Cast Iron
The
cast iron is obtainted by remelting pig iron with coke and lime stone in a
furnace known as cupola. It is primarily an alloy of iron and carbon. The
carbon contents in cast iron varies from 1.7 to 4.5%. it may be present either
as free carbon (or graphite) or combined carbon (or cementite).
Since the cast iron is a brittle material, therefore, it
cannot be used in those parts which are subjected to shocks. The properties of
cast iron which makes it a valuable material for engineering purposed are its
low cost, good casting characteristics, high compressive strength, wear
resistance and excellent machinability. The compressive strength of cast iron
is much greater than tensile strength.
The
cast iron also contains small amounts of impurities such as silicon, sulphur,
manganese and phosphorus. The effect of these impurities on cast iron are as
follows:
1.
Silicon. It may be present in cast iron upto 4%.
It provides the formation of free graphite which makes the iron soft and easily
machinable.
2.
Sulphur. It makes the cast iron hard and
brittle. It must be kept well below 0.1% for most foundry purposes.
3.
Manganese. It makes the cast iron white and
hard. It is often kept below 0.75%.
4.
Phosphorus. It aids fusibility and fluidity in
cast iron, but induces brittleness. It is rarely allowed to exceed 1%.
The important types of cast iron are as
follows:
(a)
Grey cast iron. It is an ordinary commercial
iron having 3 to 3.5% carbon. The grey colour is due to the fact that carbon is
present in the form of free graphite. It has a low tensile strength. High
compressive strength and no ductility. It can be easily machined.
according to Indian standards, grey cast iron is designated by the alphabets FG
followed by a figure indicating the minimum tensile strength in MPa or N/mm2
as minimum tensile strength.
(b)
White cast iron. It is a particular variety of
cast iron having 1.75 to 2.3% carbon. The white colour is due to the fact that
the carbon is in the form of carbide (known as cementite), which is the hardest
constituent of iron. The white cast iron has a high tensile strength and a low
compressive strength.
(c)
Chilled cast iron. It is a white cast iron
produced by quick cooling of molten iron. The quick cooling is generally called
chilling and the iron so produced is known as chilled cast iron.
(d)
Mottled cast iron. It is a product in between
grey and white cast iron in composition, colour and general properties.
(e)
Malleable cast iron. It is obtained from white
cast iron by a suitable heat treatment process (i.e. annealing). According to
Indian standard specifications, the malleable cast iron may be either
whiteheart, blackheart or pearlitic and are designated by the alphabets WM, BM
and PM respectively. These designations are followed by a figure indicating the
minimum tensile strength in MPa or N/mm2. For example ‘WM350’
denotes white heat malleable cast iron with 350 MPa as minimum tensile
strength.
(f)
Nodular or spheroidal graphite cast iron. It is
also called ductile cast iron or high strength cast iron. This type of cast
iron is obtained by adding small amounts of magnesium (0.1 to 0.8%) to the
molten grey iron just after tapping. According to Indian standard specifications,
the nodular or spheroidal graphite cast iron is designated by the alphabets
‘SG’ followed by the figures indicating the minimum tensile strength in MPa or
N/mm2 and the percentage elongation. For example, SG 400/15 means
spheroidal graphite cast iron with 400 MPa as minimum tensile strength and 15
percent elongation.
(g)
Alloy cast iron. It is produced by adding
alloying elements like nickel, chromium, molybdenum, copper and vanadium in
sufficient quantities. The alloy cast iron has special properties like
increased strength, high wear resistance, corrosion resistance or heat
resistance.
