The Iron-Iron Carbide Phase Diagram

Topics: Iron, Phase diagram, Austenite Pages: 25 (2641 words) Published: October 3, 2014
The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram


In their simplest form, steels are alloys of Iron (Fe) and Carbon (C).



The Fe-C phase diagram is a fairly complex one, but we will only consider the steel part of the diagram, up to
around 7% C b
d
Carbon.



Phases present
α-ferrite,
γ-ferrite,
δ-ferrite,
Fe3C (iron carbide
or cementite)
Fe-C liquid solution

School of Mechanical and Building Sciences, VIT University, Vellore

1

Phases in Fe–Fe3C Phase Diagram
α-ferrite - solid solution of C in BCC Fe
• Stable form of iron at room temperature.
• The maximum solubility of C is 0.022 wt%
• T
Transforms t FCC γ-austenite at 912 °C
f
to
t it t
γ-austenite - solid solution of C in FCC Fe

Fe3C (iron carbide or cementite)


This

intermetallic

compound

is

metastable, it remains as a compound
indefinitely at room T, but decomposes
(very slowly, within several years) into αFe and C (graphite) at 650 - 700 °C

• The maximum solubility of C is 2.14 wt %.
• Transforms to BCC δ-ferrite at 1395 °C
• Is

not

stable

below

the

Fe-C liquid solution

eutectic

temperature (727 °C) unless cooled
rapidly
δ-ferrite - solid solution of C in BCC Fe
• The same structure as α-ferrite
• Stable only at high T, above 1394 °C
• Melts at 1538 °C

α-ferrite

School of Mechanical and Building Sciences, VIT University, Vellore

austenite
2

1

Changes in Crystal Structure


Pure iron when heated experiences two changes in crystal structure before it melts.



At room temperature the stable form, ferrite (α iron) has a BCC crystal structure.



Ferrite experiences a polymorphic transformation to FCC austenite (γ iron) at 912 ˚C (1674 ˚F).



At 1394˚C (2541˚F) austenite reverts back to BCC phase δ ferrite and melts at 1538 C (2800˚F) 1394 C (2541 F)
1538˚C (2800 F).



Iron carbide (cementite or Fe3C) an
intermediate compound is formed
at 6.7 wt% C.



Typically, all steels and cast irons have
carbon contents less than 6.7 wt% C.



Carbon is an interstitial impurity in iron
and forms a solid solution with the
α, β, γ phases.

School of Mechanical and Building Sciences, VIT University, Vellore

3

A few comments on Fe–Fe3C system


C is an interstitial impurity in Fe. It forms a solid solution with α, β, γ phases of iron



Maximum solubility in BCC α-ferrite is limited (max. 0.022 wt% at 727 °C) – which can be p
y
p
p
,
explained by the shape and size of the BCC interstitial positions, which make it difficult to accommodate the carbon atoms. BCC has relatively small interstitial positions. Even though present in relatively low concentrations, carbon significantly influences the mechanical properties of ferrite



Maximum solubility in FCC austenite is 2.14 wt% at 1147 °C - FCC has larger interstitial positions



Mechanical properties: Cementite is very hard and brittle - can strengthen steels. Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed.



Magnetic properties: α -ferrite is magnetic below 768 °C, austenite is non-magnetic School of Mechanical and Building Sciences, VIT University, Vellore

4

2

Classification - Types of ferrous alloys


Three types of ferrous alloys:
– Iron:
• less than 0.008 wt % C in α−ferrite at room T
– Steels:
• 0.008 - 2.14 wt % C (usually < 1 wt % );
• α-ferrite + Fe3C at room T
– Cast iron:
• 2.14 - 6.7 wt % (usually < 4.5 wt %)

School of Mechanical and Building Sciences, VIT University, Vellore

5

Invariant Reactions in Fe–Fe3C System


In binary phase diagrams, a horizontal line always indicates an invariant reaction.



Three invariant reactions are present in Iron–Iron Carbide (Fe–Fe3C) Phase Diagram.

1. Peritectic reaction

δ

γ

1493 °C

2. Eutectic reaction

1150 °C

3. Eutectoid reaction

727 °C

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