Heat-treating of metal alloys, particularly steel, to reduce brittleness and restore ductility. In tempering, steel is slowly heated to a temperature between 300 and 1,300 °F (150 and 700 °C), depending on desired properties, in an oil or salt bath and held for about two hours and then allowed to air cool. As steel is physically worked (e.g., rolling, wiredrawing, hammering), hardening takes place, and it grows progressively more brittle. Similarly, but more deliberately, heating and quenching increase hardness. Combined quench-and-temper heat-treating is applied at many different cooling rates, holding times, and temperatures and is a very important means of controlling the properties of steel.
Learn more about tempering with a free trial on Britannica.com.
Tempering is a heat treatment technique for metals and alloys. In steels, tempering is done to "toughen" the metal by transforming brittle martensite into bainite or a combination of ferrite and cementite. Precipitation hardening alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal.
The brittle martensite becomes strong and ductile after it is tempered. Carbon atoms were trapped in the austenite when it was rapidly cooled, typically by oil or water quenching, forming the martensite. The martensite becomes strong after being tempered because when reheated, the microstructure can rearrange and the carbon atoms can diffuse out of the distorted BCT structure. After the carbon diffuses, the result is nearly pure ferrite.
In metallurgy, there is always a tradeoff between ductility and brittleness. This delicate balance highlights many of the subtleties inherent to the tempering process. Precise control of time and temperature during the tempering process are critical to achieve a metal with well balanced mechanical properties.
During tempering, the alloying elements will diffuse through the alloy and react to form intermetallic compounds. The intermetallic compounds are not soluble in the alloy, and will precipitate, forming small particles. These particles strengthen the metal by impeding the movement of dislocations through the crystal structure of the alloy. Careful manipulation of tempering time and temperature allows the size and amount of precipitates to be controlled, thus tailoring the mechanical properties of the alloy.
Tempering in aluminium is also referred to as "aging". Artificially aged alloys are tempered at elevated temperature, while naturally aging alloys may be tempered at room temperature.
Alloy systems with a large number of alloying elements, like some superalloys may be subjected to several tempering operations. During each operation a different precipitate is formed, resulting in a large number of different precipitates that are difficult to drive back into solution. This phenomenon contributes to the high temperature strength of precipitation hardened superalloys.
Tempering: tempering is the process of transforming metastable martensite (a super-saturated solution of carbon in iron) to tempered martensite (ferrite and carbide) at temperatures below the lower transformation temperature. (Thermal Process Guide).
Dec 01, 2002; General tempering can be defined as heating steel to any temperature below the critical temperature, [A.sub.1], (which...
Software assists in optimizing tempering process: tempering-calculator software based on the Larsen-Miller equation facilitates analysis of the heat-treating process by integrating tempering factors, which helps in the design and operation of tempering equipment and improves process control.(Process Control & Instrumentation)
Dec 01, 2005; tempering information currently is available in reference handbooks in the form of hardness-temperature curves, a...
'Energized' about tempering: Aasted introduces a new line of tempering machines that reduces energy and water consumption by at least 50%, while simultaneously enhancing quality.(International SUPPLIER SPOTLIGHT)
Apr 01, 2011; On March 26, cities throughout the world experienced Earth Hour, a global movement that requests participants to turn off lights...