Iron, the silvery-whitish metal, is the most important of metals since it
forms the basis of the spectrum of steels and cast iron. Today in
industries steel and cast iron comprise well over 80% by weight of Cast
iron and steel. Pure iron* is not an easy material to produce. Pure iron
is quite soft, weak and expensive. If carbon is added in certain quantity
in it, it will change its mechanical properties. According to carbon
content we classified iron carbon alloys into two ways:
1. S t e e l ( L e s s t h a n 2 . 1 1 % )
2. C a s t i r o n ( 2 . 1 1 - 6 . 6 7 % )
Cast irons are basically iron-carbon alloys having carbon between
2.11% and 6.67%. The industrial cast irons have carbon normally in
the range of 2.11% to 4.0%, along with other elements like silicon,
manganese, sulphur and phosphorus in substantial amounts.
Why cast iron has its name?
Higher carbon content makes them more brittle. Cast irons are brittle,
and cannot be forged, rolled, drawn, etc. but can only be ‘cast’ into
desired shape and size by pouring the molten alloy of desired
composition into a mould of desired shape and allowing it to solidify.
Due to presence of high carbon content in it machinability is poor so
casting is the only and exclusively suitable process to shape these
alloys, known as Cast iron.
Cast irons is made by remelting pig iron( C-3.5%,Si-1.9%,S-0.06%,
P-1.00%,Mn- 0.70%) often along with substantial quantities of scrap
iron and scrap steel, and taking various steps to remove undesirable
contaminants such as phosphorus and sulphur. The melting unit may be
cupola, electric arc, and induction furnaces etc. The common cast irons
are brittle and have lower strength properties than steels.
*Pure Iron-Iron contains 99.98% alpha ferrite in it. Pure iron pillars were
manufactured and situated in Delhi around 1200 AD.
Cast iron are also classified according to metallurgical point of view
• Hypo Eutectic cast iron (2.11-4.3% carbon)
• Eutectic* Cast iron (4.3% carbon)
• Hyper Eutectic cast iron(4.3-6.67% carbon)
Eutectic Cast iron- In the eutectic cast iron, there is only one phase
(liquid) of Eutectic composition at just before 1147oC. And this liquid
phase will transformed into austenite and cementite phases at 1147o C
by eutectic reaction.
L(4.3%) Austenite (2.11% C) + Cementite (6.67% C)
Hypo Eutectic Cast Iron - In the hypo eutectic cast iron, there are two
phases (i.e. austenite, liquid of Eutectic composition) at just before
1147oC. And only liquid phase will transformed into austenite and
cementite phases at 1147oC by eutectic reaction. Austenite which is
present above Eutectic temperature line is known as proeutectic or
Hyper Eutectic Cast iron - In the hyper eutectic cast iron, there are
two phases (i.e. cementite, liquid of Eutectic composition) at just
before 11470C. And only liquid phase will transformed into austenite
and cementite phases at 11470C by eutectic reaction. Cementite which
is present initially is known as proeutectic or primary cementite.
Iron bridge, made of cast iron Cover of sewerage system
*Eutectic comes from Greek word Eutectus which means “That can be easily melted”
Development of Cast Iron
Initially, there are two types of Cast iron called White cast iron and
Grey cast iron. If carbon is in form of cementite then white cast iron
forms and if carbon is in form of graphite then graphite cast forms.
White cast irons have all the carbon in the combined cementite form
(ferrite is assumed to possess negligible carbon). Cementite is a hard,
brittle, white compound. The fractured surface of white cast iron looks
silvery-white due to white cementite, and that is why the name white
cast iron is given.
Graphite is soft, brittle and gray, and thus, imparts gray colour to the
fracture. Cast irons containing graphite (as flakes) are thus, called
gray cast irons. Under microscopic graphite flakes appear as irregular
strands such as ‘corn flakes’. As shown in Figureure.1
Gray cast iron: 1 (a) Space model of flake graphite 1(b) Unetched photo-micrograph of gray cast iron
From two original cast irons, white cast iron is very brittle and
unmachinable as it is very hard due to presence of hard and brittle
cementite and thus finds very few applications. It is the gray cast iron,
the common commercial variety most extensively used in industry; due
to its certain specific properties. The compressive strength and
hardness of gray cast iron are quite high and very close to the
properties of the steel of similar composition and matrix structure.
While developing graphitic cast irons of superior properties resulted in
four more types of cast irons called meahanite iron, compacted iron,
malleable iron and S.G. iron. The microstructure of gray irons consists
of graphite flakes embedded in the steel matrix, i.e., in varying
proportions of ferrite and pearlite. The properties of Gray iron are
determined by the properties both of the matrix, and the amount, size,
shape and distribution of graphite inclusions. Graphite flakes have
weakening and embrittling effects, as graphite is soft, powdery, and
brittle, and can be considered in approximation as voids or cracks,
breaking the continuity of ductile matrix.
The properties of gray iron are determined by the properties both of the
matrix, and the amount, size, shape and distribution of graphite
inclusions. According to there graphite flakes condition, gray cast iron
is further divided into Meahanite cast iron (by making flakes finer),
S.G Cast iron (round shaped flakes), Malleable Cast iron, Mottled Cast
iron, Chilled Cast Iron etc.
Types of Cast Iron
The best method of classifying cast iron is based on type of
Two main types of Cast iron
I. White Cast Iron: Carbon is in form of White cementite
II. Grey Cast Iron: Carbon is in form of Graphite flakes
These all other cast iron except gray and white cast iron are made by
special treatment (heat treatment and by mixing chemical composition)
to enhance its properties.
III. Chilled Cast iron: Surface layers are of white cast iron with
interior of grey cast iron.
IV. Mottled iron(Mixed Iron): The transition layer between Grey
cast iron and white cast iron in chilled iron is mottled cast iron
and consists normally of graphite flakes
V. Meahanite Iron: Cast iron has very fine flakes of graphite due
to addition of calcium silicide as inoculant in melt in ladle
otherwise it would has solidified as white cast iron.
VI. Malleable Iron: These consists of structure of irregularly round
graphite particles called temper carbon and structure is obtained
by heat treatment
VII. Spheroidal graphite Iron structure of nodules embedded in steel
matrix, nodules are of more regular, shape and compact spheres.
VIII. Compacted/Vermicular Cast Iron: The graphite here is
intermediate between flakes and spheres numerous rods of
graphite. Strength and ductility is greater than gray cast iron
IX. Alloy Cast Iron: Properties and microstructure of cast iron or
any orf these is modified by addition of alloying elements.
We discussed in earlier section that cast iron classified (white and
gray) on the basis of carbon present in it. In this section we know that
how cementite* and graphite** are formed and at what condition in iron
carbon diagram. The process of direct precipitation of graphite from
liquid or by decomposition of previously formed cementite-process
• In iron carbon diagram graphite (Equilibrium state) is more
stable phase than cementite (Metastable state) but kinetically
it is easier to form cementite* than graphite** (because 6.67%
C should segregate to nucleate cementite whereas 100%
segregation of carbon is needed to nucleate graphite.
• The crystal structure of austenite (FCC) is relatively to that
cementite (complex orthorhombic***), but differ substantially
from graphite (Hexagonal layer structure).
• Cementite forms more easily from austenite or from liquid
because energy required for diffusion is much less than that
• As kinetically cementite can form more easily it is more
probable to get in microstructure Ferrite + Cementite from
austenite then Ferrite + Graphite also the liquid to form
eutectically to austenite + Cementite and not austenite than
ferrite + graphite. If kinetic factor are favourable then
graphite can form because graphite has less free energy than
Cementite. When graphite form directly from liquid is called
primary graphitisation. The formation of graphite from liquid
is takes place in a narrow range of temperature (1153-1147°C)
and also formation of graphite from austenite between 738°C
to 727°C which require slow cooling. This graphite is known
as secondary graphite.
