Metals have a crystalline structure - this is not usually
visible but can be seen on galvanized lamp posts for example.
The longer the metal takes to cool the larger the crystals grow.
These crystals form the grains in the solid metal.
Each grain is a distinct crystal with its own orientation.
The areas between the grains are known as grain boundaries.
Within each grain, the individual atoms form a crystalline lattice. Each atom will have a certain number of close neighbors with which it shares loose bonds. (The number of neighboring atoms depends upon the structure of the lattice.) When stress is applied to the metal, the atoms will start to spread apart. The atomic bonds stretch, and the attractive forces between the atoms will oppose the applied stress, like millions of tiny springs. If the metal has not yielded, the interatomic forces will pull the metal back into its original shape when the stress is removed.
So it is behaving like a piece of rubber -it is elastic!
When the metal is cold worked by forging, stamping or rolling its shape is permanently changed (DEFORMED) this is only possible because of defects (DISLOCATIONS) in the grain structure which move through the crystal structure. These dislocations or slips in the grain structure allow the overall change in shape of the metal. Each grain can have a very large number of dislocations (only visible under a powerful microscope).
Of course if the metal is hot worked there is more energy to available for the dislocations to move. This is why the strenghth of most materials falls as the temperature goes up.
Strong materials are those that can slow down or stop the movement of the dislocations.
This can be achieved by increasing the number of dislocations by cold work or work hardening (together known as stress hardening). Alloying where the other metal interacts with the crystal lattice blocking the movement of the dislocation. (Brass is a good example of this where the small percentage of zinc makes the brass stronger than either copper or zinc.)
Cold working or work hardening generates many dislocations which pile up and entangle this will prevent the further movement of dislocations. Try this by bending a paper clip back and forth - it becomes hard to bend at the same point and will eventually break if you continue.
[TEMPER is the term used to describe the amount of cold working on a metal. e.g half hard, full hard, spring temper etc.]
Cold rolling - the strip of metal passes between two rollers which exert heavy pressure. The strip is compressed and becomes much longer and thinner. The grains in the metal also become elongated. This a permanent deformation so dislocations pile up and the strenghth goes up. The larger grain boundary in the elongated strip also helps to stop the formation of further dislocations so that it becomes harder to roll a second time. The metal also becomes more brittle and is more liable to fracture as the number of dislocations goes up.
The metal actually becomes difficult to work as cold working continues. The only answer is annealling. This is a high temperature soak (keeping the metal at the same temperaure for some time). The grains recrystallise - old grains are obliterated and new new grains grow. The metal loses all the effects of cold working becoming ductle again but losing its strenghth. Annealling has to be carefully controlled sothat the grains do not become too large.
While cold forging is very useful for increasing the strength of matals - hot forging is widely used in manufacturing. The advantage is that the part can be formed without annealling and the grain structure will follow the form of the object being forged. So for example a spanner would have a central part where the grain structure is elongated but with few dislocations then the grain flows around following the form of the ends. The spanner will of course have to be heat treated to give the right degree of hardness and toughness at the right points.
Grain Flow Comparison