The term cast iron is used to describe an entire family of metals
with a broad range of properties. Like steel, it is also a generic
term that designates a family of metals. Both steels as well as cast
irons are mainly iron with carbon (C) as the prime alloying element.
While steels constitute less than 2% and generally less than 1%
carbon; all cast irons comprise more than 2% carbon. Two percent is
the maximum carbon content at which iron can become solid as a
single phase alloy with all of the carbon in solution in austenite.
Therefore, we can say that cast irons solidify as heterogeneous
alloys and always contain more than one constituent in their micro
Along with carbon, cast irons also contain silicon (Si), generally
from 1–3%, and hence, we can say they are actually
iron-carbon-silicon alloys. The high carbon and silicon content of
cast irons make them excellent casting alloys. The melting
temperature of cast irons is significantly lower than for steel.
Iron when melted is more fluid than molten steel and is less
reactive with molding materials. During the solidification process,
low density graphite is formed in the iron. This low density
graphite reduces the change in volume of the metal from liquid to
solid state and makes it possible to produce more complex castings
possible. However, as a matter of fact, cast irons do not have
adequate ductility to be forged or rolled.
Cast irons come in a variety of types, however these cannot be
specified by chemical composition because of the similarities
between the types. The table given below shows the distinctive
composition ranges for the most frequently determined elements in
the 5 generic types of cast iron.
Range of Compositions for Typical Unalloyed Cast Irons
For the commercial purposes, the range of compositions can be
categorized into a sixth type - the high-alloy irons. These have a
broad range in base composition and also constitute other elements
in significant quantities.
The presence of some minor elements also is critical to the
successful production of different types of iron. For instance,
nucleating agents, known as inoculants, are used in the production
of gray iron to control the type and size of graphite. While bismuth
and tellurium are used in little quantities to produce malleable
iron; the presence of a few hundredths of a percent magnesium (Mg)
drives the formation of the spheroidal graphite in ductile iron.
Moreover, the composition of an iron requires to be adjusted to
befit specific castings. A particular composition of metal cannot be
used to produce small and large size of castings of the same grade
of iron. Due to this reason, most of the iron castings are purchased
on the ground of mechanical properties instead of composition. An
important exception is for castings, which necessitate special
properties, such as - corrosion resistance or elevated temperature
The different types of cast irons can be classified on the basis of
their micro structure. The classification depends on the form and
shape in which the major component of carbon occurs in the iron.
This system facilitates five basic types - gray iron, ductile iron,
malleable iron, compacted graphite iron (CGI) and white iron. All
these different types of irons can be heat treated or moderately
alloyed without altering its basic classification. The high-alloy
irons, which normally contain over 3% of the added alloy, can be
separately classified as gray or ductile iron or white, however the
high-alloy irons are classified commercially as a distinguish group.