The line Q’C’R’ (1153°C) in Figure. 1.2 is for Eutectic reaction
L Austenite + graphite (Primary graphitisation)
The line is for the Eutectoid reaction
Austenite Ferrite + graphite (Primary graphitisation)
* Cementite-It is an interstitial compound of fixed carbon percentage of 6.67% carbon
** Graphite and Diamond are purest form of carbon present in nature
*** For Fe3C- Complex orthorhombic structure with 12 Fe atoms and 4 carbon atoms per unit cell at melting
point 1227°C. Crystal structure= Radius of solute atom/radius of solvent atom=0.63
Commercial cast irons contains fine particle of inclusion (like Si)
which becomes centre of graphite crystallisation and promote graphite
formation. When graphite forms from dissociation of cementite is
called secondary graphitization. Metastable cementite above
temperature 738°C decompose to Austenite+ graphite or Ferrite +
graphite below 738°C. As we know that slow cooling of liquid cast
iron leads to formation of graphite and fast cooling leads to cementite.
This is so because the formation of graphite from liquid or austenite is
very slow cooling process and takes place only at small under cooling.
FACTORS EFFECTING FORMATION OF CAST IRONS
The main factors effecting the formation of white or gray iron, i.e.,
whether carbon is present in the combined form or in the graphite form
I. Chemical composition
II. Cooling rate.
(a) Carbon: Higher is the carbon, more is graphite formed and lower
the mechanical properties. Carbons lower the melting point of metal
and act a graphitiser to favour the formation of gray cast iron.
(b) Silicon: Silicon is a strong graphitiser and increases the fluidity. It
controls the relative proportions of combined carbon and free graphite.
If silicon is present during the solidification carbon precipitates as
graphite flakes. Silicon content may vary between 1.0% to 3.5%.Silicon
shifts the graphite-eutectic line upwards. Thus during cooling from
liquid state, a larger degree of under cooling is possible with greater
chance to form graphite before cementite formation becomes possible.
(c) Sulphur and Manganese: Sulphur retards graphitisation and
increases the size of the flakes, High sulphur tends to reduce fluidity
and is often the cause of blowholes in castings. Sulphur is kept low in
amount of .06 to .12%.
Sulphur in cast iron is present either as FeS or MnS. FeS tends to
promote cementite formation, i.e., white cast iron. Mn is a mild carbide
forming element. The amount of Mn (one part of S to 1.72 part of Mn)
which combines with sulphur to form MnS particles in liquid iron and
rises to be top of melt to be removed, has not been able to have its own
effect of cementite formation, nor the lost sulphur could exert its effect
of cementite formation thus, indirectly helps to give gray iron.
Manganese in excess of what has formed MnS, weakly retards primarily
graphitisation. However, it has strong cementite stabilising effect on
(d) Phosphorus- Most cast iron contain phosphorus between .1 to .
3%.Its amount may be more than .9%, then it forms iron phosphide
(Fe3P), which form a ternary eutectic with cementite and austenite. The
ternary Eutectic is called steadite. Steadite is brittle and has a melting
point of around 960 degree. This increase the fluidity also helps in
giving good castability to the thin and intricate casting, where low
melting fluid could easily flow. However for thick and high strength
cast iron casting, brittle steadite can be avoided by maintaining
phosphorus less than 0.3%, which shall be present in dissolve state in
(d) Carbon equivalent Value: Si, P has similar effect on the
microstructure, their effect in term of carbon is important.
The carbon equivalent value (CE) = Total C% + 1/3(Si %+P %)
The carbon content of cast iron may be lower (than 4.3%), but if C.E is
4.3%, then, the cast iron is eutectic cast iron. Carbon equivalent value
for a given cooling rate, determines how close is to given composition
of cast iron to the eutectic and thus how much free graphite, it is likely
to form. This determines probable strength of a section of casting.
I I . T h e E f f e c t o f R a t e o f C o o l i n g on t h e S t r u c t u r e of C a s t
• A high rate of cooling during solidification tends to favour the
formation of cementite rather than graphite. That is, the higher
the rate of cooling for any given cast-iron composition the
'whiter' and more brittle the casting is likely to be. This effect is
important in connection with the choice of a suitable iron for the
production of castings of thin section. Supposing an iron which,
when cooled slowly, had a fine grey structure containing small
eutectic cells were chosen for such a purpose. In thin sections it
would cool so rapidly that cementite would form in preference to
graphite and a thin section of completely white iron would
result. Such a section would be brittle and useless.
• This effect is illustrated by casting a 'stepped bar' of iron of a
suitable composition. Here, the thin sections have cooled so quickly
that solidification of cementite has occurred, as indicated by the
white fracture and high Brinell values. The thicker sections, having
cooled more slowly, are graphitic and consequently softer. Due to the
chilling effect exerted by the mould, most castings have a hard white
skin on the surface. This is often noticeable when taking the first cut
in a machining operation.
Figure 3 Illustrating the effects of thickness of section, and hence rate of cooling on the
structure of a grey iron. The thinnest part of the section has cooled quickly enough to
produce a white iron structure, whilst the core of the thickest part has a grey iron
structure. The relationships between sectional thickness and microstructure are similar to
those indicated in Figure.. on the opposite page. Both micrographs x 300 and etched in
2% nital. Macro section x 3.
cooled so quickly that solidification of cementite has occurred, as
indicated by the white fracture and high Brinell values. The thicker
sections, having cooled more slowly, are graphitic and consequently
softer. Due to the chilling effect exerted by the mould, most castings
have a hard white skin on the surface. This is often noticeable when
taking the first cut in a machining operation. In casting thin sections,
then, it is necessary to choose an iron of rather coarser grey fracture
than is required in the finished casting. That is, the iron must have a
higher silicon content than that used for the production of castings of
Figure 4 The effect of thickness of cross-section on the rate of cooling, and hence upon the microstructure of a grey cast iron.
Now we discussed types of cast iron in detailed:
I. WHITE CAST IRONS
These are iron-carbon alloys having more than 2.11% carbon and all the
carbon is present in the combined cementite form, which makes the
fracture of these alloys to have dull and white colour, and that is the
reason of their name as white irons. Typical white cast iron contains
2.5 – 3.5% C, 0.4 – 1.5% Si, 0.4 – 0.6 % Mn, 0.1 – 0.4%P, 0.15%S,
and balance Fe. Figure. 3 illustrates changes occurring on cooling in
hypoeutectic white cast iron. At room temperature white cast iron is
mixture of pearlite and cementite.
Figure. 5 The metastable iron—iron carbide phase diagram.
All white cast irons are hypoeutectic alloys. The cooling of a 2.50
percent carbon alloy will now be described. The alloy, at x2 in Figure.
5, exists as a uniform liquid solution of carbon dissolved in liquid iron.
It remains in this condition as cooling takes place until the liquidus
line is crossed at x2. Solidification now begins by the formation of
austenite crystals containing about 1 percent carbon. As the temperature
falls, primary austenite continues to solidify, its composition moving
down and to the right along the solidus line toward point C. The liquid
in the meantime is becoming richer in carbon, its composition also
moving down and to the right along the liquidus line toward point E. At
the eutectic temperature, 1147°C the alloy consists of austenite
dendrites containing 2 percent carbon and a liquid solution, containing
4.3 percent carbon. The liquid accounts for (2.5—2.0)/ (4.3—2.0) or 22
percent of the alloy by weight. This liquid now undergoes the eutectic
reaction isothermally to form the eutectic mixture of austenite and
cementite known as ledeburite.
Liquid (4.3%) Austenite (2.11%) + Cementite (6.67%)
Since the reaction takes place at a relatively high temperature,
1edeburite tends to appear as a coarse mixture rather than the fine
mixture typical of many eutectics. It is not unusual for ledeburite to be
separated completely, with the eutectic austenite added to the primacy
austenite dendrites, leaving behind layers of massive, free cementite.
