Figure 13. Eutectic cell count, N as a function of holding time and temperature of bath superheating for a base cast iron is shown.39
Table 3. Effect of the Furnace Atmosphere and Type of Charge Materials on the Cell Count
This phenomenon is known in foundry practice as an inherited property of the charge materials.
2. On a number of occasions, foundry workers have expressed concern related to the likelihood that iron melted in a traditional cupola has an intrinsically higher graphite nucleation potential level than iron of the same nominal chemical composition which has been melted and held in an electric furnace.42,43 As a result, when iron from the cupola furnaces is held in electric furnaces, there should be a reduction in the graphite nucleation potential level.
3. From the experimental outcome (Table 3) it is found that the furnace atmosphere also influences the cell count. The lowest cell count is obtained when vacuum or an argon atmosphere is employed. In turn, this indicates that under vacuum the bath is purified from inclusions which can act as potential graphite nucleation agents.
4. As the temperature of bath superheating, T in- creases and the holding time, t increases, the
International Journal of Metalcasting/Summer 10
number of graphite nuclei decreases leading to a reduction in the cell count (Fig. 13). It can be observed from Fig. 13, that the dominant chang- es in cell or nodule count occur during the first 80 minutes. Beyond this time the melt stabilizes (from the viewpoint of nucleation) and the cell count does not change any further. In particular,
this is common when base irons have been held overnight or over the weekend commonly called “Monday morning” iron.44,45
5. The nucleation potential of a given melt can be influenced by certain additions, such as FeSi, SiC and graphite. This treatment is called precondi- tioning. After the Mg treatment and post-process- ing (inoculation), the higher nucleation potential of the preconditioned melt will result in a higher number of nuclei, that is a higher nodule count, or a higher eutectic cell. As an example, it is known that preconditioning (with a 0.1 wt% FeSi addi- tion) of the melt prior to treatment of the liquid with Mg41
increases the nodule count by at least 15 nodules/mm.2 45
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