it appears that the drop is caused alone by the embrittling effect of the silicon. Degenerated graphite, which acts as in- ternal notches, does not exist in the microstructure of the Y specimens. When the Si content exceeds 4.3%, elongation decreases together with the tensile strength. Above 5% Si, elongation is no longer measureable. Metallographic exami- nations have shown that the matrix is 100% ferritic within the complete alloying interval investigated. Embrittling phase components were not detectable by light or electron microscopy. The critical Si content, at which solution em- brittlement starts, is between 4.2 and 4.4 Si.
Influence of Carbide and Pearlite Stabilizing Elements
Mn, Cr and V are the elements that are most likely to lead to pearlite and carbide formation in unalloyed or low-alloyed ductile cast iron. According to Reference 2, the static me- chanical properties primarily depend on the pearlite content, while the dynamic mechanical properties mainly depend on the carbide content in the microstructure and decrease with increasing carbide content. Increasing Si contents promote the formation of carbide-free microstructures.3
Within the framework of the tests, the effect of significant contents of pearlite and carbide stabilizing elements was investigated on an exemplary basis. In these tests, alloys with compositions that were expected to cause carbide precipitations, i.e. max. 1.0% Mn, max. 0.6% Cr, max. 0.26% V and max. 0.17% Ti, were prepared and cast into Y2 specimens of 1 in. thickness and Y4 specimens of 3 in. thickness. Tensile bars and microsections were taken from these specimens. In Figure 4, the static mechanical proper- ties determined in the alloying tests are plotted against the properties of the unalloyed material. The alloys with the in- dividual elements changed within the investigated content range and cast into separate specimens did not show any significant influence on the mechanical properties com- pared to the unalloyed specimens, with the exception of chromium. With a Cr content of 0.6%, the elongations are 10% and 14%. This is lower than the value of unalloyed melts, but still in compliance with the values specified for EN-GJS-600-10 in DIN EN 1563. The metallographic examinations showed that within the investigated alloy- ing ranges carbide precipitation was found in none of the specimens. Figure 5 shows the microstructure of an alloy containing 1% of Mn. The matrix consists of 100% ferrite. The matrix of the alloy with a Cr content of 0.63% contains approximately 25% pearlite, but no precipitated carbide.
Influence of the Inoculation and Si Content on the Graphite Shape
The mechanical properties of ductile cast iron do not only depend on the structure of the metallic matrix, but also on the graphite form. Inoculation tests were conducted with the mould arrangement shown in Figure 1. All tests were
International Journal of Metalcasting/Volume 8, Issue 2, 2014
Figure 2. Tensile strength pass through a maximum in dependence on the Si content.
Figure 3. The maximum yield strength is at a higher Si content in comparison to the tensile strength.
Figure 4. This graph shows no influence of the added elements on the tensile strength.
Figure 5. Microstructure of a material with 4.03% Si; 3.01 %C; 1.0 % Mn without pearlite (Y2-block).
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