As the temperature falls, between x3 and x4, the solubility of carbon in
austenite decreases, as indicated by the Acm line CJ. This causes
precipitation of proeutectoid cementite, most of which is deposited
upon the cementite already present. At the eutectoid temperature,
727°C, the remaining austenite containing 0.8 percent carbon and
constituting (6.67—2.5)/ (6.67—0.8), or 70 percent of the alloy,
undergoes the eutectoid reaction isothermally to form pearlite. During
subsequent cooling to room temperature, the structure remains
The typical microstructure of white cast iron, consisting of dendrites of
transformed austenite (Pearlite) in a white interdendritic network of
cementite as shown in Figure. 9.
Figure 6 Changes during cooling of hypoeutectic white cast iron
Figure 7 Microstructure of white cast iron
Figure 8. Changes on cooling, of white cast irons (schematic). (a) Dendrites of
austenite font: which get broken by secondary cementite. Austenite changes to
Pearlite at eutectoid temperature. (b) Complete ledeburite forms by eutectic
reaction. Coarse ledeburite forms as temperature is high. Secondary cementite
forms reducing size of austenite particles which at eutectoid temperature
changes to Pearlite to result in complete transformed ledeburite. (c)
Cementite being a compound, feasts as plate, as primary cementite. Amount of
tertiary cementite in all these cases is negligibly small, thus. Microstructure
is same after eutectoid reaction and at room temperature.
Figure.9 Microstructure of white cast irons, (a) Microstructure of
hypoeutectic white cast iron. Carbon is close to 2.11%, as it has major amount
of broken dendrites of Pearlite and less transformed ledeburite,
(b)Microstructure of hypereutectic white cast icon having more carbon than
(a) as the amount of broken dendrites is lest,(c) eutectic cast iron having only
transformed ledeburite,(4)Hypereutectic white iron. Presence of plates of
primary cementite indicates this.
Hard and wear resistant
The hardness and brittleness increases as the carbon content increases.
Hardness Brinell 375 to 600.
Tensile strength 20000 to 70000 psi.
Compressive strength 200000 to 250000.
Because of extreme brittleness and lack of machinability, white irons
find limited engineering applications.
The parts where resistance to wear is the most important requirement
such as liners of cement mixers, ball mills, pumps, wearing plates.
Parts of sand-slingers, certain type of drawing dies, extrusion nozzles,
grinding balls. Most parts are sand-cast and don’t require much
machining, which can be done by grinding. A large tonnage of white
cast irons is used as a starting material for the production of malleable
cast iron parts.
• Brake shoes
• Shot blasting nozzles
• Mill liners
• Pump impellers and other abrasion resistant parts.
II. GRAY CAST IRON
Iron-carbon alloys containing flakes of graphite embedded in
steel matrix, which show a gray-blackish coloured fracture due to
graphite’—the free foam of carbon, are called gray cast irons.
The strength of gray iron depends on the strength of steel matrix
and the size and character of graphite flakes in it. A typical
feature of gray iron is that graphite is in the form of flakes in
microstructure, Figure 10. This microstructure represents their
appearance on a plane surface, but flakes are three dimensional
plates, sometimes connected.
Figure. – 10 Microstructure of gray cast iron
COMPOSITION OF GRAY IRONS
The gray cast irons are hypoeutectic cast irons, the total carbon content
lies between 2.4% to 3.8%. The amount of carbon does not exceed
3.8%, as more the carbon, more the eutectic liquid, which yields more
graphite as flakes, resulting in poor mechanical properties. Carbon is
kept at least 2.4%. So that cast iron has good fluidity and castability.
Silicon is kept between1.2% to 3.5%. It being a graphitiser controls
along with carbon and the rate of cooling, the nature of steel matrix. In
such iron, graphitisation of all the cementite except the eutectoid
cementite takes place. The generalised range of composition of gray
Total carbon : 2.4—3.8%
Silicon : 1.2—3.5%
Manganese : 0.5—1.0%
Sulphur : 0.06—0.12%
Phosphorus : 0.1—0.9%
In manufacturing of gray cast irons, the tendency of cementite to
separate into graphite and austenite or ferrite is favoured by
controlling alloy composition and cooling rate. These alloys solidify by
first forming primary austenite. The graphitization process is added by
high carbon content, high temperature and the proper amount of
graphitizing elements mostly silicon.
Figure.11 Iron-graphite equilibrium diagram
With proper control of above factors alloy will follow the stable iron-
graphite equilibrium diagram (Figure.11) forming austenite and
graphite at the eutectic temperature of 1154°C at any rate any
cementite which is formed will graphitize rapidly
During continuous cooling, there is additional precipitation of carbon
because of the decrease in solubility of carbon in austenite .this carbon
is precipitate as graphite
Strength of gray cast iron depends almost entirely on the matrix in
which the graphite is embedded. If the composition and cooling rate are
such that the eutectoid cementite also graphitizes, then the matrix will
be entirely Ferritic. If graphitization of the eutectoid cementite is
prevented, the Matrix will be entirely pearlitic. The graphite-ferrite
mixture is the softest and weakest gray iron, the strength and hardness
increase with the in increase in carbide, reaching a maximum with the
pearlitic gray iron.
Pearlitic matrix is obtained by proper control of alloy composition rate
of cooling or heat treatment. Properties of gray iron depend on the
nature of matrix, the size, character and amount of graphite flakes. The
classification of cast irons is based on the minimum tensile strength
possessed by a cast iron. i.e., is based on property and not the
Figure 12 Microstructure of gray irons. (a) Pearlitic gray iron, (b) Ferreto pearlitic gray iron x 250. (c)
Gray phosphoric cast iron (CE, = 4.2%) (C = 3.4%, Si = 2.4%, Mn = 0.45%, S = 0.02%, P = 1.0%.
showing ternary phosphide eutectic. Steadite, (d) Characteristic Herring bone structure of pseudo-binary
eutectic (of dark Fe3P and ferrite)
Pearlitic gray iron having high phosphorus (0.3-0.5%) used for piston
rings. High wear resistance is obtained in rings due to tine Pearlite and
uniformly distributed phosphide eutectic with few flakes of graphite.
Bearings mating with hardened (or normalised) steel shaft are of gray
iron with around 85% Pearlite. (3.2-3.6 C, 1.6-2.4% Si. 0.6-0.9% Mn).
If shaft has not been heat treated, then the composition of the bearing:
(3.2-3.8% C, 1.7-2.6% Si 0.4-0.7% Mn, 0.1% Ti, 0.3-0.5% Cu).
FORMATION OF FLAKES
Normally commercial gray iron is either hypoeutectic or eutectic in
nature. Neglecting the presence dendrites of primary austenite in
hypoeutectic iron, which imposes constraints later on in the radial
growth of the eutectic cell, Figure.13 illustrates the successive stages
in the formation of graphite flakes from the eutectic liquid present.
Once graphite has nucleated (it occurs within the interdendritic liquid
and not on austenite dendrite arms), solidification takes place at nuclei
Figure 13a, from each of which is formed a roughly spherical lump
called the eutectic cell. It grows in an approximately radial manner,
where there it simultaneous growth of austenite and graphite, the latter
being in continuous contact with the liquid. The flakes bend, twist and
branch as depicted in Figure13d.There is a continuous branched
skeleton of graphite in each eutectic cell like a cabbage. When the rate
of cooling is increased, there is more
Figure.13 (a), (b), (C): Stages in the formation of graphite flakes, (d) Growth of flake graphite eutectic
Under cooling, then the skeleton is branched more frequently with the
rapid radial growth of the cell and thus, finer graphite flakes are
observed. The diameter of the eutectic cell decreases as the number of
cells per unit volume increase, and this results in higher tensile
strength, though the soundness of the casting is affected adversely. The
number of nuclei can be increased by inoculants as well as by sulphur
(sulphur promotes constitutional supercooling, increasing the frequency
of branching i.e., cell density as well as produces coarser flakes).
