Heat treatment

Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. The most common application is 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 as incidental phases of other manufacturing processes such as hot forming or welding.

Heat treatment of metals and alloys

Metallic materials consist of a diffusion, and the rate of cooling within the microstructure.

Annealing

Main article: Annealing (metallurgy)

Annealing is a technique used to recover precipitation hardening alloys, precipitates dissolve into the matrix, "solutionizing" the alloy.

Hardening and tempering (quenching and tempering)

Main article: Quench

To harden by quenching, a metal (usually steel or cast iron) must be heated into the martensite, a hard brittle crystalline structure.

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 strength is lost.

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.

Precipitation hardening

Main article: Precipitation strengthening

Some metals are considered precipitation hardening metals. Examples include 2000 series, 6000 series, and 7000 series stainless steels. If a precipitation hardened alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Aging a "solutionized" metal (either at room temperature - "natural aging" - or at a few hundred degrees - "artificial aging") 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. 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.

Selective hardening

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 jian is one of the earliest known examples of this, and the Japanese katana the most widely known. The Nepalese Khukuri is another example.

References

"Principles of Physical Metallurgy". Reed-Hill, Robert. 3rd edition. PWS Publishing, Boston. 1994.

v • d • e Metalworking
Metalworking topics	Metallurgy • Milling • Occupations • Press tools • Pipe and Tube bending • Smithing • Terminology • Welding

Categories: Alloys

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Heat_treatment". A list of authors is available in Wikipedia.