Annealing and stress relief

Annealing is a heat treatment procedure involving heating the alloy and holding it at a certain temperature (annealing temperature), followed by controlled cooling.
Annealing results in relief of internal stresses, softening, chemical homogenizing and transformation of the grain structure into more stable state.
Annealing increases an extent of equilibrium of the metal structure resulting in softening and high ductility.
Annealing temperature and the control cooling rate depend on the alloy composition and the type of the annealing treatment.
The following types of annealing are used in heat treatment of alloys:

• Full annealing is a process in which a ferrous alloy (commonly hypoeutectoid steel) is heated to about 100°F (55°C) above the upper critical temperature, followed by soaking and slow cooling in the furnace or in some medium to a temperature below the critical temperature.

For the non-ferrous alloys full annealing means full softening after cold work in contrast to partial annealing meaning partial softening.

• Subcritical annealing is annealing of cold-worked steel below the critical temperature on the iron-carbon phase diagram.

• Recrystallization annealing is a process of heating a cold worked metal to a temperature above the recrystallization temperature followed by soaking for a time required for the grain structure transformation.

Recrystallization annealing is widely used as an intermediate softening treatment between stages of cold work (cold rolling, drawing).
Combination of recrystallization annealing and cold work allows to control the microstructure grains size.

• Stress relief (recovery) – a relatively low temperature process of reducing internal mechanical stresses, caused by cold work, casting or welding.

The stress relief temperature is lower than the recrystallization temperature.

• Spheroidizing annealing is a process of controlled heating and cooling high carbon steels (tool steels) to produce spherical (globular) form of cementite inclusions.

This treatment improves the machining characteristics of the steel.

• Bright annealing is an annealing treatment which is carried out in furnaces with reducing atmosphere preventing surface oxidation of the steel parts.

• Homogenizing annealing is a durable high temperature annealing treatment intended to decrease chemical segregation by diffusion.

Homogenizing annealing is used for steel and aluminum ingots and castings.
More homogeneous intercrystalline distribution of carbon, phosphorus sulfur and alloying elements in steel ingots is achieved in annealing at 2000°F -2370°F (1100°C - 1300°C) for 20-50 hrs.
Aluminum alloys are treated at 790°F - 970°F (420°C - 520°C) for 16-30 hrs.