Hariprasad (Asst. Professor)
Pure iron, upon heating, experiences two changes
in crystal structure before it melts.
At room temperature the stable form, called
ferrite, or iron, has a BCC crystal structure.
Ferrite experiences a polymorphic transformation
to FCC austenite, or iron, at 9120C.
This austenite persists to 1394oC, at which
temperature the FCC austenite reverts back to
a BCC phase known as ferrite, which finally
melts at 15380C
All these changes are apparent along the left
vertical axis of the phase diagram.
The -ferrite is virtually the same as -ferrite,except for the range of temperatures over
which each exists.
(a) Ferrite (90X) (b) Austenite (325X)
Maximum solubility of carbon in various single
Room temp solubility
Stable at only high temp
The solubility of carbon varies in different formsof iron.
In delta iron, maximum solid solubility of carbonis 0.1%
In gamma iron, the maximum solid solubility ofcarbon is 2.03%
Austenite is a solid solution of carbon in FCCiron and solute atoms occupy interstitialpositions in this lattice.
In alpha iron, carbon has a limited solidsolubility of about 0.008% at room temperature
The maximum solubility of carbon in ferrite is0.025%.
Description of the phases in Iron-Iron Carbide system
Interstitial solid solution of small amounts carbondissolved in BCC-Iron
Max. solubility of carbon is 0.025% at 727oC and0.008% at room temp
Softest among all the phases
Interstitial solid solution of carbon in FCC iron.
Max solubility of carbon is 2.1%
It is not stable below 727oC
Interstitial solid solution of carbon in BCC iron
Upon heating Austenite changes to BCC and gets-Ferrite
Max solubility of carbon is 0.1%-Ferrite
This is also known as Iron Carbide (Fe3C)
Its a chemical compound because it contains afixed percentage of carbon (6.67%)
It has an orthorhombic crystal structure (abc)
It is the eutectic lamellar mixture of austenite(light phase) and cementite (dark phase)
It is unstable below 727oC and transforms into-ferrite and cementite.
Austenite containing 0.8%C forms pearlite uponslow cooling below 727oC
Its lamellar structure of -ferrite and cementite. It is very bright in appearance (like a pearl)
It has a fingerprint like appearance
There are three reactions which occur in iron
cementite phase diagram.
Peritectic reaction: In the alloy containing 0.18%
carbon, the initial crystals of delta solid solution
and the whole of liquid phase is completely
transformed to from austenite on cooling at
Eutectic reaction: alloy with carbon content
4.33%, the liquid is transformed into austenite
and cementite on cooling at 11470C
The eutectic of austenite and cementite is known as
Eutectoid reaction: in Iron carbon alloy with
0.8% carbon, the austenite is transformed into
ferrite and cementite by eutectoid reaction on
cooling in 7270C
The eutectoid of ferrite and cementite is known as
If the carbon % is less than 2.14, that type of alloy
comes under the category of steels.
If the carbon % greater than 2.14, that alloy
comes under the category of cast irons
There are major categories of steels.
i. Hypoeutectoid steels
ii. Hypereutectoid steels
iii. Eutectoid steels
Steels with carbon content from 0.025% to 0.8%
are called hypoeutectoid steels.
Steels with a carbon content of 0.8% is known as
Steels with a carbon content greater than 0.8%
are called hypereutectoid steels
Hypoeutectoid steels There are solid state transformations in this steels.
They are the transformation of gamma iron to alpha
iron and the decomposition of austenite.
The limiting composition for getting pearlite is
With carbon content less than this amount, no pearlite
will be formed. The alloy will contain only ferrite
Steels containing carbon between 0.025-0.8% would
contain varying amount of ferrite and pearlite and
their relative proportions depend on carbon content
Schematic representations of the microstructures for an
ironcarbon alloy of hypoeutectoid composition
Point C: At about 8750C, point c, the
microstructure will consist entirely of grains of
the -phase, as shown in fig.
Point D: about 775oC, which is within the + phase region, both these phases will coexist as inthe schematic microstructure.
Most of the small particles will form along the
original grain boundaries
Micrograph of hypoeutectoid steel (0.34%C)
At eutectoid temperature, the composition of
austenite is 0.8% carbon
On further cooling, the entire amount of
austenite will transform to pearlite
Hence, the final microstructure consists of
pearlite and proeutectoid cementite
Schematic representations of themicrostructures for an iron
carbon alloy of hypereutectoid composition
Micrograph of hypereutectoid steel
Pearlite + proeutectoid cementite
On cooling at eutectoid point (0.8%C-7270C), all
austenite will transform into 100% pearlite.
So, the microstructure at room temperature will
reveal alternate layers of ferrite and cementite,
Schematic representations of themicrostructures for an
ironcarbon alloy of eutectoidcomposition (0.76 wt% C) above and
below the eutectoid temperature.
The microstructure for this eutectoid steel that is
slowly cooled through the eutectoid temperature
consists of alternating layers or lamellae of the two
phases ( and Fe3C) tat form simultaneously duringthe transformation
Point b, is called pearlite because it has the appearance
of mother of pearl when viewed under the microscope
at low magnifications
Photomicrograph of a eutectoid
steel showing the pearlitemicrostructure consisting of Alternating layers of -ferrite (the light phase) and Fe3C (thin layers most of which
Microstructure of a eutectoid steel The pearlite exists as grains, often termed
The thick light layers are the ferrite phase,
and the cementite phase appears as thin
lamellae most of which appear dark.
The evolution of the microstructure of hypoeutectoid and
hypereutectoid steels during cooling, in relationship to the Fe-Fe3C
Cast Irons Hypoeutectic cast iron
Eutectic cast iron
Hypereutectic cast iron
Hypoeutectic cast iron
In this case, a structure below 11470C consists of
proeutectic austenite and ledeburite (eutectic
mixture consisting of austenite and cementite).
On further cooling, in the temperature range
11470C 7230C, excess carbon comes out as
cementite from proeutectic and eutectic austenite.
Within the temp range of 1147 to 727oC upon cooling
When it cools to room temp
Eutectic cast iron (ledeburite
Austenite + Cementite
When the same cast iron is cooled to room temp, austenite
transforms to pearlite with cementite
Pearlite formed from austenite Cementite
Hypereutectic Cast Iron In this case the structure just below 11470C
consists of proeutectic cementite and ledeburite.
On further cooling in the temperature range
11470C 7270C, excess carbon comes out as
cementite from the eutectic austenite.
Calculation of relative amounts of
phases in Fe-Fe3C diagram
The relative amount of proeutectoid ferrite and
pearlite in 0.2 percent carbon steel:
Percent of ferrite: