Heat treatment

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Heat Treatment is a technique 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.

The techniques include annealing, case hardening, precipitation strengthening, tempering and quenching.

1 Heat Treatment of Metals and Alloys
2 Annealing
3 Quenching
4 Surface Hardening
5 Heat treatment of swords and knives
6 See also
7 External links
8 References

[edit] Heat Treatment of Metals and Alloys

Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) 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. In carbon and low alloy steels, fast rates of cooling result in a high degree of hardness. In precipitation hardened alloys like 2000 series, 6000 series, and 7000 series aluminium alloy, as well as some superalloys and some stainless steels, fast cooling rates result in a softer metal. At these fast cooling rates, the alloying elements are trapped in solution and require a tempering to precipitate intermetallic particles, thereby achieving maximum strength and hardness.

[edit] Annealing

Main article: Annealing (metallurgy)

Annealing is a technique used to recover cold work and relax stresses within a metal. During annealing, small grains recrystallize to form larger grains. In precipitation hardening alloys, precipitates dissolve into the matrix, "solutionizing" the alloy.

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.

[edit] Quenching

Main article: Quench

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, polymer dissolved in water, water, or brine. Upon being rapidly cooled, the austentite will transform to martensite, a hard brittle crystalline structure. 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 strength is lost.

If a precipitation hardened alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Tempering a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will fall out of solution and act as a reinforcing phase, there by increasing the strength of the alloy. This process is called "artificial aging". Conversely, some alloys may be "naturally aged" in which the intermetallic particles form at room temperature. Naturally aging alloys are often stored in a freezer to prevent them from aging until needed.

Complex heat treating schedules are often devised by metallurgists to optimize an alloy's mechanical properties. In the aerospace industry, 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.

[edit] Surface Hardening

Main article: case hardening

Surface hardening describes a variety of quenching processes that harden only the surface of the part. These processes impart a hard wear-resistant surface to a part, while maintaining a ductile (soft) core. Tools, blades, and bearings are often heat treated with surface hardening processes.

Induction hardening quickly austentizes a part before quenching.The surface endures a martensitic transformation while the core remains ductile to a heat transfer gradient through the cross section of the part.
Flame hardening (heating with an open flame before quenching) may be used to heat the surface of the part, although it can also be used to harden a part all the way through, or selectively harden only a portion of the part.
Case hardening is done on low-alloy steels that do not develop much hardness when quenched. Various methods are used to add carbon, and sometimes also nitrogen to the surface of the part. These alloying elements diffuse into the surface of the part, creating a hard, wear resistant layer.

[edit] Heat treatment of swords and knives

Usually a quenched blade is too brittle for use until tempered. Depending on the alloy used, it will be evenly heated between 200 and 500 degrees Fahrenheit (90 to 260 °C), held at that temperature (soaked) for an appropriate time (seconds or hours), then cooled slowly over an appropriate duration (minutes or hours). This heat treatment will ensure a strong blade that will hold an edge but not break by balancing the amount of hard martensite with ductile ferrite and pearlite.

In certain cases, different areas of an object will be heat treated differently. This is called differential hardening. It is common in high quality knives and swords. The Japanese katana is the best known for this. However, Chinese swords were traditionally done this way, as were Nepalese Khukuri, and many others.

[edit] See also

[edit] External links

Induction Heating Processes - How induction heating is used across an array of material heating processes.
Induction Atmospheres - has information on about induction heat treating