Annealing

Annealing is a heat-treatment process used to alter the properties of a material, typically to increase ductility, reduce hardness, relieve internal stresses, and improve machinability. The process involves heating a material to a specific temperature or range and then cooling it under controlled conditions.

In metals, annealing usually proceeds through stages of recovery, recrystallization, and grain growth. During recovery, dislocations rearrange with minimal changes to grain structure. Recrystallization forms new, strain-free grains, reducing hardness and internal stresses, while subsequent grain growth can occur if the material remains hot for extended periods. The exact temperatures, times, and cooling rates depend on the material and the desired outcome. Commonly, annealing improves formability and reduces brittleness, making metals easier to work with after cold-working.

Materials such as steel, copper, and aluminum are commonly treated by annealing, though the specifics vary. In steel, arrangements like full annealing or process annealing are used to achieve softer, more ductile structures. In copper and aluminum, annealing enhances ductility and can restore electrical conductivity that was degraded by work, sometimes requiring protective atmospheres to minimize oxidation.

Other materials are annealed as well. Glass is annealed to relieve internal stresses after forming, reducing the risk of cracking. Polymers can be annealed to modify crystallinity and reduce residual stresses. Ceramics may undergo annealing after sintering to improve microstructure and properties.

Simulated annealing is a separate concept, a probabilistic optimization technique inspired by the metal process. It uses a temperature parameter that gradually decreases to explore solution space and escape local optima, aiming to find a global optimum.

Overall, annealing balances softness, strength, and internal stress relief, with outcomes dependent on material type and processing conditions.