Introduction to Metallurgy...a Metallurgical Tutorial
Cast iron is a brittle like material that cannot absorb impact from a hammer; therefore, cast iron is little or no toughness when compared to low carbon steel. Cast iron has a fraction of the tensile strength of low carbon steels. When this material fails it will not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap. Therefore, there is no early warning of a failure. Cast iron is economical and when used right it is an excellent choice for many applications.
Cast iron is normally made from induction furnaces or cupola furnace. A cupola is like a small blast furnace for refining the iron to desired properties. Pig iron and scrape steel are used much that way primary steel is made. Pig iron is blast furnace iron. The word pig means ingot form (never making it to the BOS) or (possible original source) small pig size ingots. The scrape steel determines the desired property.
Carbon is normally in a graphite flake form that is pure carbon and acts as a natural defect in the material. The iron is so saturated with carbon that graphite forms (free carbon) and causes the cast iron to be weaker and behave as if it is weak. Much smaller amounts of carbon is combined with iron Fe in the form of FeC or a hard brittle iron carbide called Cementite named after an individual who took up the study of metallurgy in 19th century.
Cast iron is made up of principally Fe, Si, and C. The carbon content for gray cast iron is around 3.5%-C and Silicon is lowered just below 2.0% Si from the higher pig iron levels. As you can deduce most of the cast iron is in the form of Fe (Iron) and at around 94% Fe.
Machining cast iron is possible without a cutting lubricant because the graphite is a natural lubricant. Cast iron does not corrode readily on machined surfaces unless the graphite film is removed or compromised.
Gray Cast Iron - Gray Iron has a lower tensile strength and lower ductility. Very brittle.
Nodular Cast Irons:
Malleable Cast Iron - Malleable Cast Iron had reasonable good ductility and is desirable for low temperature service and small components like fittings.
Ductile Cast Iron - Ductile Iron has high strength and reasonably good ductility. This material has twice the tensile strength Gray Cast Iron.
There are other specialty cast irons like austenite gray cast iron and inoculated cast irons. Cast iron can be alloyed much like carbon steels including Chromium (Cr), Nickel (Ni), etcetera.