ZIMSEC O Level · Metal Technology and Design · Form 3, Form 4
Heat treatment
Changes the properties of metals.
- Annealing: heat to red heat and cool slowly – makes metal soft and easy to work.
- Normalising: heat and cool in still air – refines the grain, relieves stresses.
- Hardening: heat high-carbon steel to cherry red and quench in water or oil – very hard but brittle.
- Tempering: reheat hardened steel to a lower temperature (oxide colours from pale straw to blue) and quench – reduces brittleness, increases toughness. Cutting tools are tempered to straw colours; springs to blue.
- Case hardening: mild steel surface absorbs carbon (carburising), then is quenched – hard outside, tough core.
Forging
Shaping hot metal by hammering on an anvil: drawing down, upsetting, bending, twisting and punching. Tools: hearth/forge, anvil, tongs, hammers, swages and fullers.
Sheet metal work
Bending in folding bars, hollowing and raising with mallets and sandbags, wiring edges and seaming.
Finishes
Prevent corrosion and improve appearance: paint (primer then topcoat), powder coating, galvanising, electroplating, enamelling, polishing and lacquering, and oil blacking.
Exam tip: Remember the order: harden first, THEN temper.
Check yourself
- Which heat treatment makes metal soft?
- Why are hardened tools tempered?
- Name two finishes that prevent steel from rusting.
Show answers
- Annealing.
- To reduce brittleness and increase toughness.
- Any two: painting, galvanising, powder coating, electroplating, oiling.
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