3rd year undergraduate,
Department of Metallurgical and Materials Engineering
Indian Institute of Technology, Kharagpur
Topic for today
Fe-Fe3C Phase Diagram
(For a plain C steel)
(plain because no element presence other than C in Iron)
(A base for understanding how different phases of iron evolves)
Why steel is important
It is the “Gold-standard” against which emerging structural materials
“Moving standard”: exciting discoveries being made in the context of
iron and alloys, now and then.
Most successful and cost-effective structural material.
Overwhelming dominance of steel : Due to variety of microstructures
and properties generated by the solid-state transformation and
Steels-Microstructure and Properties
Honeycombe & Bhadeshia)
Presence of carbon in iron has such a great effect that even 0.1-0.2 wt%
(approx 0.5- 1.0 at % i.e. 5-10 C atoms in 1000 Fe atoms) produces
strengthening effect on ferritic iron, fact, known to smiths for over
2500 years ago, since iron heated in a charcoal fire can easily absorb
carbon through solid-state diffusion.
Carbon nano-tubes are 100 times more stronger than steels but cannot
be used in engineering scale because of inevitable defects which arise
as these tubes are grown.
Inner core of earth may consist of pure phase of iron (apart from
α,γ,δ,ε) in shape of double HCP.
What is a phase?
A homogeneous portion of the system that has a
uniform chemical and physical characteristics.
Every pure material is a phase : elements, compounds.
solid sugar is a “solid phase” and sugar-syrup solution
is a “liquid phase”
Both have different chemical composition: one is pure
sugar (C12H22O11) and the other is a solution of sugar
and water (H2O)
Every teaspoon of the sugar-syrup is always of same
It is a Binary Alloy phase diagram.
“Binary” because here only two elements, Fe and
C are considered.
The phase-diagram is only applicable for the
equilibrium cooling, meaning a very slow
(By equilibrium we mean very minute change in
the system-parameters/compositions etc for a long
period of time)
Some features of Fe-Fe3C Phase Diagram
Each Vertical Green Line represents
a different Carbon wt% Fe-C alloy
Each C wt% on the X-Axis is a
different alloy system
Tetrahedral site in BCC
Octahedral sites in FCC
Octahedral site in BCC
• Carbon goes into the octahedral sites (both in BCC and FCC) to avoid
strain due to surrounding Fe atoms in Tetrahedral sites.
(Strain due to C in octahedral site is less than tetrahedral sites due to its
• Solubility of C in Austenite is larger than Ferrite because Octahedral
site is larger in FCC than BCC.
Maximum solubility of C in α-Ferritic phase is 0.022 wt%, in γ-Austenite
phase is 2.1 wt%, in ε-phase 0.015 wt%.
It means that by heating Iron we can add more C in it and get a
homogenous solution, as the solubility of Austenite (which is stable at
higher temperature) is more than the ferrite phase( which is stable at
If we cool the iron( when it is in austenite phase) then due to less
solubility at lower temperature, C atoms will come out of the Fe Matrix
in the form of precipitates , so that the solution will be no more
Ferrite is softest whereas Cementite is the most hardest and brittle
phase in the entire phase diagram
Because presence of C enhances strength and the maximum solubility of C
in ferrite is only 0.02 wt% whereas in Cementite it is 6.67 wt% (& which is
Cementite is only a metastable phase, which will decompose into Ferrite
and graphite if heated between 650-700 for several years.
(A metastable phase may persist indefinitely and often, experiencing
only slight and imperceptible changes as time progresses)
Two types of transformation processes in α-γ phase
DISPLACIVE OR SHEAR
1. Involves diffusion of atoms (both
solute-Fe and the solvent)
2.No resultant stress in the
2. Results in combination of elastic
and plastic strain in the
3.No resultant shape change in the
3. Resultant shape change of the
4.Stable above 835˚C (in steels) 4.Stable below 835˚C (in steels)
Pearlite : A grain containing alternate lamellae of
ferrite and Cementite.
In pearlite the width ratio of α to Cementite lamellae
(Fe3C) is generally
8: 1 .
Pearlite forms through diffusion of C atoms out from
austenite making left out Fe atoms in austenite to
rearrange and transform their crystal structure from
FCC to BCC (Ferrite). C diffusing out forms
Cementite(Fe3C – 6.67 wt% C ) .
Yes, the Fe-Fe3C is just a part of the Fe-C phase diagram, the entire Fe-C
phase diagram extends to the right hand side upto 100 wt% C, which is pure
Carbon phase called Graphite. (Fe-C phase diagram has got not so significant
use in steel industry, that’s why its generally not shown).
Ferrite is a phase, Austenite is a phase, Cementite is a phase but pearlite is not
a phase, pearlite is just a colony of 2 phases-ferrite & Cementite in a grain.
We discussed the “solution” of Fe & C and definitely not a mixture. Solution
is homogenous in its composition and properties everywhere whereas a
mixture is not. (Sugar-Syrup is a solution and clay in water is a mixture).
The width of the precipitate at the grain boundary varies as a grain boundary
can be a high-angle grain boundary or a low-angle grain boundary, High angle
grain boundaries are highly preferential sites for nucleation of precipitates due
to their high incoherency.(Different orientation of grains)
Different “Heat-Treatment” processes changes the microstructure
differently for the same alloy composition. We discussed the phase diagram
and microstructures for a “Slow cooling” process. Quenching (Oil and water)
is a fast cooling process and it produces martensite (another phase of iron)
along with other phases depending on the processing parameters.