Cast iron is made from re-melting
pig iron, usually with
some quantities of scrap iron, scrap steel and some alloys, such as
Cr, Ni, Mo and Cu according to the different requirements.
Depending on the specific application,
iron foundries reduce the carbon and silicon content to
the desired levels from 2 to 3.5% and 1 to 3% respectively. Other
alloys and elements are also added during melting process according
to the different requirements. With the different melting and
inoculation processes, the different cast irons will be produced.
White Cast Iron
White cast iron is named after its white surface after fractured.
With a lower silicon content and faster cooling, the cementite which
precipitates from the melt forms as relatively large particles,
where the other phase is austenite. These eutectic carbides are much
too large to provide precipitation hardening. Rather, they increase
the bulk hardness of the cast iron simply by virtue of their own
very high hardness and their substantial volume fraction.
White iron is too brittle for use in many structural components, but
with good hardness and abrasion resistance and relatively low cost,
so the white iron castings could be used as the wear surfaces
(impeller and volute) of slurry pumps, shell liners and lifter bars
in ball mills and grinding mills, and the teeth of a backhoe's
Grey Cast Iron
Grey iron is characterized by its graphitic microstructure of
flakes, which causes fractures of the material to have a grey
appearance. Grey cast iron
is named after its grey fractured surface.
Most of grey cast irons have a chemical composition of 2.5 to 4.0%
carbon, 1 to 3% silicon, and the remainder is pure iron. Grey cast
iron has less tensile strength and shock resistance than ductile
iron or malleable iron, however its compressive strength is
comparable to low and medium carbon steel.
For the low degrees of grey cast iron, they could be used to produce
the casting parts with low requirements for tensile strength, but
have good friction and wear properties, such as protective cover,
cover, oil pan, hand wheels, frame, floor, hammer, small handle,
machine base, frame, box, knife, bed, bearing seat, table, wheels,
cover, pump, valve, pipe, flywheel, motor blocks,
gas burner, stove grates etc.
For the medium and high degrees of grey cast iron, they have certain
degree of tightness or corrosion resistance, so they could be used
to produce more important castings such as cylinder, gear,
automotive flywheels, cylinder block, cylinder liner, piston,
gear box, brake wheel,
coupling Plate, medium pressure valve, heavy machine tools, shears,
presses, automatic lathe bed, high-pressure hydraulic parts, piston
rings, force larger gear, bushings, large engine crankshaft,
cylinder block, cylinder liner, cylinder head, etc
Malleable Cast Iron
Malleable iron starts as
a white iron casting that is then heat treated at about 900
centigrade. Graphite separates out much more slowly in this case, so
that surface tension has time to form it into cottony particles
rather than flakes.
Due to their lower aspect ratio, spheroids are relatively short and
far from one another. They also have blunt boundaries, as opposed to
flakes, which alleviates the stress concentration problems faced by
grey cast iron. In general, the properties of malleable cast iron
are more like mild steel. There is a limit to how large a part can
be cast in malleable iron, since it is made from white cast iron.
However, the malleable cast iron has been replaced by the developed
ductile iron because of more good characters of ductile iron.
Ductile Cast Iron
After years development, there are
nodular iron or called as
ductile cast iron. Tiny amounts of magnesium or cerium added
to these alloys slow down the growth of graphite precipitates by
bonding to the edges of the graphite planes. Along with careful
control of other elements and timing, this allows the carbon to
separate as spherical particles as the material solidifies. The
properties are similar to malleable iron, but parts can be cast with
Ductile iron castings almost cover all major industrial sectors.
They have been applied in these sectors that require high strength,
ductility, toughness, wear resistance, resistance to severe thermal
and mechanical shock, high temperature or low temperature, corrosion
resistance and dimensional stability. To meet the conditions of use
of these changes, many of the existing grade nodular iron,
mechanical properties and provides a rational to a kiss a wide
History of Cast Iron
Cast Iron was first invented in China in the 4th century BC and
poured into moulds to make ploughshares and pots as well as weapons
and pagodas. In the west, where it did not become available till the
late 14th century, its earliest uses included cannon and shot. Henry
initiated the casting of cannon in England. Soon, English iron
workers using blast furnaces developed the technique of producing
cast iron cannons, which, while heavier than the prevailing bronze
cannons, were much cheaper and enabled England to arm her navy
better. The ironmasters of the Weald continued producing cast irons
until the 1760s and armament was one of the main uses of irons after
With its relatively low melting point, good fluidity, casting
ability, excellent machining ability, resistance to deformation and
wear resistance, cast irons have become an engineering material with
a wide range of applications and are used in pipes, machines and
automotive industry parts.