ron carbide diagram

1. 33
Dr. Mohamed Almalki
ME 204
2020
(Fe–Fe3C)

2. 34
The Iron–Carbon System
All binary alloy systems, the one that is possibly the most important is that for iron and carbon.
Both steels and cast irons, primary structural materials in every technologically advanced culture,
are essentially iron–carbon alloys
THE IRON–IRON CARBIDE (Fe–Fe3C) PHASE DIAGRAM
A portion of the iron–carbon phase diagram is presented in Figure 24. Pure iron, upon heating,
experiences two changes in crystal structure before it melts. At room temperature the stable form,
called ferrite, or iron, has a BCC crystal structure.
Ferrite experiences a polymorphic transformation to FCC austenite, or iron, at
This austenite persists to at which temperature the FCC austenite reverts back to a
BCC phase known as ferrite, which finally melts at
All these changes are apparent along the left vertical axis of the phase diagram.
Dr. Mohamed Almalki
2020

3. 35
Figure 24. The iron–iron carbide phase diagram.
35
Peritectic Reaction
Eutectoid Reaction Solid (γ ) ---------- Solid (α) + Solid (Fe3C)
Solid (δ) + Liquid (L)------- solid ( γ)
Eutectic Reaction Liquid (L)-----Solid ( ) + Solid (Fe3C)

(ledeburite)
(Pearlite)

4. 36
Dr. Mohamed Almalki
2020
0.04
0.8
L+ Ƌ
γ+ Ƌ 0.5% C
0.10
1493 °C
0.18%C

5. 37
The solubility of both C and N in austenite should be greater than in ferrite, because of the larger
interstices available. It is, therefore, reasonable to expect that during simple heat treatments,
excess carbon and nitrogen will be precipitated.
The atomic sizes of carbon and nitrogen are sufficiently small relative to that of iron to allow
these elements to enter the α- iron and & gamma- iron lattices as interstitial solute atoms.
In contrast, the metallic alloying elements such as manganese, nickel and chromium have much
larger atoms, i.e. nearer in size to those of iron, and consequently they enter into substitutional
solid solution.
Dr. Mohamed Almalki
2020
The alloys containing les than 2% C are know as steels
The alloys containing more than 2% C are know as cast irons

6. 38
Phases in Fe-C system
Phase Symbol Description
Liquid L Liquid solution of Fe and C
-Ferrite  Interstitial solid solution of C in
-Fe (high temperature, BCC phase)
Austenite  Interstitial solid solution of C in
-Fe (FCC phase of Fe)
Ferrite  Interstitial solid solution of C in
-Fe (room temperature BCC phase)
Soft and Ductile
Cementite Fe3C Intermetallic compound of Fe and C
(orthorhombic system) Hard and Brittle
Dr. Mohamed Almalki
2020

7. 39
Ferrite
Austenite
Figure 25. Photomicrographs of ferrite, pearlite and austenite.
Dr. Mohamed Almalki
2020
Pearlite

8. 40
Invariant Reactions in Fe-C system
Peritectic Reaction
)
%
18
.
0
(
)
%
5
.
0
(
)
%
1
.
0
( 1493
C
wt
C
wt
L
C
wt C
o

 
 


A horizontal line always indicates an invariant reaction in binary phase diagrams
Eutectic Reaction
)
%
67
.
6
(
)
%
1
.
2
(
)
%
3
.
4
( 3
1150
C
wt
C
Fe
C
wt
C
wt
L C
o


 
 
Eutectoid Reaction
)
%
67
.
6
(
)
%
02
.
0
(
)
%
8
.
0
( 3
725
C
wt
C
Fe
C
wt
C
wt C
o


 
 

Dr. Mohamed Almalki
2020
δ γ

9. 41
9
Eutectoid Reaction
C
Fe
C
o
3
725


 
 

