annealing (metallurgy) - wikipedia, the free encyclopedia

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Annealing (metallurgy)From Wikipedia, the free encyclopedia

Annealing, in metallurgy and materials science, is a heat treatment that alters 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 (generally

until glowing) for a while and then slowly letting it cool to room temperature in still air. Copper, silver[1]

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

worksuch as shaping, stamping, or forming.

Contents

1 Thermodynamics

1.1 Stages

2 Controlled atmospheres

3 Setup and equipment

4 Diffusion annealing of semiconductors

5 Specialized cycles

5.1 Normalization

5.2 Process annealing

5.3 Full anneal

5.4 Short cycle anneal6 Resistive heating

7 See also

8 References

9 Further reading

10 External links

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 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.[citation needed]

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.[citation needed]

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.[citation needed]

The reaction that facilitates returning the cold-worked metal to its stress-free state has many reaction

nealing (metallurgy) - Wikipedia, the free encyclopedia http://en.wikipedia.org/w/index.php?title=Annealing_(metallurgy)&pri...

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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 Ficks laws of diffusion.[2]

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.[citation needed]

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 dislocationsand 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.[3]The second stage is recrystallization, where

new strain-free grains nucleate and grow to replace those deformed by internal stresses.[3]

If annealing is

allowed to continue once recrystallization has completed, then grain growth (the third stage) occurs. In grain

growth, the microstructure starts to coarsen and may cause the metal to lose a substantial part of its originalstrength. This can however be regained with hardening.[citation needed]

Controlled atmospheres

The high temperature of annealing may result in oxidation of the metals surface, resulting in scale. If scale

must be avoided, annealing is carried out in a special atmosphere, such as with endothermic gas (a mixture

of carbon monoxide, hydrogen gas, and nitrogen gas). Annealing is also done in forming gas, a mixture of

hydrogen and nitrogen.

The magnetic properties of mu-metal (Espey cores) are introduced by annealing the alloy in a hydrogenatmosphere.

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 (salt water). 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.[citation needed]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


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Full annealing temperature ranges

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

Normalizationis 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 austinite into

ferrite, pearlite and sorbite. It involves heating the steel to 20-50 degrees 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. Ductility is important in shaping and creating a more refined piece

of work through processes such as rolling, drawing, forging, spinning, extruding and heading. The piece is

heated to a temperature typically below the austenizing temperature, and held there long enough to relieve

stresses in the metal. The piece is then furnace cooled. It can then be subjected to additional cold working.

This can also be used to ensure there is reduced risk of distortion of the work piece during machining,

welding, or further heat treatment cycles.

The temperature range for process annealing ranges from 260 C (500 F) to 760 C (1400 F), 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 an

entirely new uniform microstructure with good dynamic

properties. To perform a full anneal on steel for example,

steel is heated to 50C 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 pearlitein the steel industry as

opposed to aprocess 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


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Short cycle annealingis 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 (step 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 tais the ambient temperature:

t = ((KV )/(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 (glass)

Hollomon-Jaffe parameterLow hydrogen annealing

Tempering (metallurgy)

References

^ http://www.handyharmancanada.com/hbpm/silver/silver.htm1.

^Van Vlack, L.H.Elements of Materials Science and Engineering, Addison-Wesley, 1985, p 1342.

^ abVerhoeven, J.D. Fundamentals of Physical Metallurgy, Wiley, New York, 1975, p. 3263.

Further reading

Thesis of Degree, Cable Manufacture and Tests of General Use and Energy. - Jorge Luis Pedraz

(1994), UNI, Files, Peru.

Dynamic annealing of the Copper wire by using a Controlled Short circuit. = Jorge Luis Pedraz

(1999), Peru: Lima, CONIMERA 1999, INTERCON 99,

External links

Annealing with induction (http://ameritherm.com/overview_annealing.php): Ameritherm offers

annealing overview and Application Notes

Annealing (http://www.efunda.com/processes/heat_treat/softening/annealing.cfm):efunda -

engineering fundamentals


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Full Annealing (http://info.lu.farmingdale.edu/depts/met/met205/ANNEALING.html):Material

Science

Annealing (http://www.tinmantech.com/html/aluminum_alloys_continued.php): Aluminum and

Aircraft Metal Alloys

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Categories: Metal heat treatments

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