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Annealing

This term refers to a heat treatment process included in the production process with which optimal material properties are achieved by heating, soaking, and subsequent cooling.

A distinction is made between the following annealing processes:

Normalising usually takes place after hot forming of components. The workpieces are heated to slightly above hardening temperature and then cooled. Large-grained or non-uniform material structures can be transformed into fine structures by normalising.

Stress relief annealing is intended to reduce internal stresses in the workpiece during changes to the structure, cold deformation or thermal effects. It usually takes place over long holding times at temperatures between 450 and 650°C and then with very slow (!) cooling, so substantial changes in the material structure or its properties can be avoided.

During soft annealing, too, slow cooling takes place following treatment at temperatures below the lower transformation temperature. In this case the material should remain as soft as possible in its structure and a granular pearlite should form, which ensures optimal processing during non-cutting forming or machining.

Spheroidising means annealing on spheroidised cementite and is likewise a soft annealing process, but one which achieves the highest possible true strain of the carbides by cyclic annealing and subsequent, slow cooling, so that a material structure consisting of cementite granules in a ferritic base body is produced, which can in turn be processed optimally. This process is recommended with subsequent cold massive forming.

Full annealing takes place at a temperature above the hardening temperature in combination with an appropriate cooling step so that a coarser grain can be achieved. This process should also improve the machinability of the workpieces, which are subjected to a great deal of machining. The holding time must be long enough to ensure the coarsening of the grain. The process functions at temperatures between 950 and 1,200°C. As the desired grain growth adversely affects material properties, the condition of the structure must be made fine-grained again by hardening, temper-hardening etc.

Homogenising means annealing at very high temperatures in the (recrystallisation zone) with the aim of partially or completely reversing the changes in material properties or structure resulting from cold forming.
This process in the temperature range 1,000 – 1,300°C makes it possible to compensate the differences in the chemical composition of steels and cast materials brought about by segregation, transformation of the structure is ruled out.

Solution treatment is usually used for austenitic steels, for dissolving precipitated constituents or for eliminating stresses following strain hardening. This process is carried out at temperatures between 950 and 1,200°C for iron and between 460 and 540°C for non-ferrous metals. Uniform and homogeneous material properties are achieved with this process.

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