Annealing is a heat treatment process that changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable.
The annealing process requires the material above its recrystallization temperature for a set amount of time before cooling. The cooling rate depends upon the types of metals being annealed. For example, ferrous metals such as steel are usually left to cool down to room temperature in still air while copper, silver and brass can either be slowly cooled in air or quickly quenched in water.
The heating process cause atoms to migrate in the crystal lattice and the number of dislocations reduces, which leads to the change in ductility and hardness. The heat treated material recrystallizes as it cools. The crystal grain size and phase composition depend on the heating and cooling rates and these, in turn, determine the material properties.
Hot or cold working of the pieces of metal following annealing alters the material structure once more, so further heat treatments may be required to attain the desired properties.
However, with knowledge of material composition and phase diagram, heat treating can soften metals and prepare them for further working such as forming, shaping and stamping, as well as preventing brittle failure.
An annealing furnace works by heating a material above the recrystallization temperature and then cooling the material once it has been held at the desired temperature for a suitable length of time. The material recrystallizes as it cools once the heating process has caused atom movement to redistribute and eradicate dislocations in the workpiece.
Annealing works in three stages – the recovery stage, recrystallization stage and the grain growth stage. These work as follows:
1. Recovery Stage
This stage is where the furnace or other heating device is used to raise the temperature of the material to such a point that the internal stresses are relieved.
2. Recrystallization Stage
Heating the material above its recrystallization temperature but below its melting point causes new grains to form without any residual stresses.
3. Grain Growth Stage
Cooling the material at a specific rate causes new grains to develop. After which the material will be more workable. Subsequent operations to alter mechanical properties can be carried out following annealing.
Annealing is used to reverse the effects of work hardening, which can occur during processes such as bending, cold forming or drawing. If the material becomes too hard it can make working impossible or result in cracking.
By heating the material above the recrystallization temperature, it is made more ductile and therefore ready to be worked once more. Annealing also removes stresses that can occur when welds solidify. Hot rolled steel is also shaped and formed by heating it above the recrystallization temperature. While steel and alloy steel annealing is common, other metals can also benefit from the process, such as aluminium, brass, and copper.
Metal fabricators use annealing to help create complex parts, keeping the material workable by returning them close to their pre-worked state. The process is important in maintaining ductility and reducing hardness after cold working. In addition, some metals are annealed to increase their electrical conductivity.
Annealing can be carried out with alloys, with a partial or full anneal being the only methods used for non-heat treatable alloys. The exception to this is with the 5000 series alloys, which can be given low temperature stabillisation treatments.
Alloys are annealed at temperatures of between 300-410°C, depending on the alloy, with heating times ranging from 0.5 to 3 hours, depending on the size of the workpiece and the type of alloy. Alloys need to be cooled at a maximum rate of 20°C per hour until the temperature is reduced to 290°C, after which the cooling rate is not important.
The main advantages of annealing are in how the process improves the workability of a material, increasing toughness, reducing hardness and increasing the ductility and machinability of a metal.
The heating and cooling process also reduces the brittleness of metals while enhancing their magnetic properties and electrical conductivity.
The main drawback with annealing is that it can be a time consuming procedure, depending on which materials are being annealed. Materials with high temperature requirements can take a long time to cool sufficiently, especially if they are being left to cool naturally inside an annealing furnace.
Annealing is used across a variety of industries where metals need to be worked into complex structures or worked on several times.
Annealing dates back hundreds of years, as evidenced by the word itself, which comes from the Middle English ‘anelen,’ meaning to set on fire or kindle, as well as bake and temper.
Middle English was spoken and written in England from 1150 until 1500 and is a descendant of Old English.The term had spelling variations, such as the Middle English ‘onǣlan,’ and was used as in this instruction from 1400, ‘Take þe plates of bras pannes or of cawdrouns and anele hem in þe fire rede hoot’ (“Take the plates of brass pans or of cauldrons and anneal them in the fire red hot”).
While we do not know exactly who discovered annealing, the etymology shows that it was in practice at least 900 years ago.