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tion is consumed early during freezing. Te rapid precipitation of graphite as it floats in the liquid metal can result in insufficient graphite expansion effect during the later stages of freezing, within the last isolated pools of iron to freeze. Figure 2 shows plots of the maxi-

mum carbon equivalent data for square bars, flate plates and modulii, with three curves shown for three silicon levels. Te lines have been extended to the CE eutectic of 4.3. Tis shows all thicker sections that must be at or below 4.3% CE to avoid primary graphite precipitation, nodule flotation and higher shrinkage. Carbon precipitation as graphite nodules is required at the start of freez- ing to ensure the carbon does not take the iron carbide form as edge chill. Too much early graphite precipitation must be avoided, or too little graphite precipitation will occur during the end of freezing, when the gating system and risers can no longer deliver more liquid to compensate for contraction. Higher silicon leads directly to higher nodule count, more ferrite and early carbon precipitation. To minimize shrinkage, the CE should be on target with the maximum carbon content and the minimum silicon content. Enough silicon should be used to avoid carbides and strengthen the ferrite to meet properties but not excessively more. Too much silicon can lead to excessive initial expansion effect with too little occurring in the last iron to freeze. Nor- mally excessive nodule count that leads to shrinkage has a structure where the nodule size appears identical through- out, as all the nodules started forming early during freezing (Fig. 3). Excessive inoculant addition rate or the use of bismuth to increase nodule count can lead to high nod- ule counts with uniform sizes. High nodule counts can be useful to turn off shrinkage, but only if a wide nodule size distribution can be produced. Tis implies graphite precipitation proceeds at a steadier pace from the start to the end of freezing and not too fast during the first part of freezing.

Fig. 2. These charts show the maximum CE to avoid nodule flotation for square bars, modulii and flat plate sections for three silicon levels poured at 2,550F. The curves are extended to show the section thicknesses where the CE must be eutectic or slightly below.

Magnesium content also should be

controlled. Enough magnesium should be used to produce good nodules, but an excess of magnesium can lead to slag defects or spiky graphite formation, in addition to shrinkage problems.

3. Keep Base Sulfur Content Consistent

Te base sulfur content of iron can

have a large impact on nodule count and size distribution. For very thin cast- ings prone to carbides, some metalcast- ing facilities will intentionally operate with a higher base sulfur. Tese nodules

appear to be similar in size and may lead to shrinkage problems if the nod- ule count becomes too high, especially in heavier sections. For reproducible nodule count, the base sulfur content must be uniform from one treatment to the next. Large variations in the base sulfur, such as when converting between gray and ductile iron, could lead to variable nodule counts and nodule size distributions and shrinkage propensity.

4. Avoid Long Hold Periods

As base iron is held, carbon is lost and the state of nucleation changes over

February 2013 MODERN CASTING | 37

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