>> Part a). >> Pearlite :- Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (88 wt%) and cementite (12 wt%) that occurs in some steels and cast irons. In fact, the lamellar appearance is misleading since the individual lamellae within a colony are connected in three dimensions; a single colony is therefore an interpenetrating bicrystal of ferrite and cementite. In an iron-carbon alloy, during slow cooling pearlite forms by a eutectoid reaction as austenite cools below 727 °C (1,341 °F) (the eutectoid temperature). Pearlite is a microstructure occurring in many common grades of steels. >> Cementite :- Cementite, also known as iron carbide, is an interstitial compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking technologies. >> Austenite :- Austenite, also known as gamma-phase iron (-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with analloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1,000 K (1,340 °F; 730 °C); other alloys of steel have different eutectoid temperatures. It is Face Centred Cubic Configuration (FCC). >> Eutectoid Phase :- When the solution above the transformation point is solid, rather than liquid, an analogous eutectoid transformation can occur. For instance, in the iron-carbon system, the austenite phase can undergo a eutectoidtransformation to produce ferrite and cementite, often in lamellar structures such as pearlite and bainite. >> Proeutectoid :- When a hot steel with carbon content very close to 0.8%, is cooled down slowly, there is a temperature (723 deg C) at which a constant-temperature transformation takes place. This is called eutectoid transformation. And this results in formation of alternate layers of Ferrite and Iron-Carbide (Fe3C). But if the carbon content in this hot steel is much less than 0.8%, and it is cooled down slowly, then till the temperature reduces to 723 deg C, a part of Austenite (also called gamma iron) gets transformed to Ferrite by rejecting carbon from the solution. This is not a constant-temperature process and occurs over a range of temperature. The ferrite so formed is called Proeutectoid...At 723 deg C, all the remaining Austenite get converted to Pearlite at this constant temperature - which is nothing but alternate layers of Ferrite and cementite >> Martensite :- Martensite, most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is form.

1. >> Part a).
>> Pearlite :- Pearlite is a two-phased, lamellar (or layered) structure composed of alternating
layers of ferrite (88 wt%) and cementite (12 wt%) that occurs in some steels and cast irons. In
fact, the lamellar appearance is misleading since the individual lamellae within a colony are
connected in three dimensions; a single colony is therefore an interpenetrating bicrystal of ferrite
and cementite. In an iron-carbon alloy, during slow cooling pearlite forms by a eutectoid reaction
as austenite cools below 727 °C (1,341 °F) (the eutectoid temperature). Pearlite is a
microstructure occurring in many common grades of steels.
>> Cementite :- Cementite, also known as iron carbide, is an interstitial compound of iron and
carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By
weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard,
brittle material, normally classified as a ceramic in its pure form, though it is more important in
ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a
raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking
technologies.
>> Austenite :- Austenite, also known as gamma-phase iron (-Fe), is a metallic, non-magnetic
allotrope of iron or a solid solution of iron, with analloying element. In plain-carbon steel,
austenite exists above the critical eutectoid temperature of 1,000 K (1,340 °F; 730 °C); other
alloys of steel have different eutectoid temperatures. It is Face Centred Cubic Configuration
(FCC).
>> Eutectoid Phase :- When the solution above the transformation point is solid, rather than
liquid, an analogous eutectoid transformation can occur. For instance, in the iron-carbon system,
the austenite phase can undergo a eutectoidtransformation to produce ferrite and cementite, often
in lamellar structures such as pearlite and bainite.
>> Proeutectoid :- When a hot steel with carbon content very close to 0.8%, is cooled down
slowly, there is a temperature (723 deg C) at which a constant-temperature transformation takes
place. This is called eutectoid transformation. And this results in formation of alternate layers of
Ferrite and Iron-Carbide (Fe3C).
But if the carbon content in this hot steel is much less than 0.8%, and it is cooled down slowly,
then till the temperature reduces to 723 deg C, a part of Austenite (also called gamma iron) gets
transformed to Ferrite by rejecting carbon from the solution. This is not a constant-temperature
process and occurs over a range of temperature. The ferrite so formed is called Proeutectoid...At
723 deg C, all the remaining Austenite get converted to Pearlite at this constant temperature -
which is nothing but alternate layers of Ferrite and cementite

2. >> Martensite :- Martensite, most commonly refers to a very hard form of steel crystalline
structure, but it can also refer to any crystal structure that is formed by diffusionless
transformation. It includes a class of hard minerals occurring as lath- or plate-shaped crystal
grains. When viewed in cross section, the lenticular (lens-shaped) crystal grains are sometimes
incorrectly described as acicular (needle-shaped)
>> Bainite :-
Bainite is a plate-like microstructure or phase morphology (not an equilibrium phase) that forms
in steels at temperatures of 250–550 °C (depending on alloy content). It is one of the
decomposition products that may form when austenite (the face centered cubic crystal structure
of iron) is cooled past a critical temperature. This critical temperature is 1000K (727 °C, 1340
°F) in plain carbon steels. Davenport and Bain originally described the microstructure as being
similar in appearance to tempered martensite.
A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich
ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite
harder than it normally would be.
Part b). Two major differences are:
1) atomic diffusion is necessary for the pearlitic transformation, whereas the martensitic
transformation is diffusionless; and
2) relative to transformation rate, the martensitic transformation is virtually instantaneous, while
the pearlitic transformation is time-dependent.
>> Part c).
A proeutectoid phase (ferrite or cementite) always forms along austenite grain boundaries
because associated with grain boundaries is an interfacial energy (i.e., grain boundary energy). A
lower net interfacial energy increase results when a proeutectoid phase forms along existing
austenite grain boundaries than when the proeutectoid phase forms within the interior of the
grains.
>>The main difference between martensite and bainite is how they form. When heated austenite
tends to cool, then in first case, if it make to cool at a particular rate, then diffusion of carbon
inside its gets lowerd and carbon granules just lie inside the cementite matrix. This is known as
Bainite.
Now, if cooling rate is too high, then it will be just hardened. Its strength will be too high . This
is martensite.
>> Now, as said above, Martensite is too hard to be used. So, to make it useful, martensite is
heated for a particular time to induce some softness into it. The product formed is known as
Tempered Martensite

