Influence of Carbide-Promoting Elements on the Microstructure of High-Silicon Ductile Iron

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 solution-strengthened 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.