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Annealing is the treatment of materials at a fixed temperature, taking into consideration a definite holding period and subsequent cooling.
There are a number of annealing processes:
Normal Annealing is mainly undertaken with components which have been previously hot-worked. The component is usually heated to a temperature above the hardening temperature and is followed by cooling in a still atmosphere.
Normal Annealing should result in the production of a fine-grained ferrite-perlite structure. Large-grained and uneven structures can thereby be converted into new homogenous, fine structures This type of heat treatment Is carried out at around 20 – 50 °C above the AC3 conversion temperature, at which the austenite-ferrite conversion occurs.
Stress-relieved annealing serves to reduce internal stresses in materials arising from cold-working, changes in structure, thermal stresses or machining work. Stress-relieved annealing is normally carried out between 450 and 650 °C with a sufficiently long holding time and a subsequent very slow cooling with no significant changes in structure or mechanical properties.
Soft annealing is annealing at a temperature just below the lower conversion point followed by slow cooling to achieve the softest possible state. A granular perlite should result, with a malleable structure, which gives the optimum workability in non-cutting and cutting processes. This takes place over several hours, just below the AC1 temperature.
Spheroidization annealing is a form of soft annealing where spheroidal or globular form of cementite grains are formed. By cycling the annealing a number of times in temperature followed by a slow cooling results in a high uniformity in the carbide form and distribution. The structure comprises cementite spheroids dispersed throughout the ferrite matrix and provides the best workability (machining). This process is very important in, for example, subsequent cold-forming.
Large-grain annealing, also called high annealing, takes place at a temperature above the hardening temperature with an appropriate cooling to obtain a large grain size. The aim of large-grain annealing is to improve the tensile strength of components that will be subjected to a lot of machining and profiling It is carried out at temperatures between 950 und 1200 °C. The holding time must be sufficiently long to achieve the desired increase in grain size. Because an increase in grain size is associated with a worsening of component properties, the microstructure must be returned to a fine- grained structure, through phase transformation, during the final heat treatment process (hardening, tempering, case hardening, etc.).
Diffusion annealing is annealing at very high temperatures in the recrystallisation zone. The aim is, for example, to partially or fully reverse the changes in properties and structure that arose from cold forming. Diffusion annealing should equalise any local differences in chemical composition of cast-iron and steel materials, resulting from segregation, without any change in structure. This occurs through annealing in a temperature range of 1000-1300 °C.
Solution annealing is predominantly used for austenitic steels to dissolve precipitated components in the matrix and to eliminate stresses arising from previous work hardening. Solution annealing is used to obtain even, homogenous structural and material properties. In the case of ferrous metals annealing is carried out in a temperature range of between 950 and 1200 °C, in non-ferrous metals between 460 - 540 °C.
Advantages of Annealing
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