of steel scrap available for ductile iron foundries. The increasing concentrations of carbidepromoting elements in steel scrap are a metallurgical challenge for the production of DI. Foundries require detailed knowledge about the individual and combined impact of carbide-promoting elements to decide whether or not a steel scrap is applicable for DI production. Carbides
This paper is an invited submission to IJMC selected from presentations at the 6th Keith
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