Cast Iron

What is it?

Think of Cast Iron as steel that ate too much Carbon.

• Composition: Iron + 2-4.5% Carbon + 1-3% Silicon.
• The Matrix: Usually Ferrite or Pearlite (like steel).
• The Difference: The excess Carbon cannot remain dissolved, so it precipitates out as a second phase: Graphite or Iron Carbide (Cementite) ($F{e}_{3}C$).

• Pros:
  • Low melting point (easy to cast)
  • High fluidity (fills complex moulds)
  • Low shrinkage
  • Easy to machine.
• Cons:
  • Generally brittle
  • Has low impact resistance compared to steel.

Type	Carbon Form	Silicon Content	Key Property
White	Iron Carbide ($F{e}_{3}C$)	Low (0.5-1.5%)	Hard, Wear Resistant, Brittle
Gray	Flakes	High (1-3%)	Vibration Damping, Brittle in Tension
Ductile	Spheres (Nodules)	Med-High (1.8-2.8%) + Mg	Tough, Ductile, Strong
Malleable	Clusters (Rosettes)	Med (1.1-1.6%)	Good Toughness (Heat Treated)

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White Cast Iron

This is the “primitive” form. It forms when cooling is rapid or Silicon is low.

• Composition: Low C (2.5-3%) and Low Si (0.5-1.5%).
• Structure: Carbon does not create graphite. Instead, it forms Cementite.
• Appearance: Fracture surface looks white/shiny (crystalline).
• Properties:
  • Extremely hard and wear-resistant (due to carbides).
  • Extremely brittle. You cannot machine this; you must grind it.
• Use: Rock crushers, slurry pumps, grinding balls.

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Gray Cast Iron

This is the most common type. It forms when cooling is slow and Silicon is high (Si is a graphite stabilizer).

• Composition: C (2.5-4%) and High Si (1-3%).
• Structure: Carbon precipitates as Graphite Flakes.
• Appearance: Fracture surface looks gray (due to the graphite).
• Properties:
  • Weak in Tension: The sharp tips of the flakes act as stress concentrators (pre-existing cracks).
  • Strong in Compression.
  • Vibration Damping: The internal graphite flakes absorb energy. This is why heavy lathe beds and engine blocks are made of Gray Iron.
  • Excellent machinability (graphite acts as a lubricant).

Above is the result of chemical etching on a sample of cast iron.

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Ductile (Nodular) Cast Iron

The “We want the graphite, but we hate the flakes” cast iron.

• The Trick: We add a “nodulizing agent” (usually $Mg$ or $Ce$) to the molten mix.
• Structure: The graphite forms into Perfect Spheres (Nodules) rather than flakes. This is often called a Bull’s eye microstructure (Graphite sphere surrounded by ferrite/pearlite).
• Composition: C (3-4%) and Si (1.8-2.8%).
• Properties:
  • The spheres do not concentrate stress.
  • Result: High strength, High Toughness, and Ductility.
  • It behaves much more like steel but retains the castability of iron.
• Use: Crankshafts, gears, heavy-duty suspension parts.

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Malleable Cast Iron

• Process:
  1. Cast the part as White Iron (hard, brittle).
  2. Heat treat it (Annealing/Malleablizing) for a long time.
  3. The Iron Carbide decomposes into Irregular Graphite Clusters (Rosettes).
• Composition: C (2-2.6%) and Si (1.1-1.6%).
• Properties: Good toughness and ductility (hence “Malleable”).
• Use: Pipe fittings, tools, railway parts.