This document summarizes different types of cast irons. It discusses white cast iron which has carbon in a combined form and no free carbon. Through heat treatment, white cast iron can be converted to malleable cast iron which has spheroidal graphite and is ductile. Grey cast iron naturally forms graphite flakes during solidification, making it brittle but easily cast. Nodular cast iron is made ductile through the addition of nodulizing elements that form spheroidal graphite. The document provides examples of applications for each type of cast iron.
2. Introduction
Alloys of iron and carbon in which % C varies between 2-6.67%
Poor ductility and malleability; hence cannot be forged, rolled, drawn or
pressed into desired shape
Named “CAST IRONS”, because the components are formed by melting and
casting with or without machining to the required final shape and size
Properties:
Cheap
Lower melting temperatures (1150-1250ᵒ C) as compared to steels (1350-
1500ᵒC)
Excellent castability
Corrosion resistant
Brittle
Properties can be adjusted by suitable alloying elements and heat treatment
2Prof.Ghadage M.M.
3. Classification of cast irons
Furnace
Cupola CI
Air
furnace
CI
Electric
furnace
CI
Duplex CI
Composition and purity
Low
carbon, low
silicon CI
High
carbon, low
sulphur CI
Nickel
alloy CI
Microstructure and appearance of fracture
White CI Malleable
CI
Gray CI
Nodular
CI
Mottled
CI
Chilled CI Alloy CI
3Prof.Ghadage M.M.
4. White cast iron
Carbon present in combined form (cementite)
and there is no free carbon (graphite)
Composition: C: 2.3-3%, Si: 0.5-1.3%, S: 0.06-
0.1%, P: 0.1-0.2%, Mn: 0.5-1%
Named after its white fractured surface
No graphitisation and hence its solidification can
be represented on I-C diagram
Properties: Strong in compression (1750MPa),
hard (350-500 BHN), resistant to abrasive wear,
brittle, difficult to machine hence finishing to
final size is done by grinding only
Used to malleable CI
Applications: Pump liners, Road roller surfaces,
mill liners, grinding balls, dies and extrusion
nozzles
Microstructures of white CI:
the light cementite regions
are surrounded by pearlite,
which has the ferrite
cementite layered structure.
200x [Source: William
Callister, 2007]
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5. Malleable cast iron
Heating white cast iron around 900ᵒ C and holding
for long time (24hrs to several days), followed by
very slow cooling to room temperature produces
malleable cast iron
Contains 2.5% C and 1% Si
Cementite decomposes during the heat treatment to
more stable form (graphite)
The free carbon precipitates in the form of
spheroidal particles (temper carbon)
Properties:
Show ductility, toughness and are bendable
Good capacity to absorb shock and vibrations
NOT MALLEABLE; cannot be rolled, forged or
extruded
T. S. = 700MPa, % elongation = 10-15%, Hardness
= 80-275 BHN
More expensive than grey cast iron because of heat
treatment involved
Applications: Automobile crankshaft, chain links
and brackets, brake pedals, tractor springs, universal
joint yoke
Microstructure of
malleable CI: dark
graphite rosettes (temper
carbon) in an α-ferrite
matrix. 150x [Source:
William Callister, 2007]
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6. Malleable cast iron
Types of malleable cast iron:
Ferritic malleable
Pearlitic malleable
Pearlitic-ferritic malleable
Black heart malleable
White heart malleable
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7. Pearlitic-ferritic malleable cast iron
Produced due to intermediate cooling rate between
those to produce ferritic malleable and pearlitic
malleable cast irons
Cooling rate is slow enough to graphitise all the
proeutectoid cementite and a part of eutectoid
cementite
Since carbon itself is a graphitiser, the cementite
from pearlite adjacent to the existing rosettes of
temper carbon graphite decomposes rapidly without
graphitising cementite away from the rosettes
Thus, microstructure at room temperature shows
rosettes of temper carbon graphite surrounded by an
envelope of ferrite
Matrix is coarse pearlite or slightly spherodised due
to slow cooling
Properties: Intermediate to ferritic and pearlitic
cast iron
Applications: Machinery parts such as rolls,
pumps, nozzles, cams and rocker arms; axle and
differential housing, cam shaft and crankshaft
Microstructure of pearlitic
ferritic malleable cast iron
showing bull’s eye
structure. 100x [Source: V.
