stress relief annealing

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.

What is annealing.?

Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature and maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

In the cases of copper, steel, silver, and brass, this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool. Unlike ferrous metals—which must be cooled slowly to anneal—copper, silver[1] and brass can be cooled slowly in air or quickly by quenching in water. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming

Thermodynamcs of annealing:

Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to deform more easily, so increases their ductility.

The amount of process-initiating Gibbs free energy in a deformed metal is also reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed "stress relief".
The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow process. The high temperatures at which the annealing process occurs serve to accelerate this process.

The reaction facilitating the return of the cold-worked metal to its stress-free state has many reaction pathways, mostly involving the elimination of lattice vacancy gradients within the body of the metal. The creation of lattice vacancies is governed by the Arrhenius equation, and the migration/diffusion of lattice vacancies are governed by Fick’s laws of diffusion.

Mechanical properties, such as hardness and ductility, change as dislocations are eliminated and the metal's crystal lattice is altered. On heating at specific temperature and cooling it is possible to bring the atom at the right lattice site and new grain growth can improve the mechanical properties.

Stages of annealing:

There are three stages in the annealing process, with the first being the recovery phase, which results in softening of the metal through removal of crystal defects (the primary type of which is the linear defect called a dislocation) and the internal stresses which they cause. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains.

The second phase is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.

sedikit perkongsian mngenali stress relief annealing.