Fe3c

1. CHAPTER 6
• Dr. Talaat El-Benawy
Iron (Fe) - Iron Carbide(Fe3C)
Phase Diagram

2. Introduction
• The study of Fe-F3C alloying system is
important because it forms the basis of
commercial steels and cast irons. Moreover, the
basic features of this system influence the
behaviour of the most complex alloy steels
• Carbon is the most important alloying element
in iron, which significantly affects the allotropy ,
structure and properties of iron
• Conventionally, the complete Fe-C diagram
should extend from 100% Fe to 100% carbon but
it is normally studied up to around 6.67%
carbon, because iron alloys of practical
industrial importance contain not more than 4.5-
5.0 % carbon
• Therefore, this diagram is is usually called Fe-
Fe3C equilibrium phase diagram

3. • The iron carbide Fe3C is an intermetallic compound called
cementite has fixed carbon weight content can be
calculated as following.
• For A-B binary system, the weight percentages, wt%, can
be calculated if the atomic percentages X% and atomic
weights, a, are given and known for A and B alloying
elements through the following equations:
• If it is considered that A refers to the carbon, C, and B to
the iron, Fe. From the periodical table, it is known that
the atomic weights of C and Fe are aC = 12 and aFe = 56.
Then the Carbon atomic percentage of the Fe3C
intermetallic compound equals:
• Then iron atomic
percentage equals XFe
% = 75%
• Accordingly, carbon weight percent of 6.67% can be
calculated for the Fe3C from the previous equation.
Therefore, the Fe-Fe3C equilibrium phase diagram must
be plotted up to 6.67% carbon

4. Fe–Fe3C Phase Diagram

5. Phases in Fe–Fe3C Phase Diagram
• Interstitial solid solution of carbon in
BCC iron
• Stable form of iron at room
temperature.
• The maximum solubility of C is about
0.02 wt% at 727 °C which decreases to
negligible amount of about
< 0.00005% C at 20 °C.
• α-ferrite is ferromagnetic at low
temperatures and loses its magnetic
properties at 768 °C and sometimes
called β-ferrite instead α-ferrite.
• Transforms to FCC γ-austenite at
912 °C
• Soft and ductile phase.
1. α-ferrite - solid solution of C in BCC Fe

6. • Interstitial solid solution of
carbon in FCC iron.
• The maximum solubility of C is
2.11 wt % at 1147 °C which
decreases to 0.77% C at 727 °C.
• Transforms to BCC δ-ferrite at
1394 °C
• It is only stable above the
temperature of 727 °C and can
be obtained at room temperature
by adding Ni or Mn to the
composition (alloy steel ‫اﻟﺼﻠﺐ‬
‫)اﻟﺴﺒﺎﺋﻜﻲ‬
• It is soft, ductile, tough and non-
magnetic.
2. γ-austenite - solid solution of C in FCC Fe

7. 3. δ-ferrite solid solution of C in BCC Fe
• Interstitial solid solution of
carbon in BCC iron.
• Same structure as α-ferrite.
• The maximum solubility of C is
0.09 wt % at 1495 °C.
• Stable only at temperature above
1394 °C.

8. • Interstitial intermetallic compound
having a fixed carbon content of 6.67%,
as it was calculated before.
• It is metastable (not quietly stable ‫ﻟﯿﺴﺖ‬
‫ﻣﺴﺘﻘﺮه‬
‫ﺗﻤﺎﻣﺎ‬ ) phase where it
decomposes, very slowly (within
several years), into α-Ferrite and
Carbon (graphite) at 650-700 °C
• It has orthorhombic crystal structure
with 12 iron atoms with 4 carbon atoms.
• The stable phase melts at 1227 °C.
• It is slightly ferromagnetic up to
210 °C.
• It is very hard and very brittle phase
4. Cementite, iron carbide Fe3C intermetallic compound

9. 5. Fe-C liquid solution
• The melting temperature of the pure iron is at 1539 °C

10. Few comments on Fe–Fe3C system
• Maximum solubility in BCC α-
ferrite is limited about 0.02
wt% at 727 °C, it has to be
mentioned that BCC has
relatively small interstitial
positions.
• Maximum solubility in FCC γ-
austenite is 2.11 wt% at
1147 °C, it has to be
mentioned that FCC has
larger interstitial positions.
• Cementite, Fe3C is very hard
and brittle and it can
strengthen steels.
• α-ferrite is magnetic below
768 °C and austenite is non-
magnetic

11. Classification of the Fe-Fe3C phase diagram
• Very soft steel of carbon percentage of C < 0.008 wt%
• Steel with the following categories:
Three different types of ferrous alloys can be determined in
the Fe-Fe3C phase diagram as the following:
o Low carbon steel as carbon percentage from
0.008 wt% and up to less than 0.25 wt%.
o Medium carbon steel as carbon percentage from
0.25 wt% and up to less than 0.55 wt%
o High carbon steel as carbon percentage from
0.55 wt% and up to 2.11 wt%
• Cast-iron of carbon percentage more than 2.11 wt%, however,
usually carbon percentage between 2.25 to 3.75 is commonly
used for cast iron in practical usage

