Heat treatments

Hardening and tempering:

How it works:

1. Medium/High carbon steel is heated to a given temperature (below critical temperature) and held there for a period of time
2. It is then dropped in water/oil (often called quenching)
3. The metal has now been hardened which improves certain properties but makes the metal very brittle
4. As a result the metal is reheated to a given temperature (below critical temperature) and allowed to cool slowly

Advantages	Disadvantages
Improved tensile strength Very hard Reversible process (via annealing)	Metal becomes less ductile Metal is more likely to crack/damage in worked area Metal becomes more brittle

Uses: Screwdrivers, wrenches, hardened steel

Case hardening:

How it works:

1. Hardens the surface of steels with less than 0.4% carbon content
2. Carburising: the metal is placed in a box packed with powdered carbon and heated to 930C for a period of time
3. Carbon is then absorbed by the metal, the depth of absoprtion is determined by the length of time if carbursing
4. The metal is then quenched in water which stops the carbon escaping the surface of the metal

Advantages	Disadvantages
Greater hardness for outside surface Improved wear resistance Resistance to surface indedntations Low coefficient of friction Internal properties of metal remain unchanged	Depth of hardness is not fully known Difficult to machine metal after process

Uses: Firing pins in guns, rifle bolts, engine camshafts

Annealing:

How it works:

1. Metal is heated and then very slowly cooled
2. While similar to tempering it produces a softer and more ductile metal that can be used for products that will not undergo large stresses

Advantages	Disadvantages
Metal becomes less brittle Metal becomes more ductile Simple process that can be done in school workshops	Slow and time consuming process

Uses: Reduce hardness and increase ductility in metals such as steels

Normalising:

How it works:

1. Metal is heated to the critical temperature and held there for a set time
2. Metal is then allowed to cool slowly in air

Advantages	Disadvantages
Metal becomes easier to machine Relieves internal stress on metal Decreased hardness Improved ductility	Can't normalise non-ferous metals Decrease in hardness

Uses: To relieve stress on metal after cold working process for better physcial properties

Topic test: