The Occurrence of Chunky Graphite in Ductile Cast Iron Solution Strengthened by Silicon

Abstract

Solution strengthened ferritic cast iron is characterised in EN 1563:2018 (and also in ISO 1083:2018) as grades EN-GJS-450-18, EN-GJS-500-14 and EN-GJS-600-10. Machine and plant builders have been using this material in new designs since 2011. The strength and ductile properties, competitive in comparison with the previous classical ductile cast iron grades, make it pos¬sible to reduce the weight of castings. Particularly preferred is grade EN-GJS-600-10 with Rm min. 600 MPa and A min. 10%. Strengthening of the ferritic matrix is possible due to the content of approximately 4.2% of silicon in the cast iron. In casting practice, this material is difficult to produce in a stable manner due to the frequent occurrence of degenerate chunky graphite in the casting structure, resulting in a loss of ductile properties of the cast iron. In this article, an analysis has been made, based on literature, of the occurrence of strong negative segregation of silicon in classical ductile cast iron. The authors’ own research has confirmed the occurrence of chunky graphite in the cast. Dendritic crystallisation of austenite and strong segregation of silicon favours the formation of chunky graphite. A much stronger proportion of silicon was found in the chunky graphite zone, compared to the adjacent ball graphite zone. The introduction of micro-additives into the liquid melt eliminated the presence of chunky graphite in the casting. The applied modifiers of graphite and metal matrix (hybrid modification) seem to be an effective method to stabilise the technological process of this innovative casting material.