Cast Iron

Cast irons include many metals having a wide variety of properties. Although cast iron is often considered a simple metal to produce and to specify, the metallurgy of cast iron is more complex than that of steel and most other metals. Steels and cast irons are both primarily iron with carbon as the main alloying element. Steels contain less than 2 and usually less than 1% carbon; all cast irons contain more than 2% carbon. Two percent is about the maximum carbon content at which iron can solidify as a single-phase alloy with all the carbon solution in austenite. Thus, the cast irons, by definition, solidify as heterogeneous alloys and always have more than one constituent in their microstructure. In addition to carbon, cast irons must also contain silicon, usually from 1 to 3%; thus, they are actually iron-carbon-silicon alloys. High-carbon content and silicon in cast irons give them excellent castability. Their melting temperatures are appreciably lower than those of steel. Molten iron is more fluid than molten steel and less reactive with molding materials. Formation of lower density graphite during solidification makes production of complex shapes possible. Cast irons, however, do not have sufficient ductility to be rolled or forged. Iron's carbon content is the key to its distinctive properties. The precipitation of carbon (as graphite) during solidification counteracts the normal shrinkage of the solidifying metal, producing sound sections. Graphite also provides excellent machinability (even at wear-resisting hardness levels), damps vibration, and aids lubrication on wearing surfaces (even under borderline lubrication conditions). When most of the carbon remains combined with the iron (as in white iron), the presence of hard iron carbides provides good abrasion resistance. In some cases, iron microstructure may be all ferrite -- the same constituent that makes low-carbon steels soft and easily machined. But the ferrite