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 hardened alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal. Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. ... An alloy is a homogeneous hybrid of two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. ... For other uses, see Steel (disambiguation). ... Martensite in AISI 4140 steel 0. ... Iron-carbon phase diagram, showing the eutectoid temperature and composition, at which bainite can form. ... Iron-carbon phase diagram, showing the conditions under which ferrite (α) is stable. ... Precipitation hardening, also called Age hardening, is a heat treatment technique used to strengthen malleable materials, especially non-ferrous alloys including most structural alloys of aluminium and titanium. ... Intermetallics is concerned with all aspects of ordered chemical compounds between two or more metals and notably with their applications. ...

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. Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel. ...

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. Gold is a highly ductile metal Ductility is a mechanical property which describes how much plastic deformation a material can sustain before fracture occurs. ... “Brittle” redirects here. ...

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 quenching 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. Fig. ... A quench refers to a rapid cooling. ...

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. For the syntaxic operation, see Dislocation (syntax) For the medical term, see Dislocation (medicine) In materials science a dislocation is a linear crystallographic defect, or irregularity, in crystal structure. ...

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. It has been suggested that this article or section be merged with Superalloy. ...

Tempering in blacksmithing

The temperatures used in tempering are often too low to be gauged by song. 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 color of the oxide film formed while tempering a well-polished blade. Thin-film optics is the branch of optics which deals with very thin structured layers of different materials. ... Passivation is the process of making a material passive in relation to another material prior to using the materials together. ...

See also

• Annealing (metallurgy): slow cooling process
• Forge
• Heat treatment
• Precipitation strengthening

For other uses, see Annealing. ... For the process of shaping metal by localized compressive forces, see Forging. ... Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. ... Precipitation hardening, also called age hardening or dispersion hardening, are heat treatment techniques used to strengthen malleable materials, especially non-ferrous alloys including most structural alloys of aluminium, magnesium and titanium. ...

External links

• A thorough discussion of tempering processes

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