Tempering
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2008-06-28T04:53:59Z
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:''This article is about tempering in metallurgy. For other uses of the word, see [[temper]].''
'''Tempering''' is a [[heat treatment]] technique for metals and [[alloy]]s. In [[steel]]s, tempering is done to "toughen" the metal by transforming brittle [[martensite]] into [[bainite]] or a combination of [[ferrite (iron)|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.
==Tempering in steel==
Typically steel is heat treated in a multi-step process. First it is heated to create a [[solid solution]] of iron and carbon in a process called ''[[austenizing]]''. Austenizing is followed by [[quench]]ing to produce a martensitic microstructure. The steel is then tempered by heating between the ranges of 150°C-260°C (300°F-500°F) and 370°C-650°C (700°F-1200°F). Tempering in the range of 260°C-370°C (500°F-700°F) is sometimes avoided to reduce temper brittling. The steel is held at that temperature until the carbon trapped in the martensite diffuses to produce a chemical composition with the potential to create either bainite or pearlite (a crystal structure formed from a mixture of ferrite and cementite). It should be noted that when producing a truly bainitic or pearlitic steel the steel must be once again taken up to the austenite region (austenizing) and cooled slowly to a controlled temperature before being fully quenched to a low temperature. In banitic steels, upper banite or lower banite may form depending on the length and temperature of the tempering process. It is thermodynamically improbable that the martensite will be totally converted during tempering, so a mixture of martensite, bainite, ferrite and cementite is often formed.
==Tempering in precipitation hardened alloys==
Before a [[precipitation hardened]] alloy can be tempered, it must be "solutionized". During solutionizing, the alloy is heated to dissolve and uniformly distribute alloying elements. The alloy is then quenched at a rate of cooling high enough to prevent the alloying elements from falling out of solution. The alloy is then tempered, by heating at temperatures lower than the solutionizing temperature.
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 in blacksmithing==
The temperatures used in tempering are often too low to be gauged by the [[blackbody radiation|color of the workpiece]]. In this case, the [[blacksmith]] will heat the work piece for a known amount of time. Doing this ensures a certain degree of consistency in the tempering process from work piece to work piece. The cumulative effects of time and temperature can also be gauged by monitoring the [[thin-film optics|color]] of the [[passivation|oxide film]] formed while tempering a well-polished blade.
==See also==
*[[Annealing (metallurgy)]]: slow cooling process
*[[Forge]]
*[[Heat treatment]]
*[[Precipitation strengthening]]
==External links==
*[http://www.msm.cam.ac.uk/phase-trans/2004/Tempered.Martensite/tempered.martensite.html A thorough discussion of tempering processes]
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