ANNEALING

Annealing
The annealing of a metal part or of a material is a method corresponding to a heating cycle. This is a step gradual rise in temperature followed by controlled cooling. This procedure, common in materials science, to change the physical characteristics of the metal or the material studied. This is especially used to ease the stress relaxation that can accumulate in the heart of the matter, as a result of mechanical or thermal stresses, the steps involved in synthesis and shaping materials. At the time of annealing, the grains (single crystals) of material form again and again somehow, their "steady state".
The annealing is also used to change the magnetic properties of a room.
The crystallization annealing after cold working , aims to give the metal an optimal grain size for future use (folding, stamping , ...).

Necessity of annealing: in metallurgy 

The rolling of a cold steel contributes to hardening and loss of ductility of the metal. Grain growth is necessary to find exploitable metallurgical characteristics.
Examples of use of annealing:
adaptation of the metal grain size for optimum performance (after casting)
Elimination of residual stresses (plastic deformation)
reduction in the hardness for a machining
obtaining single-crystal pieces of unique features (eg rotor blades of turbo-machinery)
...
Process 

The annealing is obtained by raising the temperature of the metal at temperatures ranging from 500  ° C to 850  ° C . The quality of annealing requires heating cycle (time of rise in temperature, holding time) under control (it can be slow or fast).
It is necessary to respect certain values ​​coupled holding time and heating temperature for complete recrystallization.
The heating rate influences the size of grains (and their number). Depending on the original structure and the desired grain size, it will be faster or slower. The holding time, the heating temperature and the rate of cooling influencing more the grain size.
The descent is more rapid (without reaching quench rates), most grains remain small.
If quenching is desired, it can be performed instead of cooling annealing.

Slow cycle and rapid cycling 

The slow cycle of annealing a steel is performed by placing the coils in the bells for 30 to 40 hours. The continuous annealing allows for him a cycle of rapid heating (90 seconds + or - 30 seconds). the cooling step should be as slow. For example, the bell-type furnaces HICO/H2 use at the end of the heating cycle that begins with a cooler air cooling, to a temperature of 300  ° C , followed by a water spray until at a temperature of 70  ° C .

Interest of continuous annealing

Appeared in the 1960s, continuous annealing brings together on a continuous line operations annealing, cold working, inspection, oiling, marking, edge trimming and winding. It allows a significant time savings compared to the annealing bells (also called annealing base).

Microelectronics 

Annealing is a technique also used in processes for microelectronics . It allows for example:
the diffusion of dopants ( boron , arsenic , phosphorus , etc..) in a semiconductor intrinsic
in the case of one or more layers of metal (s) on a semiconductor metal diffusion in the semiconductor, thereby changing its nature and thereby enhancing or changing the nature of the junction with the metal
improvement of a ohmic contact by forming silicide - or as the term used saliciure - (case of silicon )
transformation of a Schottky contact in ohmic contact (for example by annealing a layer of aluminum on a layer of silicon)
restoring a damaged surface for a semiconductor simple: when a semiconductor single (non-binary, tertiary or quaternary, etc). as silicon or germanium is damaged, for example following a burn plasma or reactive ion etching (bombarding the surface with ions and electrons), it is possible to anneal the semiconductor in an attempt to restore the surface. This technique does not work with semiconductor binary (or tertiary, etc..) Because the heat tends to disintegrate the alloy of the two (or more) elements used.