Superheating or holding time of molten metal reduces the number of
HEAT TREATMENT OF GRAY IRON
The stress-relieving is probably the most frequently applied heat
treatment to gray irons. In the as-cast state, castings have residual
stresses developed due to differential cooling and differential
contraction, especially in non-uniform cross-sectioned castings. These
stresses are completely removed by soaking at 650°C, but grain growth
is serious at and above 600°C. Annealing of gray iron is done to
graphitise carbide, and to homogenise the castings. It softens, increases
ductility and machinability of gray iron. Castings are soaked for up to
10 hour at 850-950°C. Normalising may be done to increase the
strength and hardness of cast iron by heating at 900-930°C for a
soaking time of 2.5 m/min of maximum thickness of casting and then
air cooling. Hardening can be done by heating to and soaking at
800-850°C, and then quenching in water, oil, hot salt bath, though for
through- hardening, oil is commonly used as water quenching may
cause distortion and cracking. Tempering is done at 150 to 650°C.
Table 1 illustrates hardness of cast irons based on the microstructure.
Table 2 illustrates composition of some gray irons with some
Table 1 Hardness of Gray Iron based on Matrix Microstructure
Nature of Ferritic Soft Pearlitic Low Austenitic Martensitic Tempered
matrix Low Iron Alloy Martensite
Hardness 110-140 140-160 160-220 200-250 140-160 350-450 260-350
Table 2 Composition of Gray Irons with Applications
Applications C Si Mn P S Ni Cr Tensile
Break Drum 3.30 1.9 0.65 0.15 .08 1.25 0.5 150
Piston Ring 3.50 2.9 0.65 0.50 .06
Cylinder and 3.25 2.25 0.65 0.15 .10 To
Heavy Castings 3.25 1.25 0.50 0.35 .10 350
Clutch Casting 3.20 2.10 0.80 0.17 .05 0.32 275
Properties of Grey Cast iron:
1. Low cost of production: I n f a c t , g r a y i r o n , b e i n g t h e l e a s t e x p e n s i v e
casting material, is always considered first when a cast metal is being
chosen for a product, unless mechanical and physical properties of gray
iron are inadequate.
2. Low melting point: ( 1 1 5 0 ° — 1 2 5 0 ° C ) o f c a s t i r o n s , s e v e r a l hundred
degrees less than steel, requires simple furnaces like pit furnace,
crucible furnace, cupola, etc. which are simple, inexpensive to fun and
maintain. The control of impurities is not critical here as in steel
3. Good Castability: C a s t i r o n s h a v e e x c e l l e n t f l u i d i t y a n d t a k e g o o d
mould-impressions easily. Cast irons; as compared to steels solidity
mainly at the constant eutectic temperature—a criterion used for
choosing alloy compositions having best castability. Graphite having
low density is voluminous. Its large volume compensates for the
shrinkage. Gray iron, thus, does not need shrinkage allowance at all to
take almost exact casting impressions.
4. Good machinability of gray cast iron i s d u e t o e a s y a n d d i s c o n t i n u o u s c h i p
formation due to brittle graphite flakes. Graphite serves as a solid
lubricant decreasing coefficient of friction. It smears the cutting tool
allowing free sliding of chips increasing thus, tool life too. (White cast
irons, due to high hardness, are unmachinable).
5. Good wear resistance of gray i r o n i s d u e t o g r a p h i t e a c t i n g a s s o l i d
lubricant layer, avoiding thereby metal to metal direct contact. On
other hand, white cast irons are wear resistant due to’ their high
6. High damping capacity i s d u e t o t h e g r a p h i t e f l a k e s , w h i c h b r e a k s t h e
continuity of the metallic matrix, and thus, vibrations are not allowed
to transfer from one side of flake to other, i.e., graphitic cracks quickly
dampen the vibrations and resonance oscillations. Gray iron suits thus
the machine beds as compared to steels.
7. High compressive strength of g r a y i r o n - a l m o s t 3 t o 5 t i m e s o f i t s t e n s i l e
strength (110-350 N/mm2), and almost equal to that of steels makes it
suitable for applications, where components are subjected to
compression such as machine beds, etc.
8. High thermal conductivity, a n d h a v e a b i l i t y t o w i t h s t a n d t h e r m a l s h o c k s .
9. Good resistance to atmospheric corrosion d u e t o h i g h s i l i c o n a n d p e r h a p s
other factors, than mild steels.
10. Notch-insensitive: L a r g e n u m b e r o f f l a k e s i n g r a y i r o n a c t s a s n o t c h e s
in spite of these notches, if gray iron has the required strength, then
additional notch or notches shall have minor, or no effect, i.e., gray
iron is notch-insensitive; whereas in steels. A notch has quite a
damaging effect as it acts as stress-raiser to make the steel even brittle.
Table 3 Properties of Grey Cast Iron
Some other properties of Grey cast iron ASTM
Chemical composition: C=2.7-4%, Mn=0.8%, Si=1.8-3%, S=0.07% max, P=0.2% max
Property Value in metric unit Value unit
Density 7.06 *10³-7.34 *10³ kg/m³ 441-458 lb/ft³
Modulus of elasticity 124 GPa 18000 ksi
Thermal expansion (20 ºC) 9.0*10 ºCˉ¹ 5.0*10 in/(in* ºF)
Specific heat capacity 840 J/(kg*K) 0.2 BTU/(lb*ºF)
Thermal conductivity 53.3 W/(m*K) 370 BTU*in/(hr*ft²*ºF)
Electric resistivity 1.1*10 Ohm*m 1.1*10 Ohm*cm
Tensile strength 276 MPa 40000 psi
Elongation 1 % 1 %
Shear strength 400 MPa 58000 psi
Compressive yield strength Min. 827 MPa Min. 120000 psi
Fatigue strength 138 MPa 20000 psi
Hardness (Brinell) 180-302 HB 180-302 HB
Wear resistance Low
Corrosion resistance Low
Apart from low ductility and toughness, gray irons are section
sensitive, i.e., depending on the section thickness of the casting, the
microstructure and thus, the properties vary. Thick sections have low
strength (due to ferritic matrix) and care has to be taken, when
designing the castings.
Gray cast irons have extensive applications. The high damping capacity
and high compressive strength make them suitable for the beds and
bases of powerful machines and frames. Good wear resistance, good
machinability and damping capacity make them suitable for
applications like locomotive and internal combustion engine cylinder
blocks and heads, pistons rings, cylinders. The ease of casting and low
cost makes them suitable for counter-weights for elevators, industrial
• Fly wheels
• Guards and frames around hazardous machinery
• Gear housings
• Pump housings
• Steam turbine housings
• Motor frames
• Sewer covers
• Enclosures for electrical equipments.
III. CHILLED CAST IRON
Chilled-iron castings are made by casting the molten metal against
chillers which result in a surface of white cast iron. Chilled iron has
surface layers of white iron, while the structure of the core is that of
gray iron. Normally, chilled iron castings are obtained by casting the
molten alloy in metal mould. Chilling to certain depth (12 to 30 mm) is
because of the fast cooling (chilling) obtained due to high thermal
conductivity of metal mould, The composition of molten alloy is so
chosen that normal cooling results in gray iron in the whole section,
but fast cooling of the whole surface, or a part of the surface yield
white iron there. The fast cooling obtained by employing metal or
graphite plates—called chills in the sand mould. A chilled cast iron of
following composition can get chilled easily:
C = 2.8—3.6%; Si =0.5 to 0.8%; Mn = 0.4 — 0.6%
Where the deeper chill is needed can be increased by increasing the
thickness of the chill plates. It is possible to choose the composition of
the cast iron so that the normal cooling rate at the surface is just fast
enough to yield white iron there, and the slower cooling rate below this
surface produces mottled or mottled and gray iron. Presence of
graphitiser decreases the chill depth and the carbide forming elements
increase the chill depth.
