Lecture Notes: Iron-Carbon Phase Diagram

Introduction

• Focus on the Iron-Carbon phase diagram, crucial for material scientists.
• Importance: Provides insights into steel formation.

Key Reactions in the Iron-Carbon Phase Diagram

Eutectic Reaction

• Location: At 4.3 wt% Carbon, cooled at 1147°C.
• Reaction: Liquid → Gamma (Austenite) + Fe3C (Cementite).

Peritectic Reaction

• Location: Top left corner at 1493°C.
• Reaction: Liquid + Delta → Austenite.

Eutectoid Reaction

• Location: At 0.76 wt% Carbon.
• Reaction: Gamma (Austenite) → Alpha (Ferrite) + Fe3C.

Steel and Iron Compositions

Hypoeutectoid and Hypereutectoid

• Hypoeutectoid: Less than 0.76% Carbon.
• Hypereutectoid: More than 0.76% Carbon.

Pure Iron

• Very low carbon content (<0.008 wt%).
• Ductile, flexible (used for iron wire).

Steel

• Carbon content between 0.008 and 2.14 wt%.
• Regions:
  • Low Carbon Steel: More ductile, less hard.
  • Medium Carbon Steel: Balance of ductility and strength.
  • High Carbon Steel: Strong but less ductile.
  • Ultra High Carbon Steel: Very strong, brittle, often in cast irons.

Cast Iron

• Higher carbon content (up to 6.7 wt%, typically around 4.5).
• Less ductile, more brittle.

Microstructure Transformations

Lamellar Structure Formation

• Occurs below 727°C after eutectoid reaction.
• Combination of ferrite and cementite.
• Hypoeutectoid: Ferrite forms along grain boundaries.
  • Reason: Reduces energy by replacing existing grain boundary surfaces.
• Hyper Eutectoid: Fe3C forms instead of ferrite.

Cooling Process

• Above Eutectoid: Austenite forms throughout.
• Below Eutectoid: Mixture of ferrite and cementite arises, creating lamellar structure.

Pre-eutectoid and Eutectoid Phases

• Pre-eutectoid Phase: Ferrite forms before eutectoid transformation.
• Eutectoid Phase: Formation of lamellar structure in austenite.

Practical Implications

• Low Carbon Steels: Low cost, easy to shape, not very hard.
• Medium Carbon Steels: Good wear resistance.
• High Carbon Steels: Very strong, used for high strength applications.

Conclusion

• The iron-carbon phase diagram illustrates the transformations and properties of iron and steel alloys based on carbon content.
• Future Topics: Kinetics and microstructure variations like pearlite, bainite, and spheroidite.