Modification of Properties

Modification of Properties

Hardening

The metal is heated to a specific temperature so that the carbon structure moves out of its normal position. The material is then quenched, meaning the carbon does not have enough time to move back to its original place. This causes internal stresses which harden and strengthen the material. However, this makes the material more brittle.


Tempering

Tempering is carried out after hardening to remove any brittleness that may be present in the hardened material. The material is first cleaned to its natural shiny grey colour. The metal is then heated to a specific temperature which is shown by a colour (e.g. dark purple). When the correct colour is seen, the material is then rapidly cooled in liquid (quenched). Different colours will produce different characteristics. As the temperature rises, the material's hardness is reduced and toughness is increased.


Annealing

Annealing is a process where a metal is first heated to a specific temperature/colour where it will glow. The material is then soaked at that temperature, making it more ductile and softer. It is then allowed to cool slowly. This removes internal stresses and makes the material less brittle.


Normalising

Normalising is the same process as annealing except it only applies to ferrous metals.


Case Hardening

Used to increase the internal toughness of steel while giving it a hard outer layer. The material is first heated to a cherry red colour and plunged into case hardening compound The compound is high in carbon. It is then heated again to cherry red and quenched in water. The metal should now have a hardened outer surface and a flexible, soft interior. The process can be repeated to increase the depth of the hardened surface.

Advantages

1) Leaves a tough inner core, whilst giving it a hard surface.

2) Stops the material being brittle, makes it tougher.

Disadvantages

1) Material can start flaking off if left in for too long

2) Gain-growth can occur, which is where the material becomes larger. It then needs surface grinding to return to its original size

3) Rough surface finish