Element Chill depth Hardness of chill Other Facts
C Decrease Increase
Si Decrease Slightly Increase
Mn as Decrease Increase
MnS Increase Slightly Increase
Dissolved Decrease Increase Added unto 0.1 % P decrease chill depth
Mn 5% by 2.5% for constant C
P Decrease and Si
Ni Increase Refines Carbides Chilled
Increase structure and core. Helps to
Cr pearlitic structure in thick
Mo Decrease unto 4% then 1-4% Cr as chromium
increase Increase carbide increases hardness
Cu and wear resistance 12-35%
for corrosion and oxidation
resistance at high
Chilled layer has more
resistance to spalling, heat
It decrease mottled layer
Table 4 Effect of Elements on Chill Depth, etc. of Chilled Iron
General, chill depth is increased by increasing carbide forming
elements and decreasing carbon and silicon. Cast iron melt is allowed
so solidify in mould of shape of wedge. Figure. (14b). The Cooling rate
is faster at mold walls, which prevents graphitisation to yield white
cast iron. The cooling rate decreases as the centre of the casting is
approached, allowing graphitisation to take place to yield gray iron.
Figure. (14a) illustrates changes in hardness of the step-bar test piece
which is due to changes in microstructures. The depth of chill
decreases and the hardness of the chilled zone increases with increasing
carbon content. The depth of chill is decreased with increasing silicon
content. Phosphorus decreases the depth of chill. With carbon and
silicon constant, an increase of 0.1 % Phosphorus will decrease the
depth of chill about 0.1 in. Nickel reduces the chill depth and refines
the carbide structure. Chromium is used in small amount to control
chill depth. Manganese decreases the depth of chill until the formation
of Manganese sulphide after that increases chill depth and hardness.
Molybdenum improves the resistance of the chilled face to spalling,
pitting, chipping and heat checking.
Figure. 14(a) Effect of elements on chill depth. (b) Step bar test piece for chill depth cast iron having
Properties: C a s t i n g s h a v e s o m e g o o d p r o p e r t i e s d u e t o w h i t e i r o n
surface which are high wear and abrasion resistance, and some good
properties due gray iron core which are damping capacity, low notch
• Chilled cast irons used as
• Rail-freight car wheel
• Cane-crushing rolls
• Road rollers
• Grinding balls
• Stamp shoes and dies
• Ploughshares many other heavy-duty machinery parts
In a chilled cast iron casting, surface layers are of white iron and the
core is of gray iron, but in the
Figure. 15 Microstructure of mottled cast iron
transition region, the structure consists both of gray and white iron,
i.e., has graphite flakes, Pearlite and secondary free cementite, i.e.,
mixed iron or called mottled iron, The intermediate cooling rate for
certain carbon and silicon contents could not graphitise the free
secondary cementite, Due to incorrect foundry control for certain
compositions, The non uniform flakes increase brittleness of the
castings, apart from the extra brittleness due to the presence of
secondary cementite. Mottled cast irons, thus, don’t find applications.
If carbon and silicon content of the cast iron is increased, then the
casting shall solidify as gray iron. The thickness of the mottled zone in
chilled iron can be reduced by increasing both the graphitiser and the
carbide forming elements in the cast iron. Figure. 15 illustrates
microstructure of mottled cast iron.
V. MEAHANITE CAST IRON
The molten cast iron is treated with calcium silicides as inoculants to
produce a fine graphitic structure. The flakes are uniformly distributed
to give high mechanical properties (Tensile strength = 25 — 40
Kg/mm2). The composition is so chosen that white fracture is obtained
in the absence of any treatment, i.e., the cast iron is low in silicon
content, moderately low in carbon content about 2.5-3%. Calcium
silicides act as graphitiser, so that resulting casting is gray and
merchantable. Meahanite cast iron finds applications as a gray iron
with high mechanical strength, such as for heavy machine beds and
VI.MALLEABLE CAST IRON
Cementite (iron carbide) is actually a metastable phase. There is a
tendency for cementite to decompose into iron and carbon, but under
normal conditions it tends to persist indefinitely in its original form.
Up to this point, cementite has been treated as a stable phase; however,
this tendency to form free carbon is the basis for the manufacture of
malleable cast iron. The reaction Fe3C3Fe + C is favoured by
elevated temperatures, the existence of solid non metallic impurities,
higher carbon contents, and the presence of elements that aid the
decomposition of Fe3C On the iron—iron carbide equilibrium diagram
for the metastable system, shown in Figure. 16, are superimposed the
phase boundaries of the stable iron-carbon (graphite) system as dotted
lines. The purpose of malleabilization is to convert all the combined
carbon in white iron into irregular nodules of tamper carbon (graphite)
and ferrite. Commercially, this process is carried out in two steps
known as the first and second stages of the anneal.
White irons suitable for conversion to malleable iron are of the
following range of composition:
Phosphorus Less than 0.18
Table 5 Composition of Malleable Iron
In the first-stage annealing, the white-iron casting is slowly reheated to
a temperature between 1660 and 1750°F. During heating, the pearlite is
converted to austenite at the lower critical line. The austenite thus
formed dissolves some additional cementite as heated to the annealing
Figure. 16 The stable iron-Graphite system (dotted lines) superimposed on the
metastable iron—iron carbide system.
Figure 16 show that the austenite of the metastable system can dissolve
more carbon than can austenite of the stable system. Therefore, a
driving force exists for the carbon to precipitate out of the austenite as
free graphite. This graphitization starts at the malleabilising
temperature. The initial precipitation of a graphite nucleus depletes the
austenite of carbon, and so more is dissolved from the adjacent
cementite, leading to further carbon deposition on the original graphite
nucleus. The graphite nuclei grow at approximately equal rates in all
directions and ultimately appear as irregular nodules or spheroids
usually called temper carbon (Figure. 17). Temper carbon graphite is
formed at the interface between primary carbide and saturated austenite
at the first-stage annealing temperature, with growth around the nuclei
by a reaction involving diffusion and carbide decomposition.
Nucleation and graphitization are accelerated by the presence of sub
microscopic particles that can be introduced into the iron by the proper
melting practice. High silicon and carbon contents promote nucleation
and graphitization, but these elements must be restricted to certain
maximum levels since the iron must solidify as white iron. Therefore,
graphitizing nuclei are best provided by proper annealing practice
Figure. 17 Malleable iron, unetched. Irregular nodules of graphite called
temper carbon, box. (b) Ferritic malleable iron, temper carbon black) in a
The rate of annealing depends on chemical composition, nucleation
tendency, and temperature of annealing. The temperature of first-stage
annealing exerts considerable influence on the number of temper-
carbon particles produced. Increasing annealing temperature accelerates
the rate decomposition of primary carbide and produces more graphite
particles per unit area. However, high first-stage annealing
temperatures result in excessive distortion of castings during annealing
and the need for-straightening operations after heat treatment
Annealing temperatures are adjusted to provide maximum practical
annealing rates and minimum distortion and are therefore controlled
between 1650 and 1750°F. The white-iron casting is held at the first-
stage annealing temperature until all massive carbides have been
decomposed. Since graphitization is a relatively slow process, the
casting must be soaked at temperature for at least 20 h, and large loads
may require as much as 72 h. The structure at completion of first-stage
graphitization consists of temper-carbon nodules distributed throughout
the matrix of saturated austenite.
After first-stage annealing, the castings are cooled as rapidly as
practical to about 1400°F in preparation for the second stage of the
annealing treatment. The fast cooling cycle usually requires 2 to 6 h,
depending on the equipment used.
