Glossary

Annealing is a heat treatment process used to soften metal, relieve internal stresses, improve ductility, and refine grain structure—essentially restoring a metal’s workability and toughness after it has been hardened or deformed through processes like forging, machining, or cold working.

The annealing process involves three main stages: recovery, recrystallization, and grain growth. The metal (often steel, copper, aluminum, or brass) is first heated to a specific temperature—high enough to allow atomic diffusion and structural changes, but below its melting point. For steel, this is typically between 550°C and 750°C (1020–1380°F), depending on composition. The metal is then held (soaked) at that temperature long enough for its internal structure to homogenize, allowing new strain-free grains to form. Finally, the material is slowly cooled, usually in the furnace, to prevent new stresses or hard microstructures from developing.

In carbon and alloy steels, the goal of annealing is to transform the microstructure (often distorted ferrite and pearlite from prior processing) into a softer, more uniform arrangement. This process eliminates dislocations and internal stresses created during cold working or welding, restoring ductility and machinability. In non-ferrous metals like copper or aluminum, annealing serves a similar purpose—removing work hardening effects and returning the metal to a more pliable state for further forming or shaping.

There are several types of annealing, each tailored to specific materials and goals:

- Full annealing: Heats steel above its critical temperature (austenitizing range) and cools it slowly to produce coarse pearlite—a very soft structure ideal for machining.

- Process annealing: Uses lower temperatures to relieve stress and restore ductility without changing the metal’s core structure—often used between cold working stages.

- Spheroidizing anneal: Applied to high-carbon steels to form rounded carbides, improving machinability before hardening.

- Stress-relief annealing: Removes residual stresses after welding, casting, or machining without significantly altering strength or hardness.

The cooling rate is critical: slow cooling in the furnace allows equilibrium microstructures to form, while rapid cooling may produce harder or more brittle phases (as in quenching).