Heat Treatment Of carbon Steel

Heat treatment is a series of operations involving the heating and cooling of steel in the solid state. Its purpose is to change the mechanical properties of the steel so that it will be harder, stronger or more resistant to impact. It can also make a steel softer and more ductile.

No single heat treatment can produce all these characteristics. Some properties may be improved at the expense of others eg. when being hardened a steel may become ‘brittle’.

Heat treatments are normally carried out by heating and cooling the steel. The temperature to which the steel is heated and the rate of cooling is most important.
Plain high carbon steels can be:

• ANNEALED
• NORMALISED
• HARDENED
• TEMPERED

Annealing

This is a softening heat treatment. In general terms the steel is heated to its upper critical point and then ‘allowed to cool very slowly in the furnace’.

Normalising

This is a form of heat treatment in which the steel is heated to its upper critical point and ‘allowed to cool slowly in still air’. Normalising restores the crystalline structure and relieves stresses in the steel.

Hardening

This is a heat treatment whereby steel is made hard and brittle by heating to the upper critical point and immediately quenching in water or oil.

Tempering

This is a heat treatment process in which some of the hardness is removed from the steel to increase its toughness and decrease its brittleness. After hardening, the steel is reheated to a fairly low temperature (below the lower critical point) then quenched in water.

The temperature depends on the purpose of the tool. The higher the tempering temperature, the less the hardness but the greater the toughness. Thus the purpose of the tool or article must be considered.
The temperature required may be judged from the temper colours which appear on the bright surface of steel which is heated slowly.