High-carbon steel

Above roughly 0.45% carbon steel can be quenched to real hardness, and the 550-1400 MPa span covers everything from normalized 1045 shafting to oil-quenched and tempered spring steel. Carbon buys hardness and costs weldability: high-carbon steels crack in the heat-affected zone unless preheated and slow-cooled, which is why a broken spring is usually replaced rather than welded. Stiffness is unchanged at 200 GPa no matter what the heat treatment does, so a hardened spring bends exactly as far as a soft one under a given load — it just returns instead of staying bent.

Properties

Density

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

Melting range

1425–1500 °C

Tensile strength

550–1400 MPa

Young's modulus

200 GPa

Thermal conductivity

49 W/(m·K)

Thermal expansion

11.5 µm/(m·K)

Floats in water

No

Magnetic

Yes

What it is used for

• Springs, wire rope and piano wire
• Chisels, axes and kitchen knives
• Rail track and crane wheels
• Hand saw and file blanks

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.