12. Important Reactions in Fe-Fe3C
equilibrium phase diagram
i. Peritectic Reaction:
Peritectic reaction, in general, can be represented by equation:
L , S1 and S2 represent liquid and
two different solids of fixed
composition.
In fact, Fe-0.17% C
steel is peritectic
steel because only
this steel undergoes
above reaction
completely

13. ii. Eutectic Reaction
Eutectic reaction, in general, can be represented by equation:
where L represents a liquid of fixed
composition and S1 and S2 are two
different solids of fixed composition
The shown Figure illustrates the eutectic region of Fe-Fe3C Phase Dia.
Fe-4.3% C alloy called
eutectic cast iron where the
liquid of 4.3% C undergoes
eutectic reaction at the
eutectic temperature of
1147 °C to give a mixture
of two different solids
namely: γ-austenite (2.11%
C) and cementite, Fe3C
(6.67% C), solidifying
simultaneously. This
eutectic mixture is called
Ledeburite.

14. iii. Eutectoid Reaction
Eutectic reaction, in general, can be represented by equation:
where S1, S2 and S3 are three
different solids each of fixed
composition
The shown Figure illustrates the eutectoid region of Fe-Fe3C phase
diagram

15. During cooling, γ-austenite of 0.77% C at constant
eutectoid temperature of 727 °C undergoes eutectoid
transformation to form a mixture of α-ferrite of 0.02%
carbon and cementite, Fe3C, of 6.67% carbon in the form
of alternate lamellae of both α-ferrite and cementite. This
mixture is called pearlite because its pearly appearance
under optical microscope

16. Introduction to Development
Microstructures in the Fe-Fe3C
• The term microstructure refers to the details of a
microphotograph of metal (alloy) or similar image which is
taken through a microscope.
• An alloy normally requires metallographic preparation before its
microstructure can be seen through .
• The development microstructures of in the Fe-Fe3C alloys are
mainly depending on composition (carbon content) and heat
treatment (this will be explained in later lecture).
• Generally, the microstructure of an alloy consists of the
structure of the grains and phases, which the alloy possesses
• Microstructure of the ferrous alloys are the main parameter
affecting in most of the ordinary properties.

17. Fe–Fe3C Phase Diagram

18. Developed Microstructure due to
Peritectic Reaction
• As cooling continues more δ-ferrite solidifies as shown at points 2 and 3.
• At any temperature, lever rule helps to calculate the fraction of δ-ferrite
and liquid.
• The compositions of δ-ferrite changes with further fall of temperature.
• When the peritectic temperature of 1495 °C, is just reached, the liquid has
composition of 0.53% C and δ-ferrite has a composition of 0.09% carbon.
Therefore, the alloy undergoes the peritectic reaction completely, i.e. the
δ-ferrite reacts with the liquid to give one solid γ-austenite solution of
0.17% C as shown at point Y.

19. When γ-austenite alloy of of 0.77wt% C, is cooled slowly to the eutectoid
temperature of 727°C, it undergoes eutectoid transformation where a mixture
of layered structure of two phases α-ferrite and cementite, Fe3C, is developed.
Developed Microstructure due to
Eutectoid Reaction
This mixture is
called pearlite
because its pearly
appearance under
optical
microscope
Microstructure depends on the composition (carbon
content) as following:
i. Alloy with the eutectoid composition (0.77% C)

21. ii. Alloy with hypoeutectoid composition (0.02 - 0.77 % C)
Compositions to the left of eutectoid (0.02-0.77wt% C) is called
hypoeutectoid alloys. Where the following reactions is occurred
while the temperature is decreased:

22. The final microstructure of the hypoeutectoid alloys contain
proeutectoid α-ferrite which is formed above the eutectoid
temperature, plus the eutectoid pearlite mixture of α-ferrite and
cementite, Fe3C as shown in the figure. In the given micrograph,
the dark areas are the layers of the pearlite and the light phase is
the proeutectic α-ferrite

23. Compositions to the right of eutectoid (0.77-2.11wt% C) is called
hypereutectoid alloys. Where the following reactions is occurred
while the temperature is decreased as shown in the figure:
iii. Alloy with hypereutectoid composition (>0.77% C)

24. The final microstructure of the hypereutectoid alloys
contain proeutectoid cementite which is formed above the
eutectoid temperature, plus the eutectoid pearlite mixture
of α-ferrite and cementite, Fe3C. In the shown micrograph,
the dark areas are the layers of the pearlite and the light
phase is the proeutectic cementite, Fe3C

