Annealing (metallurgy)

Annealing is understood in materials science, the preheating, soaking and cooling of semi-finished workpieces to achieve defined material properties. Annealing is a branch of the heat treatment and is in accordance with DIN 8580 to the manufacturing process by changing the material property.

Subdivision of the annealing

It divides the annealing process in at least three phases:

Both in the warm-up and cool-down in the implementation of specific warming-up and cooling rates may be required.

Materials for high demands sometimes require a resolution of the three stages, as mentioned in other phases. Thus, there is a nine-stage heat treatment for the material 2.4669. For linguistic distinction such complex heat treatments are also called Glühvorschrift or Glühprogramm. Where Glühprogramm is homonym used and also

  • The timing of annealing of various workpieces, or
  • The compilation of the possible annealing for a product ( assortment )

Can mean.

Industrial two different methods are used for the annealing of steel strip. In the continuous annealing line, the tape is unwound and passes continuously through a several 100 m long oven. Limited by the length of the furnace annealing in this case is up to 10 minutes, the furnace temperature can be up to 950 ° C, in the production of electrical steel sheet also above. When batch annealing several coils come in a closed oven. The annealing periods may amount to up to several days, however, the heating and cooling rates are limited. The possible during batch annealing temperatures ranging from 280 to 700 ° C, at Miteinwicklung of wire even any higher - but in this case must then be cut off and scrapped the edge of the steel strip.

Subdivision according to material property

According to the desired material property, a distinction between the steel:

  • Annealing

The annealing of steel known precipitates of cementite and pearlite can be reduced to reduce the hardness and strength of the steel and to facilitate the deformation. Typical temperatures for this are 680 ° C - 780 ° C.

  • Stress-

Stress occurs at relatively low temperatures between 480 ° C and 680 ° C and causes internal stresses of the workpiece to be removed, which have been introduced by mechanical deformation or machining. Otherwise, the steel properties are not possible to be changed.

  • Normalizing ( Normalization )

Normalizing of steel is one of the most important heat treatment process. It aims at the formation of a fine-grained structure of crystallites, which are uniformly distributed over the workpiece, from. For steels with higher carbon content, the annealing temperature is just below 800 ° C; in steels with low carbon content, the temperature for annealing normally rises up to 950 ° C.

  • Coarse grain annealing

The coarse-grain annealing, the size of the individual crystallites to be increased. Thus, the strength and toughness of the material, which is desired in certain machining methods decreased.

  • Recrystallization

Under recrystallization refers to the restoration of crystallite as they have been present prior to cold working. To this end, the workpiece is usually heated to temperatures just above the Rekristallationstemperatur between 550 ° C and 700 ° C. The Rekristallationstemperatur depends on material and degree of deformation.

  • Diffusion annealing / solution annealing

Diffusion annealing takes up to 2 days, takes place at very high temperatures between 1050 ° C and 1300 ° C and is intended to provide a uniform distribution of impurity atoms in the metal lattice. The cooling rate determines the formation of the phases and thus the steel properties.

  • Put off

Hardening of carbon steel in Abschrecköfen the workpiece is first heated to a temperature between 800 ° C and 900 ° C, while that in the case of steel is present with a low carbon content of pure austenite. For alloy steels, the temperature required can vary significantly. To prevent corrosion may be used in the exothermic gas furnace. Exothermic gas is generated in a respective gas generator of hydrocarbons and contains in addition to CO, H2 and N2 and CO2, and H2O. After tempering, the steel is rapidly cooled or quenched, that a change of the carbon atoms to favorable lattice sites can not take place because the diffusion rate of carbon atoms at low temperatures is too low to allow a change in the interstitial sites. The iron bars still change with further decreasing temperature, but the lattice structure and there is the so-called martensite or martensitic steel. Because of lattice defects and tenseness martensite is very hard and strong, but also less ductile and brittle.

After quenching, the martensitic steel is very hard but also very brittle. This can called by reheating, tempering, be counteracted. First an accumulation of carbon atoms in the range of lattice defects of the martensitic steel in a temperature range below 100 ° C. At temperatures between 100 ° C and 200 ° C begin carbon atoms from unfavorable lattice sites of the iron auszudiffundieren. It starts the precipitation of iron carbide. With a further increase of the temperature of this process is accelerated. Over 320 ° C leaving virtually all carbon atoms unfavorable interstitial sites. The combination of quenching and tempering is called tempering. About 400 ° C there will be no significant structural change more and the steel is soft again. When alloyed with chromium, vanadium, molybdenum and tungsten steels, the hardness of the steel is reduced in this area but again, since carbides are precipitated. This secondary hardening is important for components to keep your hardness at operating at high temperatures.

  • Wasserstoffarmglühen

Wasserstoffarmglühen the workpieces are maintained at temperatures between 200 ° C and 300 ° C for several hours. In this escape the incorporated hydrogen atoms in the structure, which make the material brittle due to effusion of the components.

Tempering (metallurgy)
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