Generally annealing is carried out on non-ferrous metals. Annealing makes the metal soft. This property enables the worker to work upon the metal easily, and also prevents the metal to crack or fracture while being hammered, forged or rolled.
Copper tends to become hard and brittle, when hammered or cold worked. It can be softened by annealing. All plates, rods, tubes etc of copper are thoroughly annealed before use. Annealing is done by heating to a dull red colour (650°C), keeping the pieces at this temperature for a few minutes, and then quenching in water or allowing it to cool in the air. During the process of drawing into wire, copper must be annealed to avoid fracture.
Copper should not be heated in a reducing atmosphere, such as coal gas or carbon monoxide produced on the brazing hearth with a slow fire. Prolonged heating at a high temperature will also produce brittleness due to very coarse crystallization.
Brass is hardened by cold working. It can be annealed by heating between 535°C to 670°C and cooled either in air or water. Normalizing is done by heating it up to 700°C and cooling either in air or water.
Zinc is bluish white metal. It is brittle at normal temperature, but is malleable and ductile between 100°C to 150°C. Small pieces of zinc can be annealed by dipping them in boiling water. The temperature should be such that a hand touch can bear. Large jobs may be annealed by heating with a blowlamp or torch to the same temperature, which the hand can bear.
To anneal Magnesium, heat it to 300-320°C and quench in water.
Cold working, drawing, rolling etc hardens aluminium. It can be annealed by heating to a temperature of 360-400C and quenching in water or slow cooling. Drastic cooling has no effects on aluminium. It does not change its silvery colour even in its molten condition. Hence a practical method of heat treatment of aluminium is known as “Tell-tale method”. To ascertain the correct annealing temperature, for practical purposes, before it is heated, apply some lubricating oil or grease on the surface. On heating, this will first turn into black colour and on reaching the correct annealing temperature, it will turn white and finally disappear. An alternative method is to use a matchstick to check the temperature. Rubbing it on heating surface, a black mark will be obtained if the job has reached annealing temperature. These methods are employed in workshop where no other facility is available.
The heat treatments given to light alloys are normalizing and annealing. Normalized duralumin gradually becomes harder and stronger within three to four days after treatment. It is necessary after normalizing to allow the duralumin to develop its full properties by age hardening, before it is being used. If considerable bending and working of the material is required, it must be softened by annealing.
This metal is annealed by dipping in molten nitrate bath (Salt bath) at a temperature of 360°C to 420°C. The duration of immersion varies according to the thickness of the metal. The metal is then removed and quenched in clean water or cooled in air. The quenching in water will help in removing the leftover nitrates from the metal. Duralumin is normalized in a salt bath at a temperature between 485°C and 505°C, and then quenched in clean cold water.
Heat Treatment of Rivets
Duralumin is used for solid rivets because of its strength. When it becomes age hardened, such rivets cannot be used till they have been normalized. The rivet heads are liable to fracture if the rivets are used in the untreated condition. Once the rivets are normaIised they must be used within two hours. The ageing of normalised rivets can be suppressed for few hours by refrigeration.
Heat treatment of the alloy which exhibit age hardening, consist of three steps.
(a) Solution treatment.
Solution Treatment. The alloy is heated to a temperature of 420˚C at which homogeneous solid solution is obtained. The alloy is held at this temperature for homogenization. The holding time may vary from 30 minutes to several hours depending upon the alloy composition.
Quenching. After obtaining the homogeneous solid solution, the alloy is quenched generally into water, without any delay.
Ageing. After quenching the alloy is aged at a particular temperature (slightly higher than room temperature) for some time as per requirement. Temperature and time of ageing are inter related to obtain the maximum hardness. Time of aging is decreased with increasing temperature. The hardness increases with ageing, reaches a peak value and then decreased again due to over ageing.
Precipitation Treatment & Age Hardening
Precipitation is the decomposition of a solid solution into two phases of different composition, the precipitate and the solid solution. Precipitation hardening is the process whereby hardening of an alloy is caused by the precipitation of a constituent from a supersaturated solid solution by heating to some elevated temperature. Precipitation hardening is sometimes referred to as “Age Hardening” which is defined as the spontaneous increase of hardness at room temperature with lapse of time.