Iron–Carbon (Fe–Fe₃C) Phase Diagram —

Iron–Carbon (Fe–Fe₃C) Phase Diagram — Key Points

The iron–carbon phase diagram explains how steel and cast iron structures change with carbon content and temperature under equilibrium conditions.

1. Axes
• X-axis: Carbon content (0–6.67 wt% C)
• Y-axis: Temperature (°C)

2. Important Phases
i) α-Ferrite (α):
• BCC structure
• Very low carbon solubility (~0.02% at 727 °C)
• Soft and ductile

ii) γ-Austenite (γ):
• FCC structure
• High carbon solubility (up to 2.14% at 1147 °C)
• Non-magnetic

iii)δ-Ferrite (δ):
• High-temperature BCC phase
• Exists near melting point of iron
• Cementite (Fe₃C):
• Hard, brittle iron carbide
• 6.67% carbon

iv) Pearlite:
• Lamellar mixture of ferrite + cementite
• Forms at eutectoid reaction

3. Critical Reactions
i) Eutectoid Reaction (Most Important for Steel):
• 0.76% C at 727 °C
• γ (Austenite) → α (Ferrite) + Fe₃C (Cementite)
• Product: Pearlite

ii) Eutectic Reaction:
• 4.3% C at 1147 °C
• Liquid → Austenite + Cementite
• Product: Ledeburite

iii) Peritectic Reaction:
• 0.16% C at 1493 °C
• Liquid + δ-Ferrite → Austenite

4. Composition Ranges
• Steel: 0–2.14% C
• Cast Iron: 2.14–6.67% C

5. Critical Lines
• A₁ line (727 °C): Eutectoid temperature
• A₃ line: Boundary between ferrite + austenite
• Acm line: Boundary between austenite + cementite

6. Mechanical Property Trend
• Increasing carbon content:
• ↑ Hardness & strength
• ↓ Ductility & toughness

7. Practical Importance
• Basis for heat treatment (annealing, normalizing, hardening)
• Helps predict microstructure and mechanical properties
• Foundation for understanding TTT & CCT diagrams