Most common metals cannot be hardened by heat treatment. However, almost all metals are hardened - to one degree or another - as a result of forging, rolling or bending. This is called metal work hardening or cold working..
Annealing is a type of heat treatment to soften metal, which became cold-worked – riveted, so that you can continue cold processing.
Cold processing: copper, lead and aluminum
Common metals vary greatly in their degree and rate of work hardening. – slander or hardening. Copper rivets quite quickly as a result of cold forging, and, means, quickly reduces its malleability and ductility. Therefore, copper requires frequent annealing., so that it can be further processed without the risk of destruction.
On the other hand, lead can be hammered into almost any shape without annealing and without the risk of breaking. Lead has such a reserve of plasticity, which allows him to obtain large plastic deformation with a very low degree of strain hardening. but, copper, although harder than lead, generally more malleable.
Aluminum can withstand a very large amount of plastic deformation as a result of hammering or cold rolling, before it needs annealing to restore its plastic properties. Pure aluminum rivets much slower, than copper, and some aluminum alloy sheets are too hard or brittle, to allow large work hardening.
Cold working of iron and steel
Industrial pure iron can be cold worked to high degrees of deformation, before it becomes too hard for further processing. Impurities in iron or steel degrade the cold workability of the metal to such an extent, that most steels cannot be cold worked, except of course, special low-carbon steels for the automotive industry. At the same time, almost all steels can be successfully processed plastically in a red-hot state.
Why do you need annealing of metals
The exact nature of the annealing process, to which the metal is exposed, largely depends on the purpose of the annealed metal. There is a significant difference in annealing methods for its implementation between annealing in factories, where a huge amount of sheet steel is produced, and annealing in a small auto repair shop, when only one part requires such processing.
Briefly, then cold working is plastic deformation by destruction or distortion of the grain structure of the metal. During annealing, the metal or alloy is heated to a temperature, at which recrystallization occurs – education instead of old – deformed and elongated – grains of new grains – non-deformable and round. Then the metal is cooled at a given rate. In other words, crystals or grains inside the metal, which have been displaced or deformed during cold plastic working, give an opportunity to rebuild and recover to their natural state, but already at an elevated annealing temperature.
Annealing iron and steel
Iron and mild steels must be heated to about 900 degrees Celsius, and then allow to cool slowly to ensure as "soft" as possible. In doing so, measures are taken, to prevent metal contact with air to avoid oxidation of its surface. When it is done in a small body shop, then warm sand is used for this.
High carbon steels require similar processing except, that the annealing temperature for them is lower and is about 800 degrees Celsius.
Copper is annealed at a temperature of about 550 degrees Celsius, when copper is heated to dark red. After heating, the copper is cooled in water or allowed to cool slowly in air. The cooling rate of copper after heating at the annealing temperature does not affect the degree of obtained "softness" of this metal. The advantage of rapid cooling is, that at the same time the metal is cleaned of scale and dirt.
Aluminum is annealed at a temperature at a temperature 350 degrees Celsius. In factories, this is done in suitable ovens or salt baths.. In the workshop, aluminum is annealed with a gas burner. Tell, that at the same time a wooden splinter is rubbed on the surface of the heated metal. When a tree starts to leave black marks, then it means, that aluminum got its annealing. Sometimes a bar of soap is used instead of wood.: when soap starts to leave brown marks, heating must be stopped. Then the aluminum is cooled in water or left to cool in air.
Zinc becomes malleable again at temperatures between 100 and 150 degrees Celsius. It means, that it can be annealed in boiling water. Zinc needs to be processed, while he's hot: when it cools, then it greatly loses its malleability.