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Chapter 3.
Theory of Alloys and Alloys
Diagrams
Allotropic forms of Iron
IRON IRON-CARBON
DIAGRAM
L + Fe3C
2.14 4.30
6.70
0.022
0.76
Cementite Fe3C
x
x’
y
y’
z
z’
IRON IRON-CARBON DIAGRAM
Ferrite
Austenite
Steel Cast iron
Pearlite
Pearlite and
Cementine
Pearlite and
Carbide
Eutectic
eutectoid
Definition of structures
Various phases that appear on the
Iron-Carbon equilibrium phase
diagram are as under:
•Austenite
•Ferrite
•Pearlite
•Cementite
•Martensite*
•Ledeburite
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.
Definition of structures
Ferrite
 Average properties are:
 Tensile strength = 40,000 psi;
 Elongation = 40 % in 2 in;
 Hardness > Rockwell C 0 or
> Rockwell B 90
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 platelike or
lamellar mixture of ferrite and
cementite.
 The white ferritic background or
matrix contains thin plates of
cementite (dark).
Definition of structures
Pearlite
 Average properties are:
 Tensile strength = 120,000 psi;
 Elongation = 20 % in 2 in.;
 Hardness = Rockwell C 20, Rockwell B
95-100, or BHN 250-300.
Definition of structures
 Austenite is an interstitial solid solution of
Carbon dissolved in  (F.C.C.) iron.
 Maximum solubility is 2.14 % C at 1147°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.
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
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 has
 low tensile strength (approx. 5,000 psi),
but
 high compressive strength.
Definition of structures
 Ledeburite is the eutectic
mixture of austenite and
cementite.
 It contains 4.3 percent C and is
formed at 1130°C.
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
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.
Various Features of Fe-C diagram
Peritectic L + d = 
Eutectic L =  + Fe3C
Eutectoid  = a + Fe3C
Phases present
L
Reactions
d
BCC structure
Paramagnetic
 austenite
FCC structure
Non-magnetic
ductile
a ferrite
BCC structure
Ferromagnetic
Fairly ductile
Fe3C cementite
Orthorhombic
Hard
brittle
Max. solubility of C in ferrite=0.022%
Max. solubility of C in austenite=2.11%
Three Phase Reactions
 Peritectic, at 1490 deg.C, with low wt% C
alloys (almost no engineering importance).
 Eutectic, at 1130 deg.C, with 4.3wt% C,
alloys called cast irons.
 Eutectoid, at 723 deg.C with eutectoid
composition of 0.8wt% C, two-phase mixture
(ferrite & cementite). They are steels.
How to read the Fe-C phase diagram
Ferrite
Austenite
Steel Cast iron
Pearlite
Pearlite and
Cementine
Pearlite and
Carbide
Eutectic
eutectoid
The Iron-Iron Carbide Diagram
The diagram shows three horizontal lines which
indicate isothermal reactions (on cooling /
heating):
 First horizontal line is at 1490°C, where peritectic
reaction takes place:
Liquid + d ↔ austenite
 Second horizontal line is at 1130°C, where
eutectic reaction takes place:
liquid ↔ austenite + cementite
 Third horizontal line is at 723°C, where eutectoid
reaction takes place:
austenite ↔ pearlite (mixture of ferrite &
cementite)
Delta region of Fe-Fe carbide diagram
Liquid + d ↔ austenite
Ferrite region of
Fe-Fe Carbide
diagram
The Austenite to ferrite / cementite
transformation in relation to Fe-C diagram
In order to understand the transformation
processes, consider a steel of the eutectoid
composition. 0.8% carbon, being slow cooled
along line x-x‘.
 At the upper temperatures, only austenite is
present, with the 0.8% carbon being dissolved
in solid solution within the FCC. When the steel
cools through 723°C, several changes occur
simultaneously.
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.
Pearlitic structure
 The net reaction at the
eutectoid is the formation
of pearlitic structure.
 Since the chemical
separation occurs entirely
within crystalline solids,
the resultant structure is a
fine mixture of ferrite and
cementite.
Schematic picture of the formation and
growth of pearlite
Ferrite
Cementite
Austenite
boundary
Nucleation & growth of pearlite
Principal phases of steel and their
Characteristics
Phase
Crystal
structure
Characteristics
Ferrite BCC Soft, ductile, magnetic
Austenite FCC
Soft, moderate
strength, non-
magnetic
Cementite
Compound of Iron
& Carbon Fe3C
Hard &brittle
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.
