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5. Use Special Nodulizers to Avoid Shrinkage


Historically, MgFeSi alloys have been


alloyed with rare earth metals, which are used to neutralize tramp element effects to avoid edge carbides at low pouring temperatures and to optimize nodule count. For many years, mischmetal was the most common type of rare earth metal added. Special alloys have been developed that use pure lanthanum rather than the mixture of rare earth elements. An optimized amount of lanthanum results in a high nodule count with a different nodule size distribution. Te number of large, early forming nod- ules decreases slightly, and the number of medium and smaller nodules increases, which is an indication that graphite precipitation has been steadier through freezing, with more expansion effect dur- ing the latter stages of freezing. In a comparison of MgFeSi alloys using the traditional mischmetal and MgFeSi alloy using lanthanum, the ductile iron using lanthanum provided a structure with fewer large nodules and a high population of medium and small nodules. Figure 4 shows gross shrinkage in the iron treated with mischmetal and none in the iron treated with lantha- num. Conserving the expansion effect by saving some carbon precipitation to the latter stages of freezing has a pro- found effect on shrinkage. A minimized tendency of iron to shrink can be some- what correlated with a wide nodule size distribution, highly skewed to the finer sizes. Tis also can be achieved using a proprietary inoculant containing sulfur and oxygen compounds.


6. Increase Freezing Rate Rather Than Risering


Flow and solidification simula-


tion software is useful to predict where shrinkage is likely to occur in castings. At that point, metalcasters have the choice to add risering to feed more liquid iron to the spot where shrinkage is predicted or somehow make that part of the cast- ing freeze more quickly with chilling techniques. In large castings, producing structures with high nodule counts and high nodularity can be a struggle. In these situations, using chilling to avoid shrinkage is useful because it also pro- vides an improvement in the structure.


February 2013 MODERN CASTING | 39


Risers may make the structure worse by prolonging the freezing time and reduce the iron yield. Heavy shrink-prone areas can be


chilled in a number of ways. Figure 5 shows holes drilled into the cores and the subsequent cooling pins cast during mold filling. Tese iron pins serve as radiators to transfer heat from the cast- ing into the sand more rapidly, to avoid


shrinkage in those areas. In some cases, pins or fins can be added to a pattern. If they can’t be included automatically with patterns or coreboxes, they can be drilled into the pattern and then removed from the casting during or after cleaning. In Figure 6, a coiled spring is used


to rapidly freeze a section of a casting prone to shrinkage. Te greater surface area of a spring can chill the iron more


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