Beläggningshärdning

Beläggningshärdning, also known as precipitation hardening or age hardening, is a heat treatment technique used to increase the yield strength of malleable metal alloys. This process is applicable to alloys that exhibit solid-state solubility and form a supersaturated solid solution during rapid cooling. The fundamental principle involves the formation of fine, dispersed precipitates within the metal matrix.

The process typically begins with a solution heat treatment, where the alloy is heated to a temperature that dissolves the alloying elements into a single solid phase. This is followed by a rapid quench, usually in water or oil, to trap these elements in a supersaturated solid solution at a lower temperature. This supersaturated state is inherently unstable.

The next step is the aging or precipitation treatment. The alloy is then heated to a moderate temperature, below its recrystallization temperature, and held for a specific period. During aging, the dissolved alloying elements begin to precipitate out of the supersaturated solid solution, forming extremely small, uniformly distributed particles. These precipitates impede the movement of dislocations, which are line defects in the crystal lattice responsible for plastic deformation. The increased resistance to dislocation motion significantly enhances the alloy's hardness and strength.

The effectiveness of beläggningshärdning depends on factors such as the alloy composition, solution treatment temperature and time, quenching rate, and aging temperature and time. Over-aging, where precipitates grow too large or coalesce, can lead to a decrease in strength. Different alloys respond differently to this treatment, with common examples including aluminum alloys (e.g., Duralumin), some stainless steels, and nickel-based superalloys.