Methods to change the properties of metals

They could get higher strength, toughness, corrosion resistance etc depending on what is wanted for the product

• Annealing

• Cold working

• Hardening and quenching

• Normalising

• Adding/subtracting carbon to steels

• Corrosion

• Heating a metal to a suitable temperature and keeping it there to allow the grains in the metal to grow.

• This makes the metal softer and easier to work as it relieves internal stresses and allows the dislocations to move to the grain boundaries

• It’s used for metals that have been work hardened or to make metals easier to bend into complicated shapes

• Steel, aluminium, copper, brass

• Repeatedly bending/hammering a metal

• Grains in the affected area are deformed, become stretched out, thinner and smaller in that direction

• Dislocations - relocation of atoms causing the dislocations to move. When the dislocations meet, they pin each other in place

• Reduction in atoms’ ability to move reduces ductility and increases brittleness

• Steel is heated to just above the lower critical point and is then given time to soak at that temperature to make it harder

  • Gives atoms within the grains time to rearrange

• Steel then goes through quenching (rapidly cooling the metal in oil or salt water) so the atoms become fixed in place

• Heating the steel just above its upper critical point and allowing it to cool naturally in air

• Provides sufficient time for the atoms to rearrange naturally and relieves internal stresses

• Results in a tough steel with some ductility but doesn’t soften the steel as the grains don’t grow

• The surface of a metal reacts with another substance in its environment

• The oxide layer of aluminium isn’t visible to the naked eye and protects the metal against further corrosion

• With steel, the corrosion is progressive and eats away at the material, making it thinner

  • It reduces the attractiveness of a product

• Painting

• Applying a plastic coat by spraying/dipping

• Applying a layer of another metal that doesn’t react (dipping product in molten metal/ electroplating - placing product in chemical bath)

• Applying a layer of another metal that does react - the material is sacrificed instead of the product

• Cost - can’t afford corrosion resistant materials or would be too expensive for consumers

• Planned obsolescence