What is "annealing"

Annealing \An*neal"ing\, n.

1. The process used to render glass, iron, etc., less brittle, performed by allowing them to cool very gradually from a high heat.

2. The burning of metallic colors into glass, earthenware, etc.

Anneal \An*neal"\, v. t. [imp. & p. p. Annealed; p. pr. & vb. n. Annealing.] [OE. anelen to heat, burn, AS. an?lan; an on + ?lan to burn; also OE. anelen to enamel, prob. influenced by OF. neeler, nieler, to put a black enamel on gold or silver, F. nieller, fr. LL. nigellare to blacken, fr. L. nigellus blackish, dim. of niger black. Cf. Niello, Negro.]

1. To subject to great heat, and then cool slowly, as glass, cast iron, steel, or other metal, for the purpose of rendering it less brittle; to temper; to toughen.

2. To heat, as glass, tiles, or earthenware, in order to fix the colors laid on them.

n. The act of heating solid metal or glass to high temperatures and cooling it slowly so that its particles arrange into a defined lattice. vb. (present participle of anneal English)

n. hardening something by heat treatment [syn: tempering]

Annealing may refer to:

• Annealing (metallurgy), a heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility
• Annealing (glass), heating a piece of glass to remove stress
• Annealing (biology), in genetics, means for complementary sequences of single-stranded DNA or RNA to pair by hydrogen bonds to form a double-stranded polynucleotide
• Simulated annealing, a numerical optimization technique for searching for a solution in a space otherwise too large for ordinary search methods to yield results
• Quantum annealing, a method for finding solutions to combinatorial optimisation problems and ground states of glassy systems using quantum fluctuations

Annealing of glass is a process of slowly cooling hot glass objects after they have been formed, to relieve residual internal stresses introduced during manufacture. Especially for smaller, simpler objects, annealing may be incidental to the process of manufacture, but in larger or more complex products it commonly demands a special process of annealing in a temperature-controlled kiln known as a lehr. Annealing of glass is critical to its durability. Glass that has not been properly annealed retains thermal stresses caused by quenching, which indefinitely decrease the strength and reliability of the product. Inadequately annealed glass is likely to crack or shatter when subjected to relatively small temperature changes or to mechanical shock or stress. It even may fail spontaneously.

To anneal glass it is necessary to heat it to its annealing temperature, at which its viscosity, η, drops to 10 Poise ("Poise" is a measure of absolute viscosity; 1 poise = 1 dyne-second/cm²). For most kinds of glass this annealing temperature is in the range of 850–900 °F (454 - 482 °C), and is the so-called stress-relief point or annealing point of the glass. At such a viscosity the glass still is too hard for significant external deformation without breaking, but it is soft enough to relax internal strains by microscopic flow in response to the intense stresses they introduce internally. The piece then heat-soaks until its temperature is even throughout and the stress relaxation is adequate. The time necessary for this step varies depending on the type of glass and its maximum thickness. The glass then is permitted to cool at a predetermined rate until its temperature passes the strain point (η = 10 Poise), below which even microscopic internal flow effectively stops and annealing stops with it. It then is safe to cool the product to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After annealing is complete the material can be cut to size, drilled, or polished without risk of its internal stresses shattering it.

At the annealing point (η = 10 Poise) stresses relax within several minutes, while at the strain point (η = 10 Poise) stresses relax within several hours. Stresses acquired at temperatures above the strain point, and not relaxed by annealing, remain in the glass indefinitely and may cause either short-term or much delayed failure. Stresses resulting from cooling too rapidly below the strain point largely are considered temporary, although they may be adequate to promote short-term failure.

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 reduce its hardness, making it more workable. It involves heating a material to above its recrystallization temperature, maintaining a suitable temperature, and then cooling.

In annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to the change in ductility and hardness.

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 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.