This document discusses the micro-constituents and structures that form in iron-carbon alloys. It defines 9 micro-constituents - ferrite, pearlite, austenite, cementite, ledeburite, martensite, troostite, sorbite, and bainite - and describes their properties and how they form. It also discusses the transformation that occurs in eutectic, hypoeutectic, and hypereutectic cast irons upon heating and cooling.

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

5. 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.

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

7. Definition of structures
• Pearlite (α+ Fe3C) 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).

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

9. 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.

10. 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

11. 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.

12. Definition of structures
• Ledeburite (+Fe3C) is the
eutectic mixture of austenite and
cementite.
• It contains 4.3% C and is formed
at 1140°C.

13. 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

14. Troostite
• It is the radial mixture of radial lamellae of
ferrite and cementite. In fact, it differs from
pearlite only in the degree of fineness.
• It has hardness intermediate between
martensite and sorbite.

15. Sorbite
• Sorbite is the microstructure consisting ferrite,
and finely divided cementite, produced on
tempering martensite above 450ᵒC.
• Though sorbite steel is slightly less ductile
than pearlite steel, its tensile and yield
strengths are high. Thus sorbite steels are
often known as toughned steels.

16. Bainite
• Bainite is a decomposition product of
austenite, consisting of an aggregate of ferrite
and carbide.
• Bainite obtained by transformation of pearlite
at higher temperature is called upper bainite.
• Bainite obtained by low temperature is called
lowerer bainite.
• Bainite has hardness in between the hardness
of pearlite and martensite.

18. IRON-IRON CARBIDE EQUILIBRIUM DIAGRAM

21. .

22. 1.Transformation of a Eutectoid Steel

28. 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

29. • 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 cemantite , pearlite and
transformed ledubrite.

30. • 3) TRANSFORMATION OF HYPER EUTECTOID
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 ledubrite
takes place.
– Cooling below 723 C formation of pearlite takes place