heat treatment

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heat treatment

[′hēt ‚trēt·mənt]
Heating and cooling a metal or alloy to obtain desired properties or conditions.

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.

heat treatment

Heating and cooling a solid metal or alloy in order to produce changes in its physical and mechanical properties.
References in periodicals archive ?
The ethanolic extract (1 mg per ml) of ginseng that was heat-treated at 150 C for 2 hours had the highest antiproliferative activity--greater than 70%--against the gastric and breast tumor cells.
Shandong exported $430 million worth of poultry meat last year, accounting for half of the national total; and, at $330 million, 60% of the country's heat-treated poultry exports.
The $45 million facility will produce 170,000 tons per year of heat-treated plate, quenched and tempered or normalized, ranging in thickness from 3/16- to 3 inches at widths up to 123 inches.
The unit is housed in all welded steel construction with heat-treated plate in product contact areas.
Another new report will help companies understand the factors that affect the production of cakes from heat-treated flour.
It has oversized bar stock CNC-milled heat-treated slide rails; captive dual recoil spring system; oversized steel slide-stop release; forged, heat-treated and fully-machined slide; hammer-forged barrel; dovetail front and rear steel sights, and ambidextrous magazine catch.
The system also measures total case depth of heat-treated samples and other types of metallurgical structures using a linear measurement mode.
Japan now bans imports of beef and meat-based cattle feeds that are not heat-treated from countries hit by the brain-wasting illness.
The Lindsay Tribunal heard that transfusion boss Dr Jack O'Riordon got angry when he was asked to withhold Factor 9 until it was heat-treated to kill HIV.
After heat-treated coagulants were approved by the government in 1985, the Green Cross executives approved the sale of the tainted and untreated agents to avoid losing money from lost inventory, continuing to do so even after Green Cross began selling heat-treated products in early 1986.
The high vanadium P/M steels can be heat-treated from 1850F to 2150E The P/M high speed M-4 alloy and P/M T-15 alloy can be austenitized, quenched, and tempered at 2200F to 2250F.