In the second-stage annealing, the castings are cooled slowly at a rate
of 5 to 15°F/h through the critical range at which the eutectoid reaction
would take place. During the slow cooling, the carbon dissolved in the
austenite is converted to graphite on the existing temper-carbon
particles, and the remaining austenite transforms into ferrite. Once
graphitization is complete, no further structural changes take place
during cooling to room temperature, and the structure consists of
temper-carbon nodules in a ferrite matrix (Figure. 17). This type is
known as standard or Ferritic malleable iron. The changes in
microstructure during the malleabilising cycle are shown schematically
in Figure. 18.
Figure. 18 The changes in micro structure as a function of the malleabilising
cycle resulting in temper carbon in a ferrite matrix.
TYPES OF MALLEABLE CAST IRONS
1. Ferrite malleable iron: T h e s t r u c t u r e c o n s i s t s o f n o d u l e s o f
temper carbon embedded in ferrite matrix (due to slow cooling
in eutectoid temperature range). As these nodules break the
continuity to lesser damaging extent of tough ferrite. The
castings are cooled at slowly at the rate of 5 to 15 °F/hr.
Through the critical range at which the eutectoid reaction
would take place. During the slow cooling the carbon dissolved
in the austenite is converted to graphite on the existing temper
–carbon particles & remaining austenite transforms into
ferrite. Once graphitization is complete no further structural
changes takes place during cooling to room temperature and
the structure consist of temper- carbon nodules in a ferrite
matrix. This known as ferrite malleable C.I.
• In the form of compact nodules, the temper carbon does not
break up the continuity of the tough Ferritic matrix. This
results in a higher strength and ductility than exhibited by
gray cast iron. The graphite nodules also serve to lubricate
cutting tools, which accounts for the very high
machinability of malleable iron.
• The Ferritic malleable iron shows higher strength and
ductility than gray cast iron.
• Graphite nodules lubricate the cutting tools leading to good
machinability of malleable iron.
Ferritic malleable iron has been used for pipe fittings, expansion
joints, railing casting on bridges, n-hoist assemblies, bearing blocks,
valves, farm equipment, chains, automobile parts, in general hardware,
reducing gear housings, rear-axle housings, hubs, hooks, shackles,
leads, yokes, nuts, mufflers, flanges, couplings.
2. Pearlitic malleable iron: T o o b t a i n p e a r l i t i c m a t r i x , 1 % m a n g a n e s e i s
added to cast iron, or second-stage graphitisation is replaced by a
quench, usually air, which cools the castings through the eutectoid
range fast enough to retain combined carbon throughout the matrix.
The amount of Pearlite formed depends upon the temperature at which
the quench starts and rate of cooling. If the air quench produces a fast
enough cooling rate through the eutectoid rang, the matrix will be
A fully ferritic malleable iron may be converted into pearlitic
malleable iron by reheating above the lower critical temperature,
followed by rapid cooling. At higher temperature carbon will be
dissolved from the graphite nodules and subsequent cooling retains the
combined carbon. The cooling from temperature of first-stage
graphitisation (curve II in Figure 19). Normally, after air cooling, the
pearlitic malleable cast iron castings are heated to higher temperatures
(called drawing process) at 550- 650°C or so to spheroidise the Pearlite
to improve the machinability, ductility and toughness with slight drop
in hardness and strength. Pearlitic malleable iron can be hardened and
tempered. Welding of pearlitic malleable iron is rarely used due to the
formation of brittle and low strength white iron under the weld bead.
Presence of larger amount of silicon in white cast iron castings helps to
graphitise it during malleable heat treatment. But a thick casting
having higher silicon may result in gray iron in the centre while casting
it. As the casting should he of white iron up to the centre before
malleable heat treatment is given, silicon content has to be kept low in
the composition, which makes graphitisation during malleable treatment
a long and difficult process.
Figure 19 Typical Malleabilising iron
Due to high strength and hardness, pearlitic malleable iron is used for
cam shafts, crank-shafts, axles, differential housing in automobile
industry, rolls, pumps, nozzles, gears, links, sprockets, elevator
brackets in conveyer equipment, hammers, wrenches, shows, switch
gear parts, fittings for high and low voltage transmission and
distribution system, jaws of universal-joint shafts, links and rollers of
conveyer chains, bushings, couplings brake-shoes. Malleable irons are
used chiefly for thin walled castings because there are restrictions in
The malleable cast irons have reasonable ductility, high strength,
toughness and even are bendable. The main reasons of using malleable
irons are low cost and ease of machining with above properties.
Malleable iron has limitations of section thickness, lower damping
capacity and impact resistance.
VII. SPHEROIDAL GRAPHITE IRON (S.G. IRON)
This cast iron also known as nodular cast iron. In an ordinary grey cast
iron graphite is present as 'flakes' which tend to have sharp-edged rims.
Since these flakes have negligible strength they act as wide-faced
discontinuities in the structure whilst the sharp-edged rims introduce
regions of stress-concentration. In SG cast iron the graphite flakes are
replaced by spherical particles of graphite (Figure. 20 a), so that the
metallic matrix is much less broken up, and the sharp stress raisers are
Figure 20 a) A Spheroidal-graphite cast iron. Here the graphite has been made to precipitate in nodular
form by adding a nickel-magnesium alloy
b) A compacted graphite cast iron. Unetched to show the rounded edges of the graphite flakes,
The formation of this Spheroidal graphite is effected by adding small
amounts of cerium or magnesium to the molten iron just before casting.
Since both of these elements have strong carbide-forming tendencies,
the silicon content of the iron must be high enough (at least 2.5%) in
order to prevent the formation of white iron (by chilling) in thin
sections. Magnesium is the more widely used, and is usually added (as
a nickel-magnesium alloy) in amounts sufficient to give a residual
magnesium content of 0.1% in the iron. SG cast irons produced by the
magnesium process have tensile strengths of up to 900 N/mm2 or even
higher in some heat-treated irons. The term 4SG iron' really describes a
family of cast irons, which include some alloy irons, but in all cases
treatment by inoculants is employed to produce Spheroidal-graphite
particles. Some SG iron produced by using the following substances
instead of cerium or magnesium: calcium, calcium carbide, calcium
fluoride, lithium, strontium, barium and argon. Those irons consisting
of graphite nodules in a ferrite matrix will have high ductility and
toughness whilst those consisting of graphite nodules in a pearlite
matrix will be characterised by high strength. Some of these irons are
heat-treated to give even better mechanical properties. Thus, American
motor industry hardens some of their SG iron gears by the use of
'interrupted austempering'. This involves austenitising the gears at
9000C for 3.5 h in a nitrogen atmosphere followed by quenching to
235°C and holding at that temperature for 2 hour. Since transformation
from austenite occurs isothermally at 235°C there is little distortion in
shape. It is claimed that SG iron hypoid ring and pinion gears are
comparable with those of steel in terms of fatigue and also have a
greater torsional strength.
Tensile strengths of the order of 1600 N/mm2 (with an elongation of.
1%) can be obtained by austempering SG iron at 2500C, following an
initial austenitising at 9000C; whilst higher austempering temperatures
up to 4500C will yield bainitic structures of lower strengths (900-1200
N/mm2) but elongations up to 14%. SG iron crankshafts cast to near
final shape are less expensive and some 10% lighter than equivalent
forged components. They are heat-treated in a similar way to the gears
Properties of S.G. Iron:
S.G. irons have higher mechanical properties, almost equal to cast
carbon steels (thus used for pipes), such as tensile strength, ductility
and toughness (Table 6), combined with favourable properties of gray
cast irons, like good machinability, damping capacity, high wear
resistance, reasonable castability, but do not suffer from the defects of
gray irons such as growth and fire crazes, when used at elevated
temperatures, and is less section-sensitive.
