Cast Iron Properties
Cast iron is cheap, castable, and excellent in compression. Learn about grey, ductile, and white cast iron — their properties and industrial applications.
Overview
Cast iron is an iron-carbon alloy with carbon content above 2.1% (typically 2.5–4%). The high carbon content makes it easy to cast into complex shapes but brittle (cannot be forged or worked). The three main types — grey, ductile, and white — have different microstructures and properties.
Types of Cast Iron
| Type | Carbon form | Key property | Typical use |
|---|---|---|---|
| Grey | Flake graphite | Excellent machinability, damping | Engine blocks, pump bodies, valve bodies |
| Ductile (Nodular) | Spherical graphite | Ductile — can bend without fracture | Pressure pipe, pump casings, gears |
| White | Iron carbide | Hard, brittle, wear-resistant | Pump impellers, wear plates, mill liners |
Key Properties (Grey Cast Iron)
| Property | Value | |
|---|---|---|
| Density | 7200 kg/m³ | |
| Tensile strength | 150–300 MPa | |
| Compressive strength | 600–1000 MPa | 3–4× tensile |
| Melting point | 1145–1200°C | |
| Thermal conductivity | 45 W/m·K | |
| Max service temperature | 350°C |
Advantages
- Low cost: Cheaper than steel — the carbon reduces melting temperature and makes casting easy.
- Excellent castability: Flows easily into complex molds — ideal for pump casings, valve bodies, and engine blocks.
- Excellent machinability (grey): Graphite flakes act as chip breakers and lubricants — very easy to machine.
- Excellent damping capacity: Absorbs vibration — grey cast iron is used for machine tool bases and engine blocks.
- Good wear resistance: Graphite provides self-lubrication — good for sliding surfaces and cylinder liners.
Limitations
- Brittle (grey and white): Cannot be deformed — fails by fracture without warning. Not suitable for impact or shock loads.
- Cannot be welded easily: High carbon content causes cracking during welding — requires special preheat and nickel electrodes.
- Not for high pressure: Grey cast iron is limited to low pressure (up to ~15 bar). Ductile iron is used for higher pressures.
- Corrosion in water: Corrodes in water and moist environments — requires coatings for water service.
Ductile Iron — The Upgrade
Ductile (nodular) iron was developed in the 1940s by adding magnesium to the melt, which changes the graphite from flakes to spheres. This simple change gives ductile iron the strength and ductility of steel while retaining the castability of cast iron. It is the standard material for water and gas distribution pipes (DN80–DN600) and for pump and valve bodies.
Typical Applications
- Pump and valve bodies (grey/ductile)
- Water and gas distribution pipes (ductile)
- Engine blocks and cylinder heads (grey)
- Machine tool bases (grey — for damping)
- Brake discs and drums (grey)
- Manhole covers and grates (grey)
- Gear and sprocket blanks (ductile)
Chemical Compatibility
| Chemical | Rating |
|---|---|
| Fresh water | Fair — needs coating |
| Seawater | Poor |
| Steam (low pressure) | Fair |
| Dilute acids | Poor |
| Alkalis | Fair |
| Hydrocarbons | Good |
How CHI can help
Ask CHI about cast iron properties and to determine whether grey or ductile iron is appropriate for your component. The CHI assistant can recommend the right material for your specific application, explain chemical compatibility, and flag when a material is unsuitable for your operating temperature or environment.