Cast iron is actually an iron-carbon-silicon alloy containing
between 3 percent and 5.5 percent carbon, 1 percent to 3 percent
silicon, plus traces of other elements such as sulfur, magnesium
and potassium totaling about 0.5 percent. By contrast, steels
contain less than 2 percent carbon. According to the Mat Base
website, cast iron's main advantages are low price and the
capability to be molded into complex shapes in a single
production step. Cast iron is a ferrous metal--but one that has
a reasonably strong resistance to rust
Features
Cast iron is weaker than steel when under tension, but is as
strong or stronger than steel when under compression, according
to the Mat Base website. It also is more brittle than steel and
can crack or break under shock. The strength of cast iron
depends on the morphology of the carbon. In gray cast iron, the
carbon is present as plates of pure graphite. This is the
weakest form. In ductile cast iron, the carbon is present as
graphite spheres. This is the strongest form of cast iron.
Other Types
Other forms of cast iron include white iron. This is actually
iron carbide. It is a very hard but very brittle metal. Often,
parts of an iron casting that will be subject to wear are
chilled to convert that area to white iron. Then there is
malleable cast iron, which is white iron that has been subjected
to a two-stage heat treating process that produces irregular
carbon grains. It is easily machined and can be selectively
hardened after machining.
Engineering Material
Cast iron can be engineered to serve many purposes, according to
the Machine Design website. It can be machined to close
tolerances, resists warping, can be heat treated to impart
desired characteristics such as surface hardness, can be cast
with inserts of other materials, and can produce highly complex
shapes and section sizes ranging from a few ounces to over 100
tons.
Alloys
Cast iron also can be alloyed with other metals to impart
desirable characteristics impossible to achieve with cast iron
alone, according to Machine Design. Cast iron typically is
alloyed with chromium and/or nickel with anywhere from 3 percent
to 30 percent or more of the alloying metal. For instance, high
chromium iron (up to 16 percent) combines wear resistance and
corrosion resistance. High-nickel iron (over 35 percent) is
dimensionally stable under high heat, nonmagnetic and very rust
resistant.

Function
Gray cast iron, according to Machine Design, is used in industry
for engine blocks, light-duty gears, flywheels, brake discs or
drums, and machine bases. Its ability to dampen vibration makes
it valuable for precision machinery. It's also found in the home
in items such as cookware, ornamental objects and toys. Ductile
cast iron is used for engine crankshafts, heavy-duty gears and
auto door hinges. White iron is used for applications requiring
abrasion resistance, such as railroad brake shoes, mill liners
and sandblasting or shot-blasting equipment. Malleable iron
castings are used for bearing surfaces in trucks, construction
equipment, railroad rolling stock and other extreme-wear
service.
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