Ductile iron

The same iron as gray cast iron with a trace of magnesium added to the ladle, which makes the graphite form as spheres instead of flakes. That one change roughly triples the tensile strength and turns a brittle material into one that bends before it breaks, at almost no extra cost. It is why buried water mains stopped bursting when the ground shifted: they were cast iron before 1950 and ductile iron after.

Properties

Density

7,100 kg/m³ (7,000–7,200) — 7.10 g/cm³

Melting range

1150–1200 °C

Tensile strength

400–900 MPa

Young's modulus

169 GPa

Thermal conductivity

33 W/(m·K)

Thermal expansion

11 µm/(m·K)

Floats in water

No

Magnetic

Yes

What it is used for

• Water and sewer mains
• Crankshafts and steering knuckles
• Gearbox and axle housings
• Manhole covers and hydrants

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.