- Practice · 8 marks
Types of cast iron
Short answer
Grey iron: graphite flakes, good damping, used for machine beds. White iron: cementite, hard and brittle, wear-resistant. Malleable iron: temper carbon rosettes, ductile, from heat-treated white iron. Ductile iron: graphite spheres, high strength and ductility, for crankshafts. Cast iron is preferred for machine beds due to vibration damping, castability, and compressive strength.
Describe the four main types of cast iron (grey, white, malleable and ductile) with their microstructure, properties and applications. Why is cast iron preferred for machine-tool beds?
Cast iron is an iron-carbon alloy with 2.14-6.67% C (usually 2.5-4%) and 1-3% Si. Carbon may be present as free graphite or as cementite, depending on composition and cooling rate.
| Type | Carbon form | Matrix | Properties | Uses |
|---|---|---|---|---|
| Grey | Graphite flakes | Ferrite and/or pearlite | Good castability and machinability, excellent damping, high compressive strength, brittle in tension (150-400 MPa) | Machine beds, engine blocks, brake drums |
| White | Cementite (no graphite), ledeburite | Pearlite + cementite | Very hard, brittle, wear resistant, difficult to machine | Wear-resistant liners, grinding balls, base for malleable iron |
| Malleable | Temper carbon (rosettes) | Ferrite or pearlite | Good ductility, toughness and strength; made by heat treating white iron | Pipe fittings, connecting rods, hand tools |
| Ductile (nodular, SG) | Graphite spheres | Ferrite, pearlite or both | Strength like steel (400-800 MPa), good ductility, wear resistance | Crankshafts, gears, pipes, valves |
Production notes
- Grey iron: slow cooling with high C and Si, which promote graphite.
- White iron: rapid cooling (chilling) and low Si keep carbon as cementite.
- Malleable iron: white iron annealed at about 900 °C for 2 days, which decomposes cementite into temper carbon.
- Ductile iron: Mg (or Ce) added to the melt before casting turns graphite flakes into spheres, which removes the sharp stress-raising edges.
Machine-tool beds
Grey cast iron absorbs vibration (graphite flakes damp vibrations), is cheap to cast in complex shapes, machines easily, wears well on slide ways and has high compressive strength.
✓ Answer checked by Claude Opus 5.5 (high effort)
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