Cast irons | Types And Application

Cast irons

Though ferrous alloys with more than 2.14 wt.% C are
designated as cast irons, commercially cast irons contain
about 3.0-4.5% C along with some alloying additions. Alloys
with this carbon content melt at lower temperatures than
steels i.e. they are responsive to casting. Hence
casting is the most used fabrication technique for these
alloys. Hard and brittle constituent presented in these
alloys, cementite is a metastable phase, and can readily
decompose to form α-ferrite and graphite. In this way
disadvantages of brittle phase can easily be overcome.
Tendency of cast irons to form graphite is usually controlled
by their composition and cooling rate. 

Based on the form of carbon present, cast irons are
categorized as gray, white, nodular and malleable cast

Microstructure For Cast Iron.
Fig. Microstructure For Cast Iron.

1) Gray cast iron:

These alloys consists carbon in form graphite flakes, which
are surrounded by either ferrite or pearlite. Because of
presence of graphite, fractured surface of these alloys look
grayish, and so is the name for them. Alloying addition of Si
(13wt.%) is responsible for decomposition of cementite, and
also high fluidity. Thus castings of intricate shapes can be
easily made. Due to graphite flakes, gray cast irons are weak
and brittle. However they possess good damping properties,and

typical applications include: base structures, bed
for heavy machines, etc. they also show high resistance to

2) White cast iron: 

When Si content is low (less than 1%) in combination with
faster cooling rates, there is no time left for cementite to
get decomposed, thus most of the brittle cementite retains.
Because of presence of cementite, fractured surface appear
white, hence the name. They are very brittle and extremely
difficult to machine. Hence their use is limited to wear
resistant applications such as rollers in rolling
Usually white cast iron is heat treated to
produce malleable iron.

3) Nodular (or ductile) cast iron: 

Alloying additions are of prime importance in producing these
materials. Small additions of Mg / Ce to the gray cast iron
melt before casting can result in graphite to form nodules or
sphere-like particles. Matrix surrounding these particles can
be either ferrite or pearlite depending on the heat
treatment. These are stronger and ductile than gray cast

Typical applications include: pump
crank shafts, automotive
components, etc. 

4) Malleable cast iron:

 These formed after heat treating white cast iron. Heat
treatments involve heating the material up to 800-900
degreeC, and keep it for long hours, before cooling it to
room temperature. High temperature incubation causes
cementite to decompose and form ferrite and graphite. Thus
these materials are stronger with appreciable amount of

Typical applications include: railroad, connecting
rods, marine and other heavy duty services.