Anneointi

Anneointi is the Finnish term for the heat-treatment process known in English as annealing. The process alters the physical and mechanical properties of metals, glasses, and some ceramics by heating to a chosen temperature, holding for a period, and cooling slowly.

The primary aims are to relieve internal stresses and reduce hardness, thereby increasing ductility and workability. Annealing can also homogenize the microstructure and improve machinability. The specific temperature, hold time, and cooling rate depend on the material and the desired outcome.

In practice, a workpiece is heated to a predefined holding temperature, maintained for a set time, and then cooled at a controlled rate, often within a furnace or kiln. Different variants tailor different results: full annealing for maximum softness in steels; process annealing to relieve work hardening in rolled metals; stress-relief annealing to reduce residual stresses without major structural changes; spheroidizing annealing to improve machinability of high-carbon steels; and in glasses, annealing to relieve formed stresses and prevent cracking.

Materials commonly subjected to anneointi include steels, cast irons, aluminum and copper alloys, glass, and some ceramics. In steel, the outcome is a softer, more ductile metal with a refined or restored grain structure; in glass, the process reduces internal stresses after forming.

The term also appears in computer science as simulated annealing, a stochastic optimization method inspired by the physical process.