Tool steel

A family, not a material: O1 hardens in oil and is forgiving, A2 hardens in air and moves less, D2 has 12% chromium and holds an edge far longer but will not take as fine a one, and M2 high-speed steel stays hard at red heat. The 600-2000 MPa span is annealed stock through fully hardened and tempered — a tool steel is soft when you machine it and only becomes a tool afterwards. Thermal conductivity is left blank here on purpose: it runs from about 20 W/(m·K) for D2 to 46 for O1, and a single figure would mislead.

Properties

Density

7,800 kg/m³ (7,700–7,900) — 7.80 g/cm³

Melting range

1420–1500 °C

Tensile strength

600–2000 MPa

Young's modulus

200 GPa

Thermal expansion

11 µm/(m·K)

Floats in water

No

Magnetic

Yes

What it is used for

• Punches, dies and press tooling
• Milling cutters, drills and taps
• Injection mould cavities
• Woodworking chisels and plane irons

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.