3. Solution
>> Part a).
>> Pearlite :- Pearlite is a two-phased, lamellar (or layered) structure composed of alternating
layers of ferrite (88 wt%) and cementite (12 wt%) that occurs in some steels and cast irons. In
fact, the lamellar appearance is misleading since the individual lamellae within a colony are
connected in three dimensions; a single colony is therefore an interpenetrating bicrystal of ferrite
and cementite. In an iron-carbon alloy, during slow cooling pearlite forms by a eutectoid reaction
as austenite cools below 727 °C (1,341 °F) (the eutectoid temperature). Pearlite is a
microstructure occurring in many common grades of steels.
>> Cementite :- Cementite, also known as iron carbide, is an interstitial compound of iron and
carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By
weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard,
brittle material, normally classified as a ceramic in its pure form, though it is more important in
ferrous metallurgy. While iron carbide is present in most steels and cast irons, it is produced as a
raw material in the Iron Carbide process, which belongs to the family of alternative ironmaking
technologies.
>> Austenite :- Austenite, also known as gamma-phase iron (-Fe), is a metallic, non-magnetic
allotrope of iron or a solid solution of iron, with analloying element. In plain-carbon steel,
austenite exists above the critical eutectoid temperature of 1,000 K (1,340 °F; 730 °C); other
alloys of steel have different eutectoid temperatures. It is Face Centred Cubic Configuration
(FCC).
>> Eutectoid Phase :- When the solution above the transformation point is solid, rather than
liquid, an analogous eutectoid transformation can occur. For instance, in the iron-carbon system,
the austenite phase can undergo a eutectoidtransformation to produce ferrite and cementite, often
in lamellar structures such as pearlite and bainite.
>> Proeutectoid :- When a hot steel with carbon content very close to 0.8%, is cooled down
slowly, there is a temperature (723 deg C) at which a constant-temperature transformation takes
place. This is called eutectoid transformation. And this results in formation of alternate layers of
Ferrite and Iron-Carbide (Fe3C).
But if the carbon content in this hot steel is much less than 0.8%, and it is cooled down slowly,
then till the temperature reduces to 723 deg C, a part of Austenite (also called gamma iron) gets
transformed to Ferrite by rejecting carbon from the solution. This is not a constant-temperature
process and occurs over a range of temperature. The ferrite so formed is called Proeutectoid...At
723 deg C, all the remaining Austenite get converted to Pearlite at this constant temperature -

4. which is nothing but alternate layers of Ferrite and cementite
>> Martensite :- Martensite, most commonly refers to a very hard form of steel crystalline
structure, but it can also refer to any crystal structure that is formed by diffusionless
transformation. It includes a class of hard minerals occurring as lath- or plate-shaped crystal
grains. When viewed in cross section, the lenticular (lens-shaped) crystal grains are sometimes
incorrectly described as acicular (needle-shaped)
>> Bainite :-
Bainite is a plate-like microstructure or phase morphology (not an equilibrium phase) that forms
in steels at temperatures of 250–550 °C (depending on alloy content). It is one of the
decomposition products that may form when austenite (the face centered cubic crystal structure
of iron) is cooled past a critical temperature. This critical temperature is 1000K (727 °C, 1340
°F) in plain carbon steels. Davenport and Bain originally described the microstructure as being
similar in appearance to tempered martensite.
A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich
ferrite. The high concentration of dislocations in the ferrite present in bainite makes this ferrite
harder than it normally would be.
Part b). Two major differences are:
1) atomic diffusion is necessary for the pearlitic transformation, whereas the martensitic
transformation is diffusionless; and
2) relative to transformation rate, the martensitic transformation is virtually instantaneous, while
the pearlitic transformation is time-dependent.
>> Part c).
A proeutectoid phase (ferrite or cementite) always forms along austenite grain boundaries
because associated with grain boundaries is an interfacial energy (i.e., grain boundary energy). A
lower net interfacial energy increase results when a proeutectoid phase forms along existing
austenite grain boundaries than when the proeutectoid phase forms within the interior of the
grains.
>>The main difference between martensite and bainite is how they form. When heated austenite
tends to cool, then in first case, if it make to cool at a particular rate, then diffusion of carbon
inside its gets lowerd and carbon granules just lie inside the cementite matrix. This is known as
Bainite.
Now, if cooling rate is too high, then it will be just hardened. Its strength will be too high . This
is martensite.
>> Now, as said above, Martensite is too hard to be used. So, to make it useful, martensite is
heated for a particular time to induce some softness into it. The product formed is known as
Tempered Martensite