Annealing A Sword

(What an awsome parrallel this is to the biblical process of conversion.)

Annealing (metallurgy) -- a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness.  Annealing is used to soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve properties.

Annealing occurs by the realgnment of atoms within a solid material, so that the material progresses towards its equilibrium state.  Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metal. The amount of process-initiating in a deformed metal is also reduced by the annealing process. In practice, this reduction of Gibbs free energy is termed "stress relief".

The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow process. The high temperatures at which the annealing process occurs serve to accelerate this process.

Mechanical properties, such as hardness and ductility, change as dislocations are eliminated and the metal's crystal lattice is altered. On heating at specific temperature and cooling it is possible to bring the atom at the right lattice site and new grain growth can improve the mechanical properties.

The purpose of annealing may involve one or more of the following aims:
1. To soften the steel and to improve machinability.
2. To relieve internal stresses induced by some previous treatment (rolling, forging,uneven cooling).
3. To remove coarseness of grain.
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The treatment consists of:
a. heating the steel to a certain temperature,
b. "soaking" at this temperature for a time sufficient to allow the necessary changes to occur,
c. cooling at a predetermined rate.

Stages of Annealing

There are several stages in the annealing process, with the first being the phase which results in softening of the metal through removal of defects and the internal stresses which they cause. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains. The second phase is where new strain-free grains nucleate and grow to replace those deformed by internal stresses. A third phase, which is typically confined to hardenable steel, is used to refine grains which have been deformed through internal stress and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked at this heat for a period of time then allowed to cool in air. Small grains are formed which give a much harder and tougher metal. Ductility is important in shaping and creating a more refined piece of work. In this phase the piece is heated to a temperature typically below the annealing temperature and held there for long enough to relieve stresses in the metal. The piece is finally cooled slowly in to room temperature. It is then ready again for additional cold working. This can also be used to ensure there is reduced risk of distortion of the work piece.
 

Full Anneal and Normalising Treatments

For steel treatment consists in heating to about 25-50°C above the upper critical point of Fe-Fe3C equilibrium. These temperatures allow for the effects of slight variations in the impurities present and also the thermal lag associated with the critical changes. After soaking at the temperature for a time dependent on the thickness of the article, the steel is very slowly cooled. This treatment is known as full annealing, and is used for removing strains from forgings and castings, improving machinability and also when softening and refinement of structure are both required.


Normalising differs from the full annealing in that the metal is allowed to cool in still air. The structure and properties produced, however, varying with the thickness of metal treated. The tensile strength, yield point, reduction of area and impact value are higher than the figures obtained by annealing.