Mild steel

The default steel and the one meant whenever someone says "steel" without qualifying it: under about 0.25% carbon, cheap, weldable without preheat, and strong enough for most of what gets built. Its 400-550 MPa tensile range is the specified band for A36 and S275 grades; the yield strength that engineers actually design to is much lower, around 250-275 MPa. It cannot be hardened by quenching in any useful way — there is not enough carbon — which is why knives and tools are made from something else.

Properties

Density

7,850 kg/m³ (7,800–7,900) — 7.85 g/cm³

Melting range

1450–1520 °C

Tensile strength

400–550 MPa

Young's modulus

200 GPa

Thermal conductivity

50 W/(m·K)

Thermal expansion

12 µm/(m·K)

Floats in water

No

Magnetic

Yes

What it is used for

• Structural beams, columns and RSJs
• Car body panels
• Welded fabrication, gates and railings
• Reinforcing bar in concrete

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.