heat treatment

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heat treatment

[′hēt ‚trēt·mənt]
Heating and cooling a metal or alloy to obtain desired properties or conditions.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Heat treatment (metallurgy)

A procedure of heating and cooling a material without melting. Plastic deformation may be included in the sequence of heating and cooling steps, thus defining a thermomechanical treatment. Typical objectives of heat treatments are hardening, strengthening, softening, improved formability, improved machinability, stress relief, and improved dimensional stability. Heat treatments are often categorized with special names, such as annealing, normalizing, stress relief anneals, process anneals, hardening, tempering, austempering, martempering, intercritical annealing, carburizing, nitriding, solution anneal, aging, precipitation hardening, and thermomechanical treatment.

All metals and alloys in common use are heat-treated at some stage during processing. Iron alloys, however, respond to heat treatments in a unique way because of the multitude of phase changes which can be induced, and it is thus convenient to discuss heat treatments for ferrous and nonferrous metals separately.

Ferrous metals

Annealing heat treatments are used to soften the steel, to improve the machinability, to relieve internal stresses, to impart dimensional stability, and to refine the grain size.

Hardening treatments are used to harden steels by heating to a temperature at which austenite is formed and then cooling with sufficient rapidity to make the transformation to pearlite or ferrite unfavorable.

Some heat treatments are used to alter the chemistry at the surface of a steel, usually to achieve preferential hardening of a surface layer. Carburizing consists of subjecting the steel to an atmosphere of partially combusted natural gas which has been enriched with respect to carbon. In the nitriding treatment, nitrogen diffusing to the surface of the steel forms nitrides. Chromizing involves the addition of chromium to the surface by diffusion from a chromium-rich material packed around the steel or dissolved in molten lead.

Nonferrous metals

Many nonferrous metals do not exhibit phase transformations, and it is not possible to harden them by means of simple heating and quenching treatments as in steel. Unlike steels, it is impossible to achieve grain refinement by heat treatment alone, but it is possible to reduce the grain size by a combination of cold-working and annealing treatments.

Some nonferrous alloys can be hardened, but the mechanism is one by which a fine precipitate is formed, and the reaction is fundamentally different from the martensitic hardening reaction in steel. There are also certain ferrous alloys that can be precipitation hardened. However this hardening technique is used much more widely in nonferrous than in ferrous alloys. In titanium alloys, the β phase can transform in a martensitic reaction on rapid cooling, and the hardening of these alloys is achieved by methods which are similar to those used for steels.

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.

heat treatment

Heating and cooling a solid metal or alloy in order to produce changes in its physical and mechanical properties.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
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This conduit must be routed through the wall of the heat-treatment machine at a suitable point.
Figure 14 shows the relationship between the heat-treatment temperature and the total haze obtained by 10 wt% additions of L-LDPE with 898, 905, and 912 kg/[m.sup.3].
The incident at the Shimane reactor occurred last December when a person in charge of the work mistakenly operated the heat-treatment equipment and failed to quickly notice the irregularity, they said.
He was a member of the Triumph Art Society and, after being shown in various local exhibitions, the picture ended up hanging in the heat-treatment supervisor's office.
Defects such as internal or surface checks, collapse, splits, twist, and warp can occur in wood during the heat-treatment process if the heating parameters are not well chosen.
It appears that the microfibrils coalesce significantly upon heat-treatment at 600 [degrees] C.
These wood species responded to the heat-treatment technique, resulting in some density loss, making these woods more suitable for use in areas where stability is important, such as window frames.
ThermoWood[R] is a wood heat-treatment process developed at VTT, Finland.
Figure 4 shows the wood color could be influenced by the heat-treatment process.
MOE decreased with heat-treatment temperature and time, but for the less-severe heat treatments the difference was very small.
Certification, concerning either heat-treatment or fumigation for overseas shipment, is a paperless system that requires official stamps on the final product.