Description of the phase diagram,invariant reactions of Fe-FeC. Reference:Material Science and Engineering,William Callister

1. IRON-CARBON SYSTEM

2. IRON- IRON CARBIDE SYSTEM (FE - FE₃C DIAGRAM)
 Both steels and cast irons, primary
structural materials in every
technologically advanced culture, are
essentially iron–carbon alloys.
 Iron-carbon phase diagram shown in
figure is not a complete diagram.
 Part of the diagram after 6.67 wt% C is
ignored as it has little commercial
significance.
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Fe-C Phase diagram

3. MTE/III SEMESTER/MSE/MTE 2101 3
Pic Courtesy: Material Science and Engineering,
CallisterIron – Iron carbide phase diagram

4.  The left vertical axis represents 100% iron and shows
all the allotropic changes of pure iron with
temperature.
 The temperature at which allotropic changes occur in
iron changes when it is alloyed with different
amounts of carbon.
 The right vertical axis does not represent 100%
carbon but instead represents only 6.67% C by
weight.
 This is because only a maximum of 6.67% C can be
added to molten iron at which it becomes saturated.
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5.  Any further addition of carbon will not dissolve in iron but rather floats or gets blown away owing to its
very low density. Iron when contains exactly 6.67% C by weight forms an intermediate phase called
Cementite or Iron Carbide (Fe3C).
 Hence, the Iron-Carbon equilibrium diagram is actually called as the Iron – Iron Carbide Equilibrium
Diagram with pure iron and pure Fe3C (Iron Carbide) forming the extremities.
 Actually, the phase Fe3C (Iron Carbide) is called meta stable state because it decomposes with passage
of time.
 Here, the equilibrium phase diagram for iron and carbon assumes that Fe3C is stable with respect to
time. Hence, it is not a true equilibrium diagram.
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6. Characteristics of Fe-C Diagram
 The Fe-Fe₃C is characterized by five individual phases and three invariant reactions.
 Five phases that exist in the diagram are: α–ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C
solid solution, δ-ferrite (BCC) Fe-C solid solution, Fe₃C (iron carbide) or cementite - an inter-metallic
compound and liquid Fe-C solution.
 Three invariant reactions that cause transformations in the system are namely: eutectoid, eutectic, and
peritectic.
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7.  When iron solidifies first, at 1538oC, it is in the BCC
 form called ‘Delta – Iron’.
 Further cooling at 1394oC, a phase change occurs
and atoms re-arrange themselves into FCC  form
called as ‘Gamma Iron’, which is non-magnetic.
 When the temperature reaches 912oC, another
phase change occurs and atoms begin to re-
arrange themselves into  form called as ‘Alpha
Iron’, which is non-magnetic.
 Finally at 768oC, the -iron becomes magnetic
without a change in the crystal lattice structure.
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8. MTE/III SEMESTER/MSE/MTE 2101 8
Cooling Curves of Fe – C Diagram

