Annealing

The reheated of a metal part is a process corresponding to a cycle of heating, maintenance in temperature then cooling making it possible to modify the characteristics of a metal. At the time of an annealing, the grains (monocrystals) of metal are reformed. This process is used to eliminate effects of the mechanical or thermal phenomena considered as harmful.

Annealing is also used to change the magnetic properties of a part.

The purpose of the annealing of crystallization, after work hardening, is to confer on metal an optimal size of grain for its future use (folding, Emboutissage,…).

Need for annealing

The Laminage cold of a steel contributes to a hardening and a loss of Ductilité of metal. A growth of the grains is necessary to find exploitable metallurgical characteristics.

Examples of use of annealing:

  • adaptation of the size of the grain of metal for optimal performances (after a casting)

  • elimination of residual stresses (plastic deformation)
  • lowers hardness for a machining
  • obtaining single-crystal parts of exceptional characteristics (ex: paddles of rotors of turboshaft engines)

…

Process

Annealing is obtained by rise in temperature of metal at temperatures active of 500°C with 850°C. The quality of annealing requires a cycle of heating (boarding time in temperature, time of maintenance) controlled well (it can be slow or fast).

It is necessary to respect certain coupled values of time of maintenance and temperature of heating to have a complete recrystallization.

The speed of heating influences the size of the grains (and their number). According to the structure of origin and size of grain wished, it will be necessary to be more or less fast. The time of maintenance, the temperature of heating and the speed of cooling influence more still the size of the grains.

The faster the descent is (without reaching speeds of hardening), the more the grains remain small.

If a hardening is wished, it can be realized instead of the cooling of annealing.

Slow cycle and fast cycle

The slow cycle of annealing of a steel is carried out while placing the reels under bells during 30 to 40 hours. Continuous annealing allows as for him a fast cycle of heating (90 seconds about 30 seconds). the stage of cooling must be also slow. For example, the bell kilns of the type HICO/H2 use at the end of the cycle of heating a cooler which starts with an air-cooling, until a temperature of 300°C, followed by a watering to water until a temperature of 70°C.

Interest of continuous annealing

Appeared in the Sixties continuous annealing makes it possible to join together on a line uninterrupted the operations of annealing, of work hardening, inspection, oiling, marking, shearing of banks and winding. It allows an important time-saver compared to annealing under bells (also called annealing bases).

See too

Related article

  • Reheated simulated

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