Magnesium ferrosilicon (43-48% Si) is a type of alloy used to produce nodular and spheroidal graphite iron castings in the form of ductile iron. It is usually accompanied by calcium and rare earth metals. These alloys provide a variety of benefits including, but not limited to, high magnesium recovery, less reactivity and a superior microstructure for ductile cast irons.
The main ingredient in magnesium ferrosilicon is magnesium, which provides the necessary chemical interaction for spheroidization of the liquid iron into the desired graphite structure. Calcium is another element added to the magnesium ferrosilicon alloy that retards the rate of reaction between the molten iron and the magnesium. This slows the rate of onset and reduces the rate of fading during spheroidization, thus reducing the risk of producing lumpy or chunky graphite.
Several variations of magnesium ferrosilicon are commercially available. For example, Hickman, Williams & Company produces magnesium ferrosilicon in several ranges and compositions to meet customer requirements. These include 3 to 4 percent magnesium for minimal reactivity and high magnesium recovery, as well as 5 to 6 percent magnesium for normal foundry practice.
In addition to magnesium, these alloys contain varying levels of silicon and other elements to optimize the microstructure for the chosen iron type and process. These elements include aluminum, carbon, calcium, strontium, zirconium and rare earths. These additives can be injected into the base liquid iron before or during its cooling phase.
These inoculants are a key component of the metallurgical treatment of iron castings. They can be inserted into the base iron in several locations, such as the furnace, ladle, stream and mould. The exact amount can be manipulated by the foundryman and his technical specialist to obtain the desired results for the cast iron.
The dissolution rate of the magnesium ferrosilicon is relatively slow, and this requires a particular configuration of the reaction chamber in order to obtain consistent casting results. Because of this, it is important to monitor the dissolution rate as it relates to the poured weight of the cast iron. This is accomplished by placing a window in the side of the reaction chamber and monitoring its depth and the time it takes for the alloy to dissolve.
A variety of inoculants are also available to help control the matrix structure and minimize defects. These can be injected into the base liquid iron in a cored wire or injection process that can be performed horizontally or vertically near the metallurgical treatment site.
One of the more common inoculants is a 5 to 6 percent MgFeSi alloy that is accompanied by rare earth metals and/or aluminum, barium and other elements to control matrix structure, reduce risk for carbides, shrink defects and other casting defects. These alloys are offered in several rare earth ranges and compositions to satisfy a broad range of customers' needs.
The other commonly used nodulizer is a mechanical mixture of granular magnesium and granular ferrosilicon alloy (50% Si) in the weight ratio of about one part of the former to about 15 parts of the latter. While this product can be effective, it is susceptible to the same problems discussed above with magnesium ferrosilicon, and can undergo particle segregation during manufacture and shipment. It can also have poor uniformity due to the substantial disparity in density between the two components.
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