iron carbon system

1. IRON- CARBON SYSTEM

2. Allotropic Forms of pure iron

4. Micro-constituents of Iron- Carbon
alloys
1. Ferrite
2. Pearlite
3. Austenite
4. Cementite
5. Ledeburite
6. Martensite
7. Troostite
8. Sorbite
9. Bainite

6. Definition of structures
• 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.

7. Definition of structures
Ferrite
• Average properties are:
– Tensile strength = 40,000 psi;
– Elongation = 40 % in 2 in;
– Hardness > Rockwell B 90

8. Definition of structures
• Pearlite is the eutectoid mixture
containing 0.80 % C and is formed
at 723°C on very slow cooling.
• It is a very fine plate like or
lamellar mixture of ferrite and
cementite.
• The white ferritic background or
matrix contains thin plates of
cementite (dark).

9. Definition of structures
Pearlite
• Average properties are:
– Tensile strength = 120,000 psi;
– Elongation = 20 % in 2 in.;
– Hardness = Rockwell C 20

10. Definition of structures
• 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.

11. Definition of structures
Austenite
• Average properties are:
– Tensile strength = 150,000 psi;
– Elongation = 10 percent in 2 in.;
– Hardness = Rockwell C 40,
approx; and
– toughness = high

12. Definition of structures
• 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.

13. Definition of structures
• Ledeburite is the eutectic
mixture of austenite and
cementite.
• It contains 4.3 percent C and is
formed at 1130°C.

14. Definition of structures
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

17. .

18. TRANSFORMATION STRUCTURE IN Steel (EUTECToid, HYPO
EUTECToid AND HYPER EUTECToid steel)
1. TRANSFORMATION OF Eutectoid Steel
2. TRANSFORMATION OF HYPO Eutectoid Steel
3. TRANSFORMATION OF HYPER Eutectoid Steel

25. TRANSFORMATION STRUCTURE IN CAST IRON (EUTECTIC,
HYPOEUTECTIC AND HYPER EUTECTIC CAST IRONS)
• 1) TRANSFORMATION OF EUTECTIC CAST
IRON:
– CAST iron contains 4.3% C
– Heated to 1175 C
– Cooled slowly to 1148 C
– Mixture of austenite + cementite forms

26. • 2) TRANSFORMATION OF HYPO EUTECTIC
CAST IRON:
– It is having LESS THAN 4.3% C
– A 3.4% C cast iron heated up to 1350 C
– Cooled slowly to 1250 C (liquid metal and
austenite crystals forms)
– At 1148 C 50% austenite and 50% cementite
– Cooling slowly below 723 C
– Final structure as cementite , pearlite and
transformed ledeburite

27. • 3) TRANSFORMATION OF HYPER EUTECTIC CAST
IRON
– CAST IRON HAVING MORE THAN 4.3% C
– a sample of 4.8% C
– Heated to 1450 C for sufficient time
– It very cooled slowly formation of liquid phase
– Further slow cooling at 1148 C formation of
ledeburite takes place.
– Cooling below 723 C formation of pearlite takes place

29. FERRITE AND AUSTENITE STABILIZERS
• Three most common phases in Iron-Carbon
Phase diagram are Ferrite,Cementite and
Austenite. Stabilising a particular phase
primarily means making that phase more
available in room temperature. This effect may
lead to excessive changes in properties and
may make the material fit or unfit for certain
application.

30. • Ferrite generally has body centered cubic type
of structure. According to Hume Rothery
Principle a body centered cubic (BCC) material
will stabilise another body centered cubic type
of material when mixed with each other.
Therefore, Ti, Cr, Mo, Al will help in stabilising
ferrite at room temperature.

31. • Austenite generally has face centered cubic
type of structure. According to Hume Rothery
Principle a face centered cubic (FCC) material
will stabilise another face centered cubic type
of material when mixed with each other.
Therefore, Si, Mn, Ni, C will help in stabilising
austenite at room temperature.