D. Kodgire, 2009]
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8. Grey cast iron
Show gray fracture and contain graphite flakes
Flakes are curved plates, interconnected in three
dimensions
Graphite formed during freezing
Graphite flakes are sharp at their tips and act like internal
cracks or stresses
Composition: 2.5-3.8% C, 1.1-2.8% Si, 0.4-1% Mn,
0.15%P and 0.1% S
Properties: Depend upon morphology and size of
graphite flakes
Brittle, weak in tension, strong in compression (as cracks
do not propagate under compressive load)
High fluidity and hence it can be cast into complex shapes
and thin sections easily
Low shrinkage during solidification
Good wear resistance because graphite acts as lubricant
Better damping capacity than steel
Low notch sensitivity due to the presence of large number
of internal sharp notches (edges of graphite flakes) which
make the influence of external notch ineffective
Easy to machine, as chip formation is promoted by
graphite flakes. Also flakes serve as lubricant for cutting
tool
Good bearing properties
Fairly good corrosion resistance
Microstructures of Gray iron:
the dark graphite flakes are
embedded in an –ferrite
matrix. 500x [Source: William
Callister, 2007]
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9. Grey cast iron
Low ductility and impact strength
T. S. = 150-400MPa, Hardness = 150-300BHN, % elongation = < 1%
Cheaper than steel (low temperatures involved in casting and low control on
impurities as compared to steel)
Defects:
Growth
Firecracks or heat checks
These defects can be reduced by adding Cr, Mo and Ni
Applications: Manhole covers, M/c tool structures like bed, frames; Cylinder
block and head of IC engine, Gas or water pipes for underground purpose,
flywheels etc, elevators etc
Engine cylinder blockManhole covers 9Prof.Ghadage M.M.
10. Nodular (Ductile or Spheroidal) cast iron
Contains graphite in the form of spheroids
Produced from grey cast iron by adding nodulising
elements like Mg, Ca, Ba, Li, Zr or Ce
Composition: 3.2-4.2% C, 1.1-3.5% Si, 0.3-0.8% Mn,
0.08% P, 0.2% S
Since nodulising elements have strong affinity for
sulphur and they scavenge sulphur from the molten bath
as an initial step in producing nodular graphite. These
elements are expensive and hence for effective
utilization of these elements, the original grey melt must
contain less amount of sulphur (< 0.03%). Sulphur
content is reduced by treating the melt with soda ash
Properties:
More tensile strength, ductility and toughness as
compared to grey cast iron
Excellent machinability, castability and wear resistance
Do not suffer from defects like growth and firecracks
T. S. = 400-800MPa, % elongation = 10-18, Hardness =
100-300 BHN
Defects:
Blow holes
Shrinkage
Applications: Agricultural implements, industrial fan
hub, Crankshafts, gears, punch dies, sheet metal dies,
steel mill rolls and milling equipment, valves, pistons etc
Microstructure of nodular
CI: the dark graphite nodules
are surrounded by an -ferrite
matrix. 200x [Source:
William Callister, 2007]
10Prof.Ghadage M.M.
12. Mottled cast iron
Shows free cementite and graphite flakes
in its microstructure
Composition: 93.5% iron, 1.75%
graphite, remaining impurities
For a given composition, faster cooling
rates gives white structure and slow
cooling rates results in grey structure.
Intermediate cooling rates produces
mottled cast iron
Mottled structures to be avoided because
of bad properties
Can be avoided by increasing or
decreasing carbon and silicon content
Increasing carbon and silicon content
yields grey cast iron
Decreasing carbon and silicon content
yields white cast iron
Microstructure of mottled cast
iron. 500x [Source: V. D.
Kodgore, 2009]
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13. Chilled cast iron
Shows white structure at surface and grey
structure in centre
Composition is adjusted in such a way that
rapid cooling gives white structure and usual
cooling gives grey structure
Composition: % C: 3.3-3.5, %Si: 2-2.5
Surface cooled rapidly by metal or graphite
chillers or chill plates
Depth of chill can be controlled by
controlling the carbon and silicon contents
and by other alloying additions which are
either carbide formers or graphitisers
Increase in % C, silicon and graphitizers
decreases chill depth and viceversa
Properties:
Hard and wear resistant
Good machinability
Good damping capacity
Low notch sensitivity
Applications: Railway freight car wheels,
crushing balls, road rollers, hammers, dies
etc
Chiller plates
13Prof.Ghadage M.M.