Annealing (metallurgy) 3424459 222723272 2008-06-30T20:11:23Z Nahraana 6225641 Removed merge template. Reason: merge discussion closed and merge completed. {{otheruses|Annealing}} {{Unreferenced|date=February 2008}} '''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 and maintaining a suitable temperature, and then cooling. Annealing is used to induce [[ductility]], relieve internal stresses, refine the structure and improve [[cold work|cold working]] properties. In the cases of [[copper]], [[steel]], and [[brass]] this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool slowly. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming. ==Thermodynamics 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 reform 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 are governed by the [[Arrhenius equation]], and the migration/diffusion of lattice vacancies are governed by [[Fick's law of diffusion|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 (metallurgy)|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. The second phase is [[recrystallization (metallurgy)|recrystallization]], where new 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. ==Annealing in a controlled atmosphere== The low temperature of annealing (about 50 °F above C3 line) may result in oxidation of the metal’s surface, resulting in scale. If scale is to be avoided, annealing is carried out in an oxygen-, carbon-, and nitrogen-free atmosphere (to avoid oxidation, carburization, and nitriding respectively) such as [[endothermic gas]] (a mixture of carbon monoxide, hydrogen gas, and nitrogen). The [[magnet|magnetic]] properties of [[mu-metal]] (Espey cores) are introduced by annealing the alloy in a [[hydrogen]] [[atmosphere]]. ==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 annealing cycles== ===Normalization=== ''Normalization'' is an annealing process in which a metal is cooled in air after heating. This process is typically confined to hardenable steel. It is used to refine grains which have been deformed through [[cold work]], and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked for a short period then allowed to cool in air. Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum ductility achieved by annealing. ===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 work piece allowing it be worked further without breaking. Ductility is important in shaping and creating a more refined piece of work through processes such as [[Rolling (metalworking)|rolling]], [[Drawing (manufacturing)|drawing]], [[forging]], [[Metal spinning|spinning]], [[extruding]] and [[Heading (metalworking)|heading]]. The piece is heated to a temperature typically below the austenizing temperature, and held there for long enough to relieve stresses in the metal. The piece is finally cooled slowly in to room temperature. It is then ready again for 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 is ranges from 500 ºF to 1400 ºF, depending on the alloy in question. ===Full anneal=== [[image:Full_annealing_temp_range.PNG|thumb|300px|Full annealing temperature ranges]] A full anneal typically results in the most ductile state a metal can assume for metal alloy. To perform a full anneal, a metal is heated to its annealing point (about 50°c above the austenic temperature as graph shows) and held for sufficient time to allow the material to fully [[austenite|austenitize]], to form austenite or austenite-cementite grain structure. The material is then allowed to cool slowly so that the [[Phase diagram|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 that has greater [[stretch ratio]] and reduction of area properties 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 material that is annealed will be machined and then be followed by further heat treatment to obtain the final desired properties. ===Short cycle anneal=== Short cycle annealing is used for turning normal ferrite into malleabe ferrite. It consists of heating, cooling, and then heating again from 4 to 8 hours. ==See also== *[[Tempering]] *[[Annealing (glass)]] *[[Heat treatment]] *[[Hollomon-Jaffe parameter]] *[[Austenite]] ==External links== *[http://ameritherm.com/overview_annealing.php Annealing with induction]: Ameritherm offers annealing overview and Application Notes *[http://www.efunda.com/processes/heat_treat/softening/annealing.cfm Annealing]:efunda - engineering fundamentals *[http://info.lu.farmingdale.edu/depts/met/met205/ANNEALING.html Full Annealing]:Material Science [[Category:Metallurgy]] [[Category:Metals processes]] [[de:Glühen]] [[el:Ανόπτηση]] [[es:Recocido]] [[fa:آنیل‌کردن]] [[fr:Recuit]] [[it:Ricottura]] [[ja:焼きなまし]] [[pl:Wyżarzanie]] [[pt:Recozimento]] [[ru:Отжиг]] [[sk:Žíhanie]] [[fi:Metallin_lämpökäsittely]] [[sv:Anlöpning]]