Diffusion annealing
The diffusion annealing (or homogenization annealing or soaking ) is a type of heat treatment ( annealing ) of metallic materials. It serves to reduce differences in concentration in workpieces. Diffusion annealing can primarily be used to reduce structural inhomogeneities in the individual crystals ( micro-segregation ). Inhomogeneities in a workpiece ( block segregation / across crystal boundaries) cannot be completely eliminated because of the long diffusion paths.
In the process, also known as equalizing annealing, the metal is heated to just below the solidus line , as some elements such as chromium only diffuse at these temperatures, whereby the diffusion of the atoms, which is hindered by too rapid cooling, is restarted . Inhomogeneous mixed crystals are transformed into homogeneous ones . As with all annealing processes, this does not significantly affect the mechanical properties of the alloy . Due to the high temperature and the long annealing times, however, it should be noted that coarse grains can arise, which must be removed again by normal annealing .
Since diffusion processes in solids are strongly temperature-dependent, diffusion annealing is carried out at very high temperatures (for steel mostly between 1100 and 1300 ° C) and often over long annealing times (up to 50 hours).
Diffusion annealing is not very economical and is therefore only used when segregation cannot be avoided, for example in ingot casting of hypereutectoid bearing steels.
literature
- Zoch, Spur: Handbook Heat Treatment and Coating , 2015, p. 348.