Heat treatments (hardening, tempering, annealing, normalising) that change a metal's properties and surface treatments and finishes that protect or improve a surface.

What this dot point is asking

AQA wants you to name the main heat treatments and say how each changes a metal, and to name surface treatments and finishes and say how each protects or improves a surface. You should be able to match a treatment to a stated need and explain the purpose, not just list names.

Heat treatments

• Hardening: heat steel above its critical temperature, then quench (cool quickly in water or oil) to make it hard but brittle. Only steels with enough carbon will harden this way.
• Tempering: reheat hardened steel to a lower temperature and cool slowly to reduce brittleness and add toughness, at the cost of a little hardness.
• Annealing: heat then cool very slowly (often in the furnace) to soften a metal, relieve work hardening and make it easier to bend and machine.
• Normalising: heat then cool in still air to relieve internal stress and refine the grain, leaving the metal slightly harder and tougher than annealing does.

The link between hardening and tempering is the most examined idea: hardening alone leaves steel too brittle for tools, so it is almost always followed by tempering to balance hardness with toughness.

Surface treatments and finishes

Surface treatments only change the outside of the part, so they protect against corrosion and wear and improve appearance without changing the bulk properties. This is the key contrast with heat treatment, which changes the whole metal throughout.

Matching a treatment to a need

Try this

Q1. State what annealing does to a metal. [1 mark]

• Cue. Softens it and relieves work hardening so it is easier to bend and machine.

Q2. Name a surface treatment that protects steel from rusting. [1 mark]

• Cue. Galvanising (zinc coating), painting or powder coating.