In materials science, annealing is understood to mean the heating, warming and cooling of semi-finished products and workpieces to achieve defined material properties . Annealing is a branch of heat treatment and, according to DIN 8580, is one of the manufacturing processes by changing the material properties .
Subdivision of the glow process
The annealing process is divided into at least three phases:
Warming (including warm-up or high-soaking)
in the warm-up phase is brought the workpiece to the holding temperature.
In the hold phase, the workpiece is held at a constant hold temperature. It serves to equalize the temperature in the workpiece and to establish equilibrium between chemical and physical processes in the material. The duration required for this is called the holding time and depends not only on the result to be achieved but also on the workpiece geometry and the arrangement of the workpieces in the annealing furnace or the heat treatment system.
In the cooling down phase, the workpiece is brought back to ambient temperature.
It may be necessary to adhere to specific heating and cooling speeds in both the warm-up and cool-down phases.
Materials for high requirements sometimes require the three phases mentioned to be broken down into further sub-phases. There is a nine-stage heat treatment for the material 2.4669. Such complex heat treatments are also called annealing specifications or annealing programs for the purpose of linguistic differentiation. Whereby the annealing program is used homonymously and also
- the chronological sequence of annealing of different workpieces or
- the compilation of possible anneals for a product (range)
Two different methods of annealing steel strip are used industrially:
- The strip is unwound in the continuous annealing line and runs continuously through a furnace several hundred meters long. Due to the overall length of the furnace, the annealing time is a maximum of 10 minutes, the furnace temperature can be up to 950 ° C, and above that in the production of electrical steel. Typical annealing conditions: 60 seconds at 700 ° C for recrystallization , then 1 to 3 minutes at 400 ° C for overaging .
- During hood annealing , several coils are placed in a closed furnace. The glow times can be up to several days, but the heating and cooling speeds are limited. The possible temperatures for hood annealing range from 280 to approx. 700 ° C, and even higher if the wire is wrapped in. In this case, however, the edge of the steel strip must then be cut off and scrapped. In the case of high-priced steels such as grain-oriented electrical steel, a separating MgO layer can also be used as an alternative to wire wrapping.
Subdivision according to material properties
Depending on the desired material property, a distinction is made for steel between:
Soft annealing of steel reduces existing precipitates of cementite or pearlite in order to reduce the hardness and strength of the steel and to make it easier to deform. Typical temperatures for this are 650 ° C - 750 ° C.
Stress relief annealing takes place at relatively low temperatures between 480 ° C and 680 ° C and has the effect that internal stresses in the workpiece that were introduced by mechanical deformation or machining are eliminated. Otherwise the steel properties should not be changed if possible.
- Normalizing (Normalization)
The normalizing of steels is one of the most important heat treatment processes. It aims at the formation of a fine-grain structure of crystallites that are evenly distributed over the workpiece. For steels with a higher carbon content, the annealing temperature is just below 800 ° C; For steels with a low carbon content, the temperature for normalizing increases to 950 ° C.
In coarse grain annealing, the size of the individual crystallites should be increased. This lowers the strength and toughness of the material, which is desirable for certain machining methods.
Recrystallization annealing means the restoration of crystallite forms as they existed before cold deformation. For this purpose, the workpiece is heated to temperatures just above the recrystallization temperature, usually between 550 ° C and 700 ° C. The recrystallization temperature depends on the material and the degree of deformation.
Diffusion annealing lasts up to 2 days, takes place at very high temperatures between 1050 ° C and 1300 ° C and is intended to ensure an even distribution of foreign atoms in the metal lattice. The cooling rate determines the formation of the phases and thus the steel properties.
With low hydrogen annealing, the workpieces are kept at temperatures between 200 ° C and 300 ° C for several hours. The hydrogen atoms embedded in the structure, which make the material brittle, escape from the components through effusion .