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
Thanks

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Phase Diagram Analysis of Iron-Carbon Alloys

  • 1. Chapter 3. Theory of Alloys and Alloys Diagrams
  • 4. L + Fe3C 2.14 4.30 6.70 0.022 0.76 Cementite Fe3C x x’ y y’ z z’
  • 5. IRON IRON-CARBON DIAGRAM Ferrite Austenite Steel Cast iron Pearlite Pearlite and Cementine Pearlite and Carbide Eutectic eutectoid
  • 6. Definition of structures Various phases that appear on the Iron-Carbon equilibrium phase diagram are as under: •Austenite •Ferrite •Pearlite •Cementite •Martensite* •Ledeburite
  • 7. 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.
  • 8. Definition of structures Ferrite  Average properties are:  Tensile strength = 40,000 psi;  Elongation = 40 % in 2 in;  Hardness > Rockwell C 0 or > Rockwell B 90
  • 9. 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 platelike or lamellar mixture of ferrite and cementite.  The white ferritic background or matrix contains thin plates of cementite (dark).
  • 10. Definition of structures Pearlite  Average properties are:  Tensile strength = 120,000 psi;  Elongation = 20 % in 2 in.;  Hardness = Rockwell C 20, Rockwell B 95-100, or BHN 250-300.
  • 11. Definition of structures  Austenite is an interstitial solid solution of Carbon dissolved in  (F.C.C.) iron.  Maximum solubility is 2.14 % C at 1147°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.
  • 12. 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
  • 13. 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 has  low tensile strength (approx. 5,000 psi), but  high compressive strength.
  • 14. Definition of structures  Ledeburite is the eutectic mixture of austenite and cementite.  It contains 4.3 percent C and is formed at 1130°C.
  • 15. 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
  • 16. 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.
  • 17. Various Features of Fe-C diagram Peritectic L + d =  Eutectic L =  + Fe3C Eutectoid  = a + Fe3C Phases present L Reactions d BCC structure Paramagnetic  austenite FCC structure Non-magnetic ductile a ferrite BCC structure Ferromagnetic Fairly ductile Fe3C cementite Orthorhombic Hard brittle Max. solubility of C in ferrite=0.022% Max. solubility of C in austenite=2.11%
  • 18. Three Phase Reactions  Peritectic, at 1490 deg.C, with low wt% C alloys (almost no engineering importance).  Eutectic, at 1130 deg.C, with 4.3wt% C, alloys called cast irons.  Eutectoid, at 723 deg.C with eutectoid composition of 0.8wt% C, two-phase mixture (ferrite & cementite). They are steels.
  • 19. How to read the Fe-C phase diagram Ferrite Austenite Steel Cast iron Pearlite Pearlite and Cementine Pearlite and Carbide Eutectic eutectoid
  • 20. The Iron-Iron Carbide Diagram The diagram shows three horizontal lines which indicate isothermal reactions (on cooling / heating):  First horizontal line is at 1490°C, where peritectic reaction takes place: Liquid + d ↔ austenite  Second horizontal line is at 1130°C, where eutectic reaction takes place: liquid ↔ austenite + cementite  Third horizontal line is at 723°C, where eutectoid reaction takes place: austenite ↔ pearlite (mixture of ferrite & cementite)
  • 21. Delta region of Fe-Fe carbide diagram Liquid + d ↔ austenite
  • 22. Ferrite region of Fe-Fe Carbide diagram
  • 23. The Austenite to ferrite / cementite transformation in relation to Fe-C diagram In order to understand the transformation processes, consider a steel of the eutectoid composition. 0.8% carbon, being slow cooled along line x-x‘.  At the upper temperatures, only austenite is present, with the 0.8% carbon being dissolved in solid solution within the FCC. When the steel cools through 723°C, several changes occur simultaneously.
  • 24. 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.
  • 25. Pearlitic structure  The net reaction at the eutectoid is the formation of pearlitic structure.  Since the chemical separation occurs entirely within crystalline solids, the resultant structure is a fine mixture of ferrite and cementite.
  • 26. Schematic picture of the formation and growth of pearlite Ferrite Cementite Austenite boundary
  • 27. Nucleation & growth of pearlite
  • 28. Principal phases of steel and their Characteristics Phase Crystal structure Characteristics Ferrite BCC Soft, ductile, magnetic Austenite FCC Soft, moderate strength, non- magnetic Cementite Compound of Iron & Carbon Fe3C Hard &brittle
  • 29. 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.
  • 30. 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