0.8 0.02 6.67
cool
Pearlite
Dr. Mohamed Almalki
2020
Figure 26

10. 42
Eutectoid Reaction
C
Fe
C
o
3
725


 
 

0.8 0.02 6.67
cool
Pearlite
Ammount of Fe3C in Pearlite
Red Tie Line below eutectoid temp
117
.
0
65
.
6
78
.
0
02
.
0
67
.
6
02
.
0
8
.
0
3





pearlite
C
F
f
Dr. Mohamed Almalki
2020

11. 43
Figure 27. Schematic representation of the formation of pearlite from austenite; direction of carbon diffusion indicated by arrows.
Dr. Mohamed Almalki
2020

12. 44
Hypoeutectoid Alloys
Figure 28. Schematic representations of the microstructures for an iron–
carbon alloy of hypoeutectoid composition (containing less than 0.76 wt% C)
as it is cooled from within the austenite phase region to below the eutectoid
temperature.
Dr. Mohamed Almalki
2020
 Curve MN represents the carbon change % (phase change) from
ferrite to austenite.
 Curve MO represents the carbon change % (phase change) from
austenite to ferrite.
 Cooling from point d to e, the fraction of α phase increases, the α
particles will have grown larger
 The phases are determined by constructing a tie line at the eutectic TE
temperature, the α phase will contain 0.022 wt% C, while the γ phase
will be of the eutectoid composition, 0.76 wt% C.
α+γ

13. 45
Proeutectoid
Ferrite
Pearlite
Figure 29. Photomicrograph of a 0.38 wt% C steel having a microstructure consisting of pearlite and proeutectoid ferrite.
Dr. Mohamed Almalki
2020
Micrograph at point F

14. 46
Tie-Line above the eutectoid temperature TE
Dr. Mohamed Almalki
2020
54
.
0
78
.
0
42
.
0
2
.
0
8
.
0
38
.
0
8
.
0





pearlite
f
0.02
ferrite
0.022 0.8
0.38
α Eutectoid point
(pearlite)

15. 47
Lever rule expression for computation of pearlite mass fraction composition
Lever rule expression for computation of proeutectoid ferrite mass fraction
Figure 30. A portion of the Fe–Fe3C
phase diagram used in computations for
relative amounts of proeutectoid and
pearlite microconstituents
Dr. Mohamed Almalki
2020

16. 48
Hypereutectoid Alloys
Figure 31. Schematic representations of
the microstructures for an iron–carbon
alloy of hypereutectoid composition C1
(containing between 0.76 and 2.14 wt%
C), as it is cooled from within the austenite
phase region to below the eutectoid
temperature.
Dr. Mohamed Almalki
2020
The composition of the austenite phase will
move along line PO toward the eutectoid
As the temperature is lowered through the
eutectoid to point i, all remaining austenite
of eutectoid composition is converted into
pearlite

17. 49
Figure 32. Microstructure of a hypereutectoid steel, 1.4 wt. % C
Proeutectoid cementite
on prior austenite grain
boundaries
Pearlite
Dr. Mohamed Almalki
2020

18. 50
10
.
0
87
.
5
6
.
0
8
.
0
67
.
6
8
.
0
4
.
1





cementite
id
proeutecto
f
Dr. Mohamed Almalki
2020
10
.
0
87
.
5
6
.
0
8
.
0
67
.
6
8
.
0
4
.
1





cementite
id
proeutecto
f
Pearlite
6.67 1.4 5.27
0.898
0.8 6.67
1.4 Fe3C

19. 0.022 6.70
0.35
α Fe3C

20. 0.022 6.70
0.35
α Fe3C
0.76

21. 51
Eutectoid
steel
+Fe3C
Pearlite
Hypoutectoid
steel
+Fe3C
Pearlite + proeutectoid
ferrite
Hypereutectoid
steel
+Fe3C
Pearlite + proeutectoid
cementite
Dr. Mohamed Almalki
2020
Figure 33