25. Calculation of the Relative Amounts of
Proeutectoid Phases
The relative amounts of the proeutectoid phases α-ferrite or Fe3C
as well as the pearlite can be calculated for steels of composition
C0 and C1 by applying the lever rule as shown in the figure:
First, draw the tie line
between α and pearlite
(α + Fe3C) that extends
from the eutectoid
composition of 0.77% C
to αboundary of 0.02% C
for hypoeutectoid alloys
and the tie line between
pearlite and Fe3C that
extends from the
eutectoid composition of
0.77% C to Fe3C
boundary of 6.67%C, for
hypereutectoid alloys

26. Fraction of total α phase (proeutectoid phase and the one that
existed in pearlite mixture) is determined by application of the
lever rule between α and Fe3C phase. Example of the calculations
made for hypereutectoid alloy of composition C1 that is shown as
follows:

27. Fraction of pearlite:
Fraction of proeutectoid
cementite:
Fraction of total α-ferrite:
Fraction of total cementite:

29. Example 1
Determine and show the transformation
experienced by slowly cooling plain carbon
steel containing 0.13% C and 0.7% from the
liquid stat to γ-austenite phase.

33. Example 2
Determine and calculate the amounts of
phases in Fe-0.35% C alloy just above and just
below 727 °C. For the same alloy calculate the
total amount of the existed phases below
727 °C

37. Example 3
Calculate the amounts of α-ferrite and
cementite phases in pearlite mixture of the
eutectoid alloy

39. Example 4
Determine and calculate the amounts of
phases in Fe-1.25% C alloy just above and just
below the eutectoid temperature (727 °C)

41. Example 5
Slowly cooled plain carbon steel shows
proeutectoid ferrite to be 15% by the weight of
the microstructure. Estimate the carbon
percent of the steel

43. Home Work
Problem 1:
The given above figures indicate
the following:
1) Eutectoid reaction
2) (Liquid + g) zone
3) (Ledeburite + Fe3C) zone
4) (Pearlite + a-ferrite) zone
5) (g + Fe3C) zone
6) Eutectic reaction
7) (Ledeburite + g) zone
8) d phase zone
9) (Pearlite + Fe3C) zone
10)Peritectic reaction
It is required to the put
corresponding correct number
below each given figure

44. Problem 2:
The given figures indicate the following:
1) Microstructure of hypoeuectoid steel.
2) Tempered Martensite.
3) Microstructure of hypereuectoid
steel.
4) Martensite microstructure.
5) Microstructure of eutectoid steel.
6) γ-austenite microstructure.
It is required to the put corresponding
correct number below each figure.
‫ھﺎﻣﺔ‬ ‫ﻣﻠﺣوظﺔ‬
:
‫ﺗﺗﻌرض‬ ‫إﻻ‬ ‫و‬ ‫اﻟﺷﻛل‬ ‫ﺗﺣت‬ ‫ﻧوع‬ ‫أي‬ ‫ﻣن‬ ‫ﻛﺗﺎﺑﺔ‬ ‫أي‬ ‫ﺗﺿﻊ‬ ‫ﻻ‬
‫اﻟﺷﻛل‬ ‫ﺗﺣت‬ ‫اﻟرﻗم‬ ‫ﺿﻊ‬ ‫ﻓﻘط‬ ،‫اﻟﺳؤال‬ ‫درﺟﺔ‬ ‫ﻟﺧﺻم‬
‫اﻟﻣرادف‬

45. Problem 3
Slowly cooled plain carbon steel shows
proeutectoid ferrite to be 10% by the weight of
the microstructure. Estimate the carbon percent
of the steel

46. Problem 4
Put sign )
√
( in the front of the correct answer: ‫ﻋﻼ‬ ‫ﺿﻊ‬
‫ﻣﺔ‬
)
√
(
‫اﻟﺻﺣﯾﺔ‬ ‫اﻹﺟﺎﺑﺔ‬ ‫أﻣﺎم‬
Steel can be existed only at carbon content:
(………………) less than 0.53 wt% (………………) less than 2.11 wt%
(………………) more than 2.11 wt% (………………) more than 0.09 wt%
Pearlite mixture consisting of layers from:
(………………) g-ferrite and Cementite (………………) g-austenite and a-ferrite
(………………) d-ferrite and Cementite (………………) a-ferrite and Cementite
The equation of Liquid phase Solid phase 1 + Solid phase 2 represents:
(………………) ledburite reaction (………………) eutectoid reaction
(………………) peritectic reaction (………………) eutectic reaction
For hypoeutectoid steel the existed proeutectoid phase is:
(………………) g-austenite (………………) d- ferrite
(………………) Fe3C (………………) a- ferrite
For hypereutectoid steel the existed proeutectoid phase is:
(………………) g-austenite (………………) d- ferrite
(………………) Fe3C (………………) a- ferrite

47. Problem 5
Put the corresponding number in the circles of the given figures, which
contains different transformation reaction and different important lines: