DRUŠTVO LIVARJEV SLOVENIJESLOVENIAN FOUNDRYMEN SOCIETY
Član The World Foundry OrganizationMember of World Foundry Organization

Due to its favorable combination of castability, mechanical properties, machinability and a comparatively low price, the production volume of cast iron with spheroidal graphite, so called ductile iron (DI), increased in Germany between 2009 and 2012 by 35 % to 1.6 million tons per year. Improving the mechanical properties of DI will help to strengthen its position as a high- performance-material, to fulfill future requirements and to withstand the competition to welded constructions and forgings. The as-cast mechanical properties of conventional ductile irons are mainly adjusted by the ferrite and pearlite ratio of its microstructure, which is influenced by alloying. A fully ferritic ductile iron exhibits the highest ductility and lowest strength. By increasing the pearlite content the tensile strength is increased accompanied by a strong reduction of the elongation at fracture. In contrast to this, "Second Generation" DI with a silicon content of up to 4.3 wt.% exhibits a fully ferritic matrix, which is solution-strengthened by the substitutionally solved silicon. The resulting solution-strengthened grades like the EN-GJS-450- 18, the EN-GJS-500-14 and the EN-GJS-600-10 offer superior combinations of strength, ductility and excellent machinability due to a fully ferritic matrix [Bjorkegren, L.E., K. Hamberg, Keith Millis Symposium on Ductile Cast Iron, 2003]. In addition a high silicon content leads to a higher tolerance against higher amounts of carbide-forming elements like e.g. vanadium, chrome and titanium [Löblich, final report of AIF Project 41EN, 2012]. However, the maximum strength is limited to 600 MPa at a silicon concentration of 4.3 wt.-%. At higher silicon contents the tensile strength as well as elongation at fracture are dramatically decreased due to the presumed formation of a silicon long range order [Löblich, final report of AiF Project 41EN, 2012]. The effect of additional solution strengthening elements on fully ferritic ductile iron is hardly addressed in literature. In order to close this current research gap, ferritic DI with a silicon content of 3.8 and 4.3 wt.-% silicon were alloyed with 2 and 4 wt.-% cobalt and 1.5 and 3 wt.-% nickel in the present study. According to a full factorial experimental design, the effects of cobalt and nickel were evaluated with the aid of thermal and microstructural analysis and tensile tests. Further, these results were statistically proofed using analysis of variances. In this article the results of the microstructural analyses are presented.