Table 6 Properties of S.G. Iron
Grade Minimum Minimum, BHN % Matrix Heat
Tensile 0.2% Elongations Treatment
350/22 350 220 130 22 Ferrite Annealed
420/12 420 270 150 12 Mainly Ferritic Annealed
500/7 500 320 275 7 Ferrite+Pearlite Annealed
800/7 800 480 320 2 Tempered Quenched
400/40 400 200 130 40 Austenite Cast
STEPS IN PRODUCTION OF S.G. IRON
1. D e s u l p h u r i s a t i o n :
Sulphur helps to form graphite as flakes.
Thus, the raw material for producing iron should have low sulphur (less
that 0. 1%), or remove sulphur from iron during melting, or by mixing
iron with a desulphurising agent such as calcium carbide, or soda ash
2. N o d u l i s i n g :
Magnesium is added to remove sulphur and oxygen
still present in the liquid alloy and provides 0.04% magnesium, which
causes growth of graphite to be Spheroidal. Magnesium treatment
desulphurises the iron to below 0.02% S before alloying with it.
Magnesium and such elements have strong affinity for sulphur, and thus
scavenge sulphur from the molten alloy’s an initial step or, producing
S.G. iron. These additions are expensive to increase the cost of S.G.
iron produced. Thus, sulphur S molten alloy (or the raw material used),
before nodulising, should be kept low.
Magnesium is added when melt is near 1500°C but magnesium vaporises
at 1 150°C. Magnesium, being lighter floats on the top of the bath, and
being reactive burn off at the surface. In such cases magnesium is
added as Ni-Mg, Ni-Si-Mg alloy or magnesium coke to reduce violence
of reaction and to have saving in magnesium. Magnesium metal can be
added as metal itself The method of addition include ladle transfer,
covered ladle technique, porous plug stirring, and in-mould technique.
Addition of magnesium and ferrosilicon is done shortly before casting.
3 . Inoculation : A s m a g n e s i u m i s c a r b i d e f o r m e r , f e r r o s i l i c o n i s
added immediately as inoculant. Remelting causes reversion to flake
graphite due to the loss of magnesium. Stirring of molten alloy after
addition of nodulising element evolves a lot of gas, which gets
dissolved in liquid alloy, and forms blow-holes in solid casting. The
contraction during solidification of nodular cast iron castings is much
greater than of gray iron castings, which needs careful design of
moulds to avoid shrinkage cavities in solidified castings.
In spite of these drawbacks, nodular cast iron is replacing gray iron and
steels in applications. A nodule of graphite (having minimum surface
area per unit volume) weakens the steel matrix to a lesser extent than
gray iron flakes. The nodules don’t act very much as stress-raisers.
Figure.19 Illustrates range of carbon and silicon in S.G. iron. One of
the composition of S.G. iron can be:
C = 3.7%, Si = 2.5%, Mn = 0.3% S = 0.01%, P = 0.01%, Mg = 0.04%
Range of carbon and silicon for S.G irons. Figure 21
Helpful Neutral Inhibitors
Mg,Ce,Ca,Ba,Li,Zr Fe,C,Ni,Si,Mo Al,Ti,Sb,As,Pb,Bi
Table 7 illustrates effect of some elements in the production of S.G. iron.
Figure. 22 illustrates various C.E.V. depending on the maximum thickness of
the casting in sand moulds and in metal moulds.
The matrix of as-cast S.G. iron depends on the composition and the
rate of cooling. Complete Ferritic or matrix having a maximum of 10%
Pearlite, still called Ferritic matrix, possesses maximum ductility,
toughness and machinability, Figure. 23(a). A largely pearlitic matrix
23(b) , obtained in as-cast, or by normalising (air cooling from 850 to
900°C) makes S.G. iron stronger but less ductile. Oil or water
quenching from 900°-950°C yields martensite matrix, which is
tempered to desired strength, hardness and toughness. Austenitic
ductile matrix, Figure.21 (d) (which can be retained up to 25°C) is
obtained by alloying (15-36% Ni, 1.8-6% Cr) the cast iron to have high
corrosion resistance and good creep resistance at high temperatures.
Figure.21(c) illustrates, bulls eye S.G. iron, where ferrite in immediate
vicinity of graphite is present in mainly Pearlite matrix.
Figure.23 Microstructures of S.G. irons. (a) Ferrite S.G. iron. x 250, (b) Pearlitic S.G. Iron. x 500, (c)
Bull’s eye S.G. Iron. x 100, (d) Austenitic S.G. iron (Ni-Resist 21.06% Ni, 2.20% Cr,0.06% Mg) as cast. X
Application of S.G. Iron:
S.G. iron is used for gear pumps for processing and transport of
sulphuric acid, pumps and valves in sea water applications, components
used in steam services, and in the handling of alkali, caustic and
ammonia-cal solutions, and for pumping and handling of sour crude oils
in petroleum industry. Other wide applications are-
• Pistons and cylinder heads in automobile and diesel engines
• Pressure castings like gears and roller slides
• Steering knuckles
• Rocker arms
• Paper mill dryer rolls
VIII. COMPACTED/VERMICULAR CAST IRON
This is the latest member to join the family of cast irons in which
graphite occurs as worm-like blunt-edged stubby flakes (rounded rods,
which are interconnected within eutectic cell); embedded in steel
matrix, Figure.24 The formation of compacted iron depends on the
chemical composition, section thickness. and the process used for
production. Normally, 10-20% of the spheroidal graphite may be
present, which requires C.E.V. of 4.00, and flake-graphite should be
avoided. In one of the production methods, nitrogen (—0.015%) is
added to liquid alloy in ladle by adding nitride Ferro-manganese
Figure.24 Microstructure of compacted vermicular cast iron.
(80% Mn, 4% N, rest Fe). This method gives non-uniformity of
structure and unsoundness in castings. In another method, an alloy
(4-5% Mg, 8.5-10.5% Ti, 4-5.5% Ca, 1-1.5% Al, 0.2-0,5% Ce, 48-52%
Si, rest Fe) in amounts 0.6-1.6% is added, as additions are made to
produce S.G. iron. Sulphur content of iron should not be more than
0.035%. This method is section-sensitive as spheroids get formed in
thin sections. The compacted graphite permits strength. Stiffness and
ductility that exceeds those of gray iron.
Compacted cast iron to retain good damping capacity, and thermal
conductivity. Its resistance to crazing, tracking and distortion is
superior to both S.G. iron and gray iron. As the shrinkage during
casting is less than in S.G. iron. This cast iron having inferior
mechanical properties hut similar production costs as S.G. Iron has
limited replacement potential to S.G. iron parts. However, because of
greater strength and toughness, it can replace more expensive alloyed
gray cast irons.
Compacted cast iron is used for making thick sections.
• Hydraulic valves
• Ingot moulds
• Cylinder heads
• Exhaust manifolds,
• Brake drums
• Discs and piston rings are made from this iron as it has good
elevated temperature properties.
Figure. 25 Types of cast irons (a) Gray iron, (b) White iron, (c) Malleable iron, (d) S.G. iron, compacted
IX. ALLOYED CAST IRONS
One or more of the elements like, Ni, Cr, Cu, Si, Mo, V etc. (> 3%) are
added into graphite free, or graphite-bearing cast irons to improve
corrosion, elevated temperature and wear and abrasion resistance
1. Ni-hard: In the white iron composition, 3-5% Ni and 1-3% Cr are
added, producing a microstructure consisting of massive continuous
carbides in the matrix of martensite and some retained austenite on
cooling after solidification. Martensite is obtained due to increased
hardenability, due to the presence of these elements, which along with
carbon lower the Mf temperature to below room temperature to retain
some austenite. Hardness attained is 550-700 BHN. As nickel is half as
powerful a graphitiser as silicon, the risk of graphitisation is prevented
by adding carbide-former, chromium. The poor impact strength and
fatigue resistance due to the continuous network of carbides can be
improved by increasing Ni and Cr content. The modified Ni-hard having
4-8% Ni and 4-15% Cr, after heat treatment has a microstructure of
discontinuous carbides in the matrix of tempered martensite and
F i g u r e . 2 6 N i - H a r d c a s t i r o n m i c r o gr a p h y
• Very good usury strength until 700°C
• These cast irons have excellent wear resistance.
