What is cast iron?
Alloys of iron and carbon with
more than 2.11% carbon are
called cast irons.
Cast iron
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Family of ferrous alloys
• Cast into desired shape – not worked
• 2-4% C and 1-3% Si
• Instability of Fe3C:
– Cementite / graphite flakes / graphite
nodules
Classification of cast iron
Type of cast Graphite
iron
White
No
Ductility
No
Fast cooling rates
Gray
Flake
No
Slow cooling rates
Malleable
Spherical Yes
aggregates
Nodular
Nodular
Yes
White iron +
annealing heat
treatment
Additions made so
that nodules of
graphite form
instead of flakes
Gamma+Fe3c

Iron rich end of the Fe-C phase diagram
White cast iron
• Fe3C + pearlite
• Hard, brittle
• Shows a “white” crystalline
fractured surface
• Excellent wear resistance
• High compressive stress
White Cast Iron
Fe3C
Pearlite
White cast iron (Contd.)
 Has excellent wear resistance
 But is very brittle
 Finds use as
 balls for grinding mills,
 liners for cement mixers and
 rolls for paper manufacture
Gray cast iron
During slow solidification carbon in
Fe separates or graphitizes to form
separate graphite flakes
Microstructure of gray
cast iron
Separate graphite
flakes form
X100
X500
Ferritic vs.Pearlitic gray iron
 If all the carbon is in the form of graphite, we
have ferritic gray iron, where the graphite
flakes are embedded in a matrix of ferrite
 If only a part of the carbon is in the form of
graphite, we have the pearlitic gray iron,
with pearlite as the matrix.
General characteristics/advantages
of gray cast iron
• Cheaper than steel, as temperature to be
attained for making it is several hundred
degrees lower than for casting steel. Also
control of impurities is not critical here, as in
steel making.
• It has excellent fluidity, even large complex
shapes can be cast advantageously.
• Excellent machinability, as chip formation is
promoted by the graphite flakes. In addition the
flakes serve as a lubricant for the cutting tool.
General characteristics/advantages
of gray cast iron
 The wear resistance of gray iron is very good,
as graphite flakes act as lubricant.
 The damping capacity (ability to damp
vibrations) of gray iron is superior to that of
steel
 Can be alloyed to improve properties, e.g.
Nihard iron with 4%Ni and 1.5%Cr has
excellent wear resistance.
 Graphite flakes are sharp at their tips and act
like internal cracks or stress raisers. For this
reason gray iron is brittle and shows only about
0.5% elongation in tension.
Great at dampening!
Relative ability of ferrous metals to dampen
vibrations. The energy absorbed per cycle, or
specific damping capacity of these can differ by
more than 10 times.
Factors that promote the formation
of graphite
Slow cooling
 Thick cross sections or castings in sand moulds
tend to have graphite, as the cooling rate is slow.
 Chill castings (in metal moulds) and thin cross
sections tend to have cementite.
 This effect can also be seen in the fracture
appearance across the cross section varying from
white at the surface to gray inside. The transition
region has the “mottled” appearance
Factors that promote the formation
of graphite
Alloying elements influence graphitisation:
 Silicon has a very high effect.
 Effect of alloying elements other than Si is
described in terms of
Si equivalent =
%Si+3(%C)+0.3(%Ni)+0.3(%Cu)+0.5(%Al)0.25(%Mn)-0.35(%Mo)-1.2(%Cr)
 Increasing carbon in the melt tends to
promote graphite formation
Silicon promotes graphitization
Stress-strain curves in tension and compression
for Class 20 and Class 40 cast irons
Uses of gray iron
 The good damping capacity and the high compressive
strength make it suitable as a base for erection of
machinery.
 Ease of machining, good wear resistance and
damping capacity are utilized in applications such as
locomotive and internal combustion engine cylinder
blocks and heads
 Ease of casting and low cost make it suitable for
flywheels and counterweights for lifts
 Niresist with 20%Ni and 2%Cr has excellent corrosion
resistance and heat resisting properties and is used
for handling alkalis at high temperatures.
Malleable cast iron
• White cast iron (typical composition 2.5%C and
1%Si)+ prolonged heat treatment at 900-950oC
followed by very slow cooling
• During this treatment cementite decomposes to
the more stable form (graphite). The free carbon
precipitates in the form of spheroidal particles
(nodules)
Temper graphite in
malleable iron (Fe2.9%C-1.5%Si0.53%Mn-0.06%P0.22%S-0.08%Ni0.1%Cu-0.09%Cr0.003%Bi)
The casting was
annealed
at 950 °C, held 10 h,
furnace cooled to
720 °C, held 16 h,
and air cooled.
Malleable cast iron-Contd.
 Has a tensile strength up to 700 MPa,
with an elongation of 10-15%
 They are more expensive than gray
irons, because of the heat treatment
involved.
 They are used in applications such as
automobile crankshafts, chain links and
brackets.
Ductile/nodular/spheroidal
graphite(SG) cast iron
• Small quantities of Mg (modifier)
added to the melt to produce this iron
•The basic composition of the melt is 3-4%C and
2.5%Si
•The fairly high Si equivalent produces
graphitisation during solidification.
•The modifier has the effect of making the growth
rate of graphite same in all directions, so that a
spherical shape results
Contrasting gray and nodular/ductile
cast iron
Separate graphite
flakes form
Mg added to molten iron –
helps spherodise graphite
Low levels of minor elements
such as S and P
X100
X500
X100
Gray
Ferritic
vs
Nodular
cast iron
Pearlitic
Gray – graphite as
flakes
Brittle
Nodular – graphite
as nodules
Ductile
Nodular iron (Contd.)
 Nodular iron is a major engineering material,
as it combines the advantages of steel with
the processing economies of iron
 Tensile strength ranges from 400 to 700
MPa, with elongation in the range 10-18%
 Agricultural components, industrial fan hubs,
coke oven doors, crankshafts and gears are
some of the applications
Applications of ductile cast irons
Great at dampening!
Relative ability of ferrous metals to dampen
vibrations. The energy absorbed per cycle, or
specific damping capacity of these can differ by
more than 10 times.