Annealing, in metallurgy and materials science, is a heat treatment that alters the physical
and sometimes chemical properties of a material to increase its ductility and to make it more
workable.
It involves heating a material to above its critical temperature, maintaining a suitable
temperature, and then cooling.
Annealing can 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 heating
the material for a while and then slowly letting it cool to room temperature in still air.
Copper, silver and brass can be cooled slowly in air, or quickly by quenching in water,
unlike ferrous metals, such as steel, which must be cooled slowly to anneal.
In this fashion, the metal is softened and prepared for further work—such as shaping,
stamping, or forming.
Thermodynamics Annealing occurs by the diffusion of atoms
within a solid material, so that the material progresses towards its equilibrium state.
Heat increases the rate of diffusion by providing the energy needed to break bonds.
The movement of atoms has the effect of redistributing and eradicating the dislocations in metals
and in ceramics.
This alteration to existing dislocations allows a metal object to deform more easily, so increasing
its 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 annealing occurs serve to accelerate this process.
The reaction that facilitates returning 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.
Hardness decreases and ductility increases, because dislocations are eliminated and the
metal's crystal lattice is altered.
On heating to a specific temperature atoms will migrate within the lattice and the adjusted
grain can improve the mechanical properties.
Stages The three stages of the annealing process
that proceed as the temperature of the material is increased are: recovery, recrystallization,
and grain growth.
The first stage is recovery, and it results in softening of the metal through removal
of primarily linear defects called dislocations and the internal stresses they cause.
Recovery occurs at the lower temperature stage of all annealing processes and before the
appearance of new strain-free grains.
The grain size and shape do not change.
The second stage 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 completed, then grain growth occurs.
In grain growth, the microstructure starts to coarsen and may cause the metal to lose
a substantial part of its original strength.
This can however be regained with hardening.
Controlled atmospheres The high temperature of annealing may result
in oxidation of the metal’s surface, resulting in scale.
If scale must be avoided, annealing is carried out in a special atmosphere, such as with
endothermic gas.
Annealing is also done in forming gas, a mixture of hydrogen and nitrogen.
The magnetic properties of mu-metal are introduced by annealing the alloy in a hydrogen atmosphere.
Setup and equipment Typically, large ovens are used for the annealing
process.
The inside of the oven is large enough to place the workpiece in a position to receive
maximum exposure to the circulating heated air.
For high volume process annealing, gas fired conveyor furnaces are often used.
For large workpieces or high quantity parts, car-bottom furnaces are used so workers can
easily move the parts in and out.
Once the annealing process is successfully completed, workpieces are sometimes left in
the oven so the parts cool in a controllable way.
While some workpieces are left in the oven to cool in a controlled fashion, other materials
and alloys are removed from the oven.
Once removed from the oven, the workpieces are often quickly cooled off in a process
known as quench hardening.
Typical methods of quench hardening materials involve media such as air, water, oil, or
salt.
Salt is used as a medium for quenching usually in the form of brine.
Brine provides faster cooling rates than water.
This is because when an object is quenched in water air bubbles form on the surface of
the object reducing the surface area the water is in contact with.
The salt in the brine reduces the formation of air bubbles on the objects surface, meaning
there is a larger surface area of the object in contact with the water, providing faster
cooling rates.
Quench hardening is generally applicable to some ferrous alloys, but not copper alloys.
Diffusion annealing of semiconductors In the semiconductor industry, silicon wafers
are annealed, so that dopant atoms, usually boron, phosphorus or arsenic, can diffuse
into substitutional positions in the crystal lattice, resulting in drastic changes in the
electrical properties of the semiconducting material.
Specialized cycles Normalization
Normalization is an annealing process applied to ferrous alloys to give the material a uniform
fine grained structure and make it less brittle.
It is used on steels of less than 0.4% carbon to transform austenite into ferrite, pearlite
and sorbite.
It involves heating the steel to 20-50 Kelvin above its upper critical point.
It is soaked for a short period at that temperature and then allowed to cool in air.
Smaller grains form that produce a tougher, more ductile material.
It eliminates columnar grains and dendritic segregation that sometimes occurs during casting.
Normalizing improves machinability of a component and provides dimensional stability if subjected
to further heat treatment processes.
Process annealing Process annealing, also called intermediate
annealing, subcritical annealing, or in-process annealing, is a heat treatment cycle that
restores some of the ductility to a product during the process of cold working, so it
can be worked further without breaking further heat treatment cycles.
The temperature range for process annealing ranges from 260 °C to 760 °C, depending
on the alloy in question.
Full anneal
A full anneal typically results in the second most ductile state a metal can assume for
metal alloy.
It creates a new uniform microstructure with good dynamic properties.
To perform a full anneal on steel for example, steel is heated to 50°C above the austenic
temperature and held for sufficient time to allow the material to fully form austenite
or austenite-cementite grain structure.
The material is then allowed to cool slowly so that the equilibrium microstructure is
obtained.
In some cases this means the material is allowed to air cool.
In other cases the material is allowed to furnace cool.
The details of the process depend on the type of metal and the precise alloy involved.
In any case the result is a more ductile material but a lower yield strength and a lower tensile
strength.
This process is also called LP annealing for lamellar pearlite in the steel industry as
opposed to a process anneal, which does not specify a microstructure and only has the
goal of softening the material.
Often the material to be machined is annealed, and then subject to further heat treatment
to achieve the final desired properties.
Short cycle anneal Short cycle annealing is used for turning
normal ferrite into malleable ferrite.
It consists of heating, cooling, and then heating again from 4 to 8 hours.
Resistive heating Resistive heating can be used to efficiently
anneal copper wire; the heating system employs a controlled electrical short circuit.
It can be advantageous because it does not require a temperature-regulated furnace like
other methods of annealing.
The process consists of two conductive pulleys, which the wire passes across after it is drawn.
The two pulleys have an electrical potential across them, which causes the wire to form
a short circuit.
The Joule effect causes the temperature of the wire to rise to approximately 400 °C.
This temperature is affected by the rotational speed of the pulleys, the ambient temperature,
and the voltage applied.
Where t is the temperature of the wire, K is a constant, V is the voltage applied, r
is the number of rotations of the pulleys per minute, and ta is the ambient temperature:
t =/(r))+ta The constant K depends on the diameter of
the pulleys and the resistivity of the copper.
Purely in terms of the temperature of the copper wire, an increase in the speed of the
wire through the pulley system has the same effect as an increase in resistance.
Therefore, the speed of the wire can be varied quadratically as the voltage is applied.
See also Annealing
Hollomon-Jaffe parameter Low hydrogen annealing
Tempering References
Further reading Thesis of Degree, Cable Manufacture and Tests
of General Use and Energy.
- Jorge Luis Pedraz, UNI, Files, Peru.
Dynamic annealing of the Copper wire by using a Controlled Short circuit.
= Jorge Luis Pedraz, Peru: Lima, CONIMERA 1999, INTERCON 99,
External links Annealing with induction: Ameritherm offers
annealing overview and Application Notes Annealing:efunda - engineering fundamentals
Full Annealing:Material Science Annealing: Aluminum and Aircraft Metal Alloys