2. Ni-Resist: N i ( 1 3 - 3 6 % ) a n d C r ( 1 . 8 - 6 % ) a r e a d d e d t o p r o d u c e
austenitic matrix with flake or Spheroidal graphite, to get good
corrosion resistance. The latter offers better mechanical properties but
are more expensive. Ni being austenite stabiliser makes the matrix
austenitic, and thus, these are called austenitic cast irons. The
concentration of the elements depends on the nature of the corrosion
environment. Chromium in combination with nickel forms an effective
oxidation resistant scale. Ni-resists combine good corrosion resistance,
excellent erosion resistance to the flow of liquids with heat resisting
properties. Some Ni-resists contain 5.5-8.0% copper. Though, these
alloys could be used up to 800°C, but after stabilisation at 950°C.
These alloys could be used at temperatures higher than 800°C. Average
T.S. = 247 — 485 MN&2
BHN = 120 - 250
Important applications are gear pumps (for processing and transport of
sulphuric acid), pumps and valves in sea-water applications, parts used
in steam and for handling of alkali, caustic, for pumping and handling
of sour crude oils in petroleum industry. Furnace parts, cylinder liners,
exhaust manifolds, etc.
F i g u r e . 2 7 N i - R e s i s t c a s t i r o n m i c r o g r ap h y
3. Silal and Nicrosilal: S i l a l i s t h e c h e a p e s t o x i d a t i o n a n d g r o w t h -
resistant cast iron, particularly the low carbon cast iron resists up to
750°C. Thy composition on an average is:
C = 2.3%; Si=5.5-7.0%; Mn=0.5-0.8%; S=0.06%; P=0.1-0.3%
T.S. = 139—263 Nmm-2
BHN 220 — 255
The microstructure of Silal consists of ferrite and fine graphite ‘D’
type flakes. These cast irons are very brittle. Silicon increase oxidation
resistance by forming a resist it oxide film, and with more silicon, an
impermeable silicate film. Nicrosilal is Ni-Cr added. Silal which gives
austenitic matrix reducing the brittleness, and can be used at
650-900°C. The composition is:
C 1.5-2.0%; Si = 4.5-5.0%, Mn = 0.6-1.0%; S = 0.10%; p <0.1%,
Ni = 18— 23%; Cr = 2-2.4%
T.S. 139-247 Nmm-2
BHN = 150-200
Nicrosilal offers excellent corrosion resistance. Common applications
are: Ingot-moulds. Cylinder heads exhaust manifolds, aluminium
melting crucibles, retorts, glass-moulds, gas-turbine parts.
Table (8) Comparative qualities of cast irons
Comparative qualities of cast irons
Name Nominal Form and Yield Tensile Elongation Hardness Uses
composition condition strength strength [% (in [Brinell
[% by ksi [ksi] 2 inches)] scale]
Grey cast C 3.4, Si 1.8, Cast — 25 0.5 180 Engine
iron Mn 0.5 cylinder
White cast C 3.4, Si 0.7, Cast (as — 25 0 450 Bearing
iron Mn 0.6 cast) surfaces
Malleable C 2.5, Si 1.0, Cast 33 52 12 130 Axle
iron Mn 0.55 (annealed) bearings,
(ASTM track wheels,
Ductile or C 3.4, P 0.1, Cast 53 70 18 170 Gears,
nodular Mn 0.4, camshafts,
iron Ni 1.0, crankshafts
Ductile or — cast 108 135 5 310 —
Ni-hard C 2.7, Si 0.6, Sand-cast — 55 — 550 High strength
type 2 Mn 0.5, applications
Ni 4.5, Cr 2.0
Ni-resist C 3.0, Si 2.0, Cast — 27 2 140 Resistance to
type 2 Mn 1.0, heat and
Ni 20.0, corrosion
HEAT TREATMENT OF CAST IRONS
The common heat treatments given to cast irons are:
I. STRESS-RELIEVING TREATMENT
Residual-stresses develop during solidification and differential cooling
and thus cause differential contraction. Thermal gradients and residual-
stresses are more pronounced in castings with non-uniform cross-
sections. Phase transformations accompanied with volume changes
aggravate the situation further. Castings are slowly heated to a
temperature 480-650°C, normally at 600°C and then furnace cooled to
200°C, followed by air cooling.
The aim is to decompose carbides and Pearlite from the as cast-
structure. This gives graphite in Ferritic matrix. Gray cast iron and
S.G. irons get softened increasing ductility and machinability. White
cast iron gets malleablised.
A typical two stage process particularly for S.G. iron could be used:
First austenitising at 900°C and then cool to transform to Pearlite to
675°C and then ferritization of Pearlite is done at 760°C. Air cooling
may be done unless casting is susceptible to residual-stresses.
It is heating the castings to temperatures above the critical range,
soaking at it and cooling in still air as induced by large fans.
Normalising gives higher hardness and strength by obtaining fine
pearlitic matrix. Table 9 gives normalising temperature range for some
Malleable Iron High Strength Low strength S.G. Iron
Gray Iron Gray iron
Temperature 800-830°C 810-870°C 840-900°C 820-900°C
IV. HARDENING AND TEMPERING
Hardening and tempering induce higher strengths, and good wear
resistance. The time and temperature austenitising depends on the
original matrix of the cast iron. The temperature is up to 50°C above
critical temperature range, but time is important in low combined-
carbon-matrix and thus, soaking is continued till desired amount of
carbon has been dissolved in austenite from free graphite. High silicon
cast irons are less responsive to quenching and prone to cracking as
silicon reduces solubility of carbon in austenite necessitating high
temperatures of austenitising, but which can cause cracking due to more
severe quenching. Water quenching of castings (complex shapes and
different sectioned) causes quench cracks. Oil quench is normally used
or even air-quench, if large amounts alloying elements are present.
Tempering improves tensile strength, reducing hardness, though
depends on tempering temperature and type of iron.
It reduces chances of distortion and cracks. Thin-walled cylinder liners
for diesel engines (BHN needed 390-430) are martempered. The casting
is quenched in a hot salt bath, or oil kept slightly above Ms,
temperature (from austenitising temperature) till the centre of the
casting too attains the bath temperature, and then air cooled. Tempering
may be done as usual.
Castings austenitised at 850-950°C, quenched into, salt or oil bath,
kept at temperature 450-250°C for around 4 hrs. Lower S.G. iron is
twice as strong with same toughness. As it approaches properties of
steels, crank shafts, camshafts, gears of S.G. iron are used in
VII. SURFACE HARDENING
It is an economical method to get wear resistance in selected areas.
Excepting white and highly alloyed cast irons, most cast irons could be
surface hardened by induction, flame, laser etc. Ferritic matrix is not
used, necessitating a pearlitic (even bulls eye), or tempered martensitic
matrix. Flame hardening requires combined carbon of 0.5-0.7% in
matrix. Induction hardening is good for mass production. Electron
beam, plasma and laser are increasingly used methods for surface
hardening. General Motors have been using gray iron diesel engine
cylinder liners, which are laser hardened.
1. Physical Metallurgy by Vijendra Singh (Standard publications)
2. A Introduction to Physical Metallurgy by Sidney H Avner (Tata McGraw-Hill
3. Physical Metallurgy for Engineers by Clark Donald
4. Engineering metallurgy Raymond Higgins