9. The diagram shows 3 horizontal lines (isotherms)
which represent 3 invariant reactions:
(a) The first isotherm at 1493oC indicates the
peritectic reaction. This region where the peritectic
reaction takes place is called the Delta region.
(b) The second isotherm at 1147oC indicates the
eutectic reaction. This region where the eutectic
reaction takes place is called the Eutectic region.
(c) The third isotherm at 727oC indicates the
eutectoid reaction. This region where the eutectoid
reaction takes place is called the Eutectoid region.
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10.  Carbon is an interstitial impurity in iron and forms a solid solution with each of α and δ ferrites, and also
with austenite, as indicated by the α,γ and δ single phase fields.
α - Ferrite Phase (912ᵒC-768ᵒC):
 In the BCC α ferrite, only small concentrations of carbon are soluble; the maximum solubility is 0.022
wt% at 727°C.
 The interstitial position in BCC, α – Ferrite is very small to accommodate carbon atoms. It is because of
this reason the solubility is less .
 This particular iron–carbon phase is relatively soft, may be made magnetic at temperatures below 768°C
and has a density of 7.88 g/cm³.
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11. γ – Austenite Phase (1394ᵒC-912ᵒC):
 The austenite, or γ phase of iron, when alloyed with carbon alone, is not stable below 727°C.
 The maximum solubility of carbon in austenite, 2.14 wt%, occurs at 1147°C.
 This solubility is approximately 100 times greater than the maximum for BCC ferrite, since the FCC
interstitial positions are larger, and, therefore, the strains imposed on the surrounding iron atoms are
much lower.
 Austenite is nonmagnetic.
δ Phase (1538ᵒC-1394ᵒC):
 The δ ferrite is virtually the same as ferrite, except for the range of temperatures over which each
 δ ferrite is stable only at relatively high temperatures,
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12. Cementite (Fe₃C):
 It is formed when the solubility limit of carbon in α ferrite is exceeded below 727°C (for compositions
within the α+Fe₃C phase region).
 Fe₃C will also coexist with the phase between 727°C and 1341° C.
 Mechanically, cementite is very hard and brittle; the strength of some steels is greatly enhanced by its
presence.
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13. THE INVARIANT REACTIONS IN THE PHASE DIAGRAM
(1) THE DELTA REGION:
 The first horizontal line MB at 1495oC indicates the Peritectic
reaction. This region is called as the delta region because of
the solid solution .
 The point P is known as the peritectic point at 1495oC &
0.18%C.
 The peritectic reaction may be written as:
Cooling
Liquid +   (Austenite)
Heating
 The maximum solubility of carbon in BCC  Fe is 0.1% (point
M).
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14. (2) THE EUTECTIC REGION:
 The Eutectic reaction takes place at 1148oC.
 The Eutectic point ‘E’ is at 1148oC and at 4.33% C.
 Whenever an alloy crosses the line line CED, and Eutectic
takes place, giving rise to a fine mixture of Austenite + Cementite
( + Fe3C).
 This eutectic mixture is commonly called a Ledeburite.
 This reaction can be expressed as:
Cooling
Liquid (L) Austenite (γ) + Cementite (Fe₃C)
Heating
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15. (3) THE EUTECTOID REACTION:
 A small solid solution region to the left of the line GH, which
-Fe (Ferrite), is a solid solution with small amounts of carbon
in BCC Fe.
 At 912oC, the FCC -Fe changes to BCC -Fe.
 Line HJK represents the Eutectoid reaction taking place at 727oC.
 The Eutectoid point is ‘J’ which has 0.8%C & 727oC.
 Whenever an alloy crosses the line HJK, the Eutectoid reaction takes
place giving rise to the fine Eutectoid mixture of Ferrite + Cementite
( + Fe3C) commonly known as Pearlite.
 The reaction is :
Cooling
Austenite (γ) Ferrite (α )+ Cementite (Fe3C)
HeatingMTE/III SEMESTER/MSE/MTE 2101 15

16. MTE/III SEMESTER/MSE/MTE 2101 16
PHASES PRESENT:
PHASES STRUCTURE PROPERTY
Liquid Phase (1538ᵒC) - -
δ Phase (1538ᵒC-1394ᵒC) BCC Structure -
γ – Austenite Phase (1394ᵒC-912ᵒC) FCC Structure Non-magnetic property, Fairly ductile
α - Ferrite Phase (912ᵒC-768ᵒC) BCC Structure Non-magnetic, Ductile
α - Ferrite Phase(768ᵒC) BCC Structure Magnetic
REACTIONS TAKING PLACE:
i. Peritectic: L+δ=γ
ii. Eutectic: L= γ+Fe₃C
iii. Eutectoid: γ = α + Fe3C
Three types of ferrous alloys:
i. Iron: less than 0.008 wt % C in α−ferrite at room T
ii. Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ): α-ferrite + Fe₃C at room
T
iii. Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)