The orderly arrangement of atoms and molecules which compose substances are called "Crystals". Iron (metal) is a composite crystal composed of many smaller crystals.

When high pressure (force) is applied to iron at room temperature, the crystals cause sliding and twin crystals to deform.

If this deformation is smooth, the energy of applied force is used for the flow of atoms, but if the deformation is not performed smoothly, the energy produced remains in the crystal.

The remaining force is called "Residual Stress".

Due to this residual stress, the crystal remains deformed, and becomes very hard.

The larger the deformation, the harder the crystal. However, if it gets too hard, it starts cracking. The cracking can be prevented by designing a process to produce the ideal sliding of crystals.

The iron hardened by this process can regain its original malleability by heating it to the required temperature. This is a very typical property of metal, and this happens because the heat changes the crystals to their normal shape.

This is called "Recrystellization".

The temperature which generates the recrystellization is called "Recrystellization temperature". In the case of iron, it is about 350~450℃.

The steel (metal) which was formed by cold forging becomes hard because the forming is performed without reaching this temperature. This effect is not obtained by hot or warm forging.