Heat treatments for steel
- Annealing: heat the steel and cool it very slowly. It becomes softer, more ductile and easier to work, and internal stresses are relieved.
- Normalising: heat and cool in still air. It refines the grain structure, making the steel tougher.
- Hardening: heat medium or high carbon steel until red hot, then quench it quickly in water or oil. It becomes very hard, but brittle.
- Tempering: reheat hardened steel to a lower temperature, then cool. It reduces brittleness and increases toughness, for tools such as chisels.
- Case hardening: add carbon to the surface of low carbon steel, then heat and quench. The part has a hard, wear-resistant surface and a tough core.
- Work hardening: bending or hammering metal makes it harder and more brittle; annealing reverses this.
Surface finishes
- Finishes protect against corrosion and wear, and improve appearance.
- Painting and powder coating: powder coating sprays charged powder onto metal, then bakes it into a tough layer.
- Galvanising: dipping steel in molten zinc. The zinc protects steel even if scratched.
- Anodising: thickening the oxide layer on aluminium, which can be dyed.
- Electroplating: using electrolysis to coat a part with a thin layer of another metal, such as chrome.
Other finishes and choosing one
- Plastic dip coating: dipping heated metal into plastic powder, as on tool handles.
- Polishing and lacquering give a shiny finish; oiling protects tools.
- Choose a finish by thinking about where the product will be used (indoors or outdoors), the material, how hard-wearing it must be, how it should look, and cost. A garden gate might be galvanised then powder coated; a bike frame might be powder coated; an aluminium torch might be anodised.
Key terms
- Annealing
- Heating and slowly cooling metal to soften it and relieve stress.
- Hardening
- Heating steel and quenching it to make it hard.
- Quenching
- Cooling hot metal rapidly in water or oil.
- Tempering
- Reheating hardened steel to reduce brittleness.
- Case hardening
- Hardening only the surface of low carbon steel.
- Galvanising
- Coating steel with zinc to prevent rusting.
- Anodising
- Thickening the oxide layer on aluminium.
- Powder coating
- Baking a charged powder onto metal to form a tough coating.