Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as
glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, temperingand quenching. It is noteworthy that while the term "heat treatment" applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
Heat treatment of metals and alloys
Metallic materials consist of a
microstructureof small crystals called "grains" or crystallites. The nature of the grains (i.e. grain sizeand composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure.
Complex heat treating schedules are often devised by
metallurgists to optimize an alloy's mechanical properties. In the aerospaceindustry, a superalloy may undergo five or more different heat treating operations to develop the desired properties. This can lead to quality problems depending on the accuracy of the furnace's temperature controls and timer.
Annealingis a technique used to recover cold workand relax stresses within a metal. Annealing typically results in a soft, ductile metal. When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment. When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment. During annealing, small grains recrystallize to form larger grains. In precipitation hardeningalloys, precipitates dissolve into the matrix, "solutionizing" the alloy.
Typical annealing processes include, "normalizing", "stress relief" annealing to recover cold work, and full annealing.
Hardening and tempering (quenching and tempering)
To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other
gas(such as nitrogen), oil, polymerdissolved in water, or brine. Upon being rapidly cooled, a portion of austentite (dependent on alloy composition) will transform to martensite, a hard brittle crystalline structure. The quenched hardness of a metal depends upon its chemical composition and quenching method. Cooling speeds, from fastest to slowest, go from polymer (i.e. silicon), brine, fresh water, oil, and forced air. However, quenching a certain steel too fast can result in cracking, which is why High-tensile steels like AISI 4140 should be quenched in oil, tool steels such as 2767 or H13 hot work tool steel should be quenched in forced air, and low alloy or medium-tensile steels such as XK1320 or AISI 1040 should be quenched in brine or water. However, metals such as austenitic stainless steel (304, 316), and copper, produce an opposite effect when these are quenched; they anneal. Austenitic stainless steels must be quench-annealed to become fully corrosion resistant, as they work-harden significantly.
Untempered martensite, while very hard and strong, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. Most applications require that quenched parts be tempered (heat treated at a low temperature, often three hundred degree Fahrenheit or one hundred fifty degrees Celsius) to impart some
toughness. Higher tempering temperatures (may be up to thirteen hundred degrees Fahrenheit, depending on alloy and application) are sometimes used to impart further ductility, although some yield strength is lost.
Some metals are classified as "precipitation hardening metals". When a precipitation hardening alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will nucleate and fall out of solution and act as a reinforcing phase, thereby increasing the strength of the alloy. Alloys may age "naturally" meaning that the precipitates form at room temperature, or they may age "artificially" when precipitates only form at elevated temperatures. In some applications, naturally aging alloys may be stored in a freezer to prevent hardening until after further operations - assembly of rivets, for example, may be easier with a softer part.
Examples of precipitation hardening alloys include 2000 series, 6000 series, and 7000 series
aluminium alloy, as well as some superalloys and some stainless steels.
Some techniques allow different areas of a single object to receive different heat treatments. This is called
differential hardening. It is common in high quality knives and swords. The Chinese jianis one of the earliest known examples of this, and the Japanese katanathe most widely known. The Nepalese Khukuriis another example.
"Principles of Physical Metallurgy". Reed-Hill, Robert. 3rd edition. PWS Publishing, Boston. 1994.
* [http://www.metalimprovement.com/heat_hi.php Heat treating basics video]
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