The Iron-Iron Carbide Diagram
A map of the temperature at which different phase changes occur on very slow heating and cooling in relation to Carbon, is called Iron- Carbon Diagram.
Iron- Carbon diagram shows
the type of alloys formed under very slow cooling,
proper heat-treatment temperature and
how the properties of steels and cast irons can be
radically changed by heat-treatment.
Cooling curve for pure iron
IRON-CARBON (Fe-C) PHASE DIAGRAM
Pure iron: 3 solid phases
BCC ferrite (α)
FCC Austenite (γ)
BCC δ
Beyond 6.7% C, cementite (Fe3C)
Eutectic: 4.3% C (L - γ + Fe3C)
(L -solid + solid)
Eutectoid: 0.76% C (γ + α + Fe3C)
(solid - solid + solid
How to read the Fe-C phase diagram
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| How to read the Fe-C phase diagram |
Definition of structures
Various phases that appear on the Iron- Carbon equilibrium phase diagram are as under:
- Ferrite
- Pearlite
- Austenite
- Cementite
- Ledeburite
- Martensite
Ferrite
Ferrite is known as α solid solution.
It is an interstitial solid solution of a small amount of carbon dissolved in α (BCC) iron.
stable form of iron below 912 deg.C
The maximum solubility is 0.025 % C at 723 C and it dissolves only 0.008 % C at room temperature.
It is the softest structure that appears on the diagram.
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| Ferrite |
Pearlite
Pearlite is the eutectoid mixture containing 0.80 % C and is formed at 723°C on very slow cooling.
It is a very fine platelike or lamellar mixture of ferrite and cementite.
The white ferritic background or matrix
contains thin plates of cementite (dark).
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Pearlite |
Austenite
Austenite is an interstitial solid solution of Carbon
dissolved in (F.C.C.) iron.
Maximum solubility is 2.0 % C at 1130°C.
High formability, most of heat treatments begin with this single phase.
It is normally not stable at room temperature. But,
under certain conditions it is possible to obtain
austenite at room temperature.
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Austenite |
Cementite
Cementite or iron carbide, is very hard, brittle intermetallic compound of iron & carbon, as Fe3C, contains 6.67 % C.
It is the hardest structure that appears on the diagram, exact melting point unknown.
Its crystal structure is orthorhombic.
It is has low tensile strength (approx. 5,000 psi), but high compressive strength.
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| Cementite |
Ledeburite
Ledeburite is the eutectic mixture of austenite and cementite.
It contains 4.3 percent C and is formed at 1130°C.
Martensite
Martensite - a super-saturated solid solution of carbon in ferrite.
It is formed when steel is cooled so rapidly that the change from austenite to pearlite is suppressed.
The interstitial carbon atoms distort the BCC ferrite into a BC-tetragonal structure (BCT).; responsible for the hardness of quenched steel
Various Features of Fe-C diagram
Max. solubility of C in ferrite=0.022%
Max. solubility of C in austenite=2.11%
Three Phase Reactions
A horizontal line always indicates an invariant reaction in binary phase diagrams
The Iron-Iron Carbide Diagram
Cast Irons
-Iron-Carbon alloys of 2.11%C or more are cast irons.
-Typical composition: 2.0- 4.0%C, 0.5-3.0% Si, less than 1.0% Mn and less than 0.2% S.
-Si-substitutes partially for C and promotes formation of graphite as the carbon rich component instead Fe3C.
The Austenite to ferrite / cementite transformation in relation to Fe-C diagram
The Austenite to ferrite / cementite transformation in relation to Fe-C diagram
The iron wants to change crystal structure from the FCC austenite to the BCC ferrite, but the ferrite can only contain 0.02% carbon in solid solution.
The excess carbon is rejected and forms the
carbon-rich intermetallic known as cementite.
The Austenite to ferrite / cementite transformation in relation to Fe-C diagram
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The Austenite to ferrite / cementite transformation in relation to Fe-C diagram
Hypo-eutectoid steels: Steels having less than 0.8% carbon are called hypo-eutectoid steels (hypo means "less than").
Consider the cooling of a typical hypo-eutectoid alloy along line y-y‘.
At high temperatures the material is entirely austenite.
Upon cooling it enters a region where the stable phases are ferrite and austenite.
The low-carbon ferrite nucleates and grows, leaving the remaining austenite richer in carbon.
The Austenite to ferrite / cementite transformation in relation to Fe-C diagram
Hyper-eutectoid steels (hyper means "greater than")
are those that contain more than the eutectoid amount of Carbon.
When such a steel cools, as along line z-z' , the process is similar to the hypo-eutectoid steel, except that the primary or pro-eutectoid phase is now cementite instead of ferrite.
Principal phases of steel and their
Characteristics
Applications
It is used tailor properties of steel and to heat treat them.
It is also used for comparison of crystal structures for metallurgists in case of rupture or fatigue