The most effective elements for Gray Iron are Carbon and Silicon. When the objective is to cast the iron at the eutectic to prevent iron carbides from forming, it is possible to use ladle additions of Silicon to modify the molten alloy so that a eutectic equivalent is always achieved. Using the CE calculation and confirming the appropriate amount of Ferro-silicate additions, a wedge block is used to confirm the gray iron is at the desired CE level.
Compared to common steel grades, the carbon content in gray iron is about ten times higher. With scrap steel being a large part of the melt charge for an iron foundry, carbon usually has to be added at some point in the process, either in the main charge or after the iron is in the molten state. Since there is very little carbon in the scrap steel charge materials, the metallurgist needs to take into account all of the metallic charge materials (steel, scrap iron, pig iron) in the main furnace. Carbon raising additions depend heavily on the melting method (cupola melting with coke will elevate carbon), the amount of silicon used and the availability of low-cost graphite. However, as the iron foundry industry transitions from cupola to inductive melting with similar high additions of steel scrap in the charge, alternative ladle, in-stream or in-mold additions to achieve Type A flakes in Gray iron are required.
With the switch to inductive melting, foundries carefully charge theire induction ladles with carefully weighed amounts of scrap steel, scrap iron and the more expensive pig iron. Today, higher amounts of lower carbon steel can be accommodated by adding a ferrosilicon (FeSi) inoculants. The inoculation process involves an addition of between 0.05 to 1% of a specialized FeSi alloy containing controlled amounts of one or more carefully selected elements to further refine the graphite morphology. The explanation of how these carefully selected elemental additions, including Al, Ca, Ba, Sr, Ce, La, Mn, Bi, S, O, and Zr can be found on Table 1.0 Structural Effects of Elemental Additions to Cast Iron, shown below. In addition to raising the Si level, the inoculant provides nucleation sites that promote graphite precipitation and growth, together with iron solidification based on a stable Fe-C system.
When Carbon needs to purchased for addition, the addition material is generally in the form of graphite. Graphite additions frequently come from carbon electrodes, previously used in steel arc melting furnaces.
The purpose of inoculation is to promote heterogeneous graphite nucleation by introducing elements that form suitable substrates that will act as nuclei and initiate the desired graphite formation. By promoting a stable eutectic solidification, inoculation enables the C to come out of solution in a favorable form of graphite and not as iron carbide.
With careful control, the use of an inoculate addition will help:
The effect of inoculation is presented in the figure below (Inoculation Effect), where the cooling curves for an un-inoculated iron are indicated with a black-dotted line and an inoculated iron are indicated as a blue solid line.
Inoculation can take place either at tapping, in the ladle, in the stream during casting, or even inside the mold. Inoculating alloys are available in granular form, packed in a wire, or cast/pressed into various shapes. The size is adjusted based on the point of addition and the time and temperature available for dissolution into the molten iron. As a rule, additions can be reduced when inoculation takes place as close as possible to the pouring of iron into the mold, which is why “in-stream” inoculation with FeSi granules is the most common method of adding.
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