EFFECT OF SILICON

Sources

Silicon is found in almost all charge materials. Like carbon, there is very little in most steels, but it should be accounted for in charge calculations. It is also possible to buy pig iron with relatively low silicon; however, most pig irons, cast scrap and returns have higher silicon contents than steels. (It is sometimes possible to get a source of high silicon steel scrap. This can be an economical charge material.)

Silicon is also purchased as an addition. Care should be used when selecting the addition material. Inoculating grades of ferro-silicon should only be used when inoculation is desired. The aluminum and calcium in those grades can cause additional slag when introduced in induction furnaces

Effects and Comments

Silicon is like carbon in many respects. It has been long recognized as an important element, and, therefore, controls are typically adequate. While silicon control in a cupola can be difficult, in induction melting it is relatively easy.

Also like carbon, the higher the silicon the more likely larger graphite will occur as well as more ferrite in the matrix. This, of course, will generally be a weaker iron but with less likelihood of having carbides to degrade machinability. When silicon becomes very high it hardens the ferrite and can increase the hardness of the iron.

In ductile iron the impact transition temperature is affected by the silicon content. The impact transition temperature is the temperature at which ductile iron changes from being a

ductile material to a brittle material. With silicon contents around 2.40% the temperature is about -40o F. With silicon contents over 3% that temperature can be raised so high that castings will be brittle at room temperature.

Recovery of silicon alloys when added to a ladle is usually around 90%.