The Iron-Iron Carbide Phase Diagram
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
•
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