Ferro Silicon Magnesium is an alloying agent used in a variety of iron and steel production processes, most notably as an additive in the manufacturing of spheroidal graphite iron castings. It offers a range of desirable characteristics for this application, including high tensile strength at high temperatures, increased resistance to corrosion and improved hardness.
Traditionally, magnesium ferrosilicon (43-48% Si) alloys have been used as nodulizers to convert carbon-containing iron into nodular graphite cast iron by way of a gating system which includes at least one intermediate reaction chamber. During the casting process, the nodulizing agent is present in a particulate form and dissolves rapidly in the molten iron as the metal passes through the intermediate reaction chamber to produce nodular graphite in the cast iron.
The dissolution rate of magnesium ferrosilicon alloys depends on three important factors - the melting point of the alloy, the exothermic effect of silicon in iron and the amount of magnesium in the alloy. As the silicon content increases, so does the melting point and heat of solution. The resulting maximum overall dissolution rate is about 65-75% silicon, although the exact percentage of magnesium can be varied to achieve desired results.
There are many types of nodulizing alloys available for use in ductile iron foundries. They are generally divided into two categories: ferrosilicon alloys and rare earth metals. The latter category are typically added to the nodulizing alloy to counteract the effect of tramp elements on the nodular iron, which may occur in a variety of ways.
For example, cerium can be added to the nodulizing alloy to neutralize the effects of lead on the graphite formation. This allows for the use of a wider range of nodulizing alloys in this application, as the lead can be removed more easily and at lower costs than it would be without the addition of cerium.
Alternatively, other nodulizing alloys can be applied to the iron, such as nickel-magnesium (Ni-Mg) alloys. These usually contain a mixture of magnesium and other nodulizing ingredients, such as cerium, calcium or aluminum. These are used in conjunction with other nodulizing ingredients to provide optimum microstructure for ductile iron castings, such as SG Iron and specialty grey iron.
Nodularisation of ductile iron requires a large degree of control of the temperature, reactivity and added rate of the nodulizer. Therefore, it is important that the treatment process be carried out at a suitable rate and with the required accuracy. The choice of the nodulizing alloy is also dependent on several other factors, including the capacity of the foundry and the type of ductile iron it produces.
In most ductile iron foundries, the main nodulizing alloy is an Fe-Si-Mg alloy. The magnesium content of this alloy can be selected from a wide range, depending on the treatment method and degree of reactivity. Other nodulizing ingredients include calcium, which retards the reaction of magnesium with molten iron and is useful in reducing the rate of onset of the magnesium-iron reaction and the subsequent fade rate.
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