Methods to change the properties of metals

How do the properties of metals change when other metals are added to make alloys

How do the properties of metals change when other metals are added to make alloys

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

State the methods that affect grain size

• Annealing

• Cold working

• Hardening and quenching

• Normalising

• Adding/subtracting carbon to steels

• Corrosion

Explain the effect and uses of annealing

• 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

Explain the effect and uses of cold working

• 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

Explain the effect and uses of hardening and quenching

• 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 and form ‘autensite’

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

Explain the effect and uses of normalising

• 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

Explain the effect and uses of corrosion on aluminium and steel

• 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

Explain the ways to reduce corrosion

• 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

Explain the reasons for choosing materials that corrode easily

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

• Planned obsolescence