Case D. An even more complicated graphite nodule size dis- tribution was observed (Fig. 9) in low Si (3.6%C. 1.6%Si) ductile iron solidified in a heavy section (4” wall thickness casting poured into no-bake sand mold). Three major classes of 3D nodule size distribution were observed: large (40-90 µm), medium (10-20 µm) and very fine (1-5 µm). In this case, a large amount of small graphite nodules promotes fer- ritization of the matrix during austenitic decomposition and decreases alloying element segregation in the as-cast condi- tion, which is important for low temperature toughness.9
Discussion
The thermodynamic simulation was used to predict the pos- sible sequences of transformations which could take place in reaction zones,10,11
where active elements, such as Mg,
Ca, Si and Al react when dissolved in the melt impurities S and O. FACTSAGE software and databases were used for thermodynamic description of the component activities in Fe-melt, mono- and complex liquid and solid phases (ox- ides and sulfides). Gibes free energy (∆Gi
Statistics of non-metallic inclusions inside graphite in the metal matrix showed a higher probability of complex (Ca+S)-contain- ing inclusions inside graphite nodules (compare Fig. 3 and Fig. 4). This experimental data support described thermodynamic considerations. Direct high resolution observation showed the complex nature of heterogeneous nuclei in ductile iron.12
Active
heterogeneous nuclei provide continuous nucleation of spheri- cal graphite during solidification resulting in bi-modal 3-D graphite nodule distributions in the casting. This effect became more prominent at a longer local solidification time.
Thermal analysis is a powerful tool to collect information about solidification because the precipitation of phases from the melt is associated with a significant amount of latent heat. In the Case B, thermal curves were collected directly from the castings and reprocessed using a computer assisted single thermocouple method.13
Liquid and solid phase seg- ) minimization was
used to obtain final equilibrium phases after completion of the possible parallel reactions. In this article, different simu- lated scenarios included variations in sulfur in the melt and calcium in Mg-additions.
winite (Ca3
At low S in the melt and low Ca in Mg-addition, MgO and more complex oxides such as forsterite (Mg2
200-300˚C above ductile iron solidification interval (Fig. 10a). These reaction products could be partially removed from the melt by flotation. A similar situation takes place in a low S, high Ca scenario (Fig. 10b). In high the S and low Ca scenario (Fig. 10c), Ca and especially Mg sulfides are still liquid during ductile iron solidification and cannot be active as heterogeneous nuclei. From this standpoint, the scenario with residual S in the melt and Ca in the Mg-addition (Fig. 10d) looks more promising to enhance heterogeneous nucle- ation. The possible sequence of reactions will include early forming MgO and complex Mg-Si-O oxides (forsterite, for example) with sequential “coating” inclusions by sulfides (CaS) just before graphite eutectic solidification.
MgSi2 O8
Nucleation potential depends on several factors, including re- quirements of minimal interfacial energy between nuclei and growing phase which could be less for sulfides when com- pared to oxides. Also important is temperature-time condition for inclusions precipitation in the melt: small and fresh formed inclusions with a minimal interfacial to graphite energy will have a higher graphite phase nucleation potential.
) could be formed at high temperature SiO4) and mer-
ments of cooling curve were used for development of “zero” line, and solid fraction curve was calculated from deviation of experimental data from “zero” line between liquidus and solidus. The calculated solidification kinetics (solid phase volume fraction versus temperature) from the original ther- mal curves is given in Fig. 11 for untreated (Ladle 1) and post-inoculated ductile irons (Ladle 3) poured at lower tem- perature. Post-inoculation of ductile iron shifted the shape of solidification curves from near linear with a negative slope to more complicated curves with two segments including a short linear negative segment at a higher temperature (start solidification) followed by a large segment with positive slope. It is interesting to correlate the shape of solidification
Figure 9. Three-dimensional graphite nodule distributions in heavy section (4” wall thickness) low silicon ductile iron. Red points show total 3D distribution and black dashed lines indicated three groups of normal distributions.
Table 1. Nodule Graphite Two-Dimensional (n and d) and Three-Dimensional (N) Parameters in Step Plate
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International Journal of Metalcasting/Volume 8, Issue 2, 2014
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