Mechanical Magic Mechanical Learning Tutorials

Topic: Iron Carbon Diagram

Key Reactions in Iron Carbon Diagram

1. Peritectic Reaction

   • Occurs at high temperatures (~1493°C)
   • Low carbon content (0.18% C)
   • Reaction: Liquid + δ-iron → γ-austenite (100% solid steel)
   • Above 723°C

2. Eutectoid Reaction

   • After peritectic reaction, γ-austenite forms
   • Cooling at ~723°C with 0.8% carbon
   • Reaction: γ-austenite → α-ferrite + Fe₃C (cementite)
   • Forms pearlite (mixture of α-ferrite + cementite)

3. Eutectic Reaction

   • Occurs at ~1147°C
   • 4.3% carbon
   • Reaction: Liquid → γ-austenite + Fe₃C
   • Forms ledeburite (mixture of γ-austenite + cementite)

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Drawing the Iron Carbon Diagram

• Axes:

  • X-axis: Weight percentage of carbon
  • Y-axis: Temperature (°C)

• Key Points on X-axis (Carbon %):

  • 0.0008%
  • 0.18%
  • 0.8%
  • 4.3%
  • 6.67%

• Key Points on Y-axis (Temperature in °C):

  • 723°C
  • 1147°C
  • 1493°C

Important Points

• Peritectic Point

  • 0.18% carbon, 1493°C
  • Horizontal line up to 0.5% carbon

• Eutectoid Point

  • 0.8% carbon, 723°C
  • Horizontal line at 0.025% carbon

• Eutectic Point

  • 4.3% carbon, 1147°C
  • Horizontal line up to 2% carbon

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Phases in Iron Carbon Diagram

• Liquidus Line

  • 100% liquid phase

• Peritectic Reaction Phase Change

  • Liquid + δ → γ-austenite
  • Forms δ at high, α-ferrite at lower temperatures

• Eutectoid Transformation

  • γ-austenite → α-ferrite + cementite (pearlite)
  • No α-ferrite after 0.8% carbon

• Eutectic Transformation

  • Liquid → γ-austenite + cementite (ledeburite)
  • Divides into γ-austenite + ledeburite, ledeburite + Fe₃C

Other Concepts

• Hypo-Eutectoid Reaction

  • <0.8% carbon, forms before eutectoid point

• Hyper-Eutectoid Transformation

  • 0.8% to 2% carbon
  • Forms after eutectoid point

• Hypo-Eutectic Transformation

  • 2% to 4.3% carbon

• Hyper-Eutectic Transformation

  • 4.3% to 6.67% carbon

• Steel Category

  • Up to 2% carbon

These notes cover the essential points of drawing and understanding the iron-carbon phase diagram.