Gray cast iron

The 2-4% carbon that makes it castable comes out as graphite flakes, and those flakes are simultaneously its great strength and its fatal weakness: they damp vibration superbly, which is why machine tool beds are cast iron, and they act as internal cracks, which is why it snaps rather than bends. It is roughly 10% less dense than steel because the graphite is light. Melting 350 °C below steel is what made it the first mass-produced metal casting, and is also why a cast iron pan can be left on a fire that would ruin an aluminum one.

Properties

Density

7,150 kg/m³ (7,050–7,300) — 7.15 g/cm³

Melting range

1150–1200 °C

Tensile strength

150–400 MPa

Young's modulus

100 GPa

Thermal conductivity

46 W/(m·K)

Thermal expansion

10.5 µm/(m·K)

Floats in water

No

Magnetic

Yes

What it is used for

• Engine blocks and cylinder liners
• Machine tool beds and lathe ways
• Drain covers, pipe and radiators
• Frying pans and Dutch ovens

How it compares

Does it rust?

Yes, and readily. Rust is iron oxide, and it is flaky rather than adherent — it does not seal the surface, so oxygen and water keep reaching fresh metal underneath and the process continues until the section is gone. That non-protective quality is what makes rust different from the oxide layers on other metals.

Is it magnetic?

Iron, nickel and cobalt are the three common ferromagnetic elements, and anything based mostly on them is strongly attracted to a magnet.