Tempering
Encyclopedia
Tempering is a heat treatment
technique for metals, alloy
s and glass
. In steel
s, tempering is done to "toughen" the metal by transforming brittle martensite
or bainite
into a combination of ferrite
and cementite
or sometimes Tempered martensite. Precipitation hardening alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal. Tempering is accomplished by a controlled reheating of the work piece to a temperature below its lower critical temperature.
The brittle martensite becomes tough 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 body-centred-tetragonal (BCT) structure. After the carbon diffuses, the result is nearly pure ferrite with body-centred structure.
In metallurgy and materials science, there is always a trade-off between strength
and ductility
. 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.
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 150–260 °C (302–500 F) and 370–650 °C (698–1,202 F). Tempering in the range of 260–370 °C (500–698 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). 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 bainitic steels, upper bainite or lower bainite may form depending on the duration and temperature of the tempering process. It is thermodynamically impossible that the martensite will be totally converted during tempering, so a mixture of martensite, bainite, ferrite and cementite is often formed.
or by a quench and self-temper (QST) process. After the bar exits the final rolling pass, where the final shape of the bar is applied, the bar is then sprayed with water which quenches the outer surface of the bar. The bar speed and the amount of water are carefully controlled in order to leave the core of the bar unquenched. The hot core then tempers the quenched skin, leaving a bar with high strength but with a degree of ductility.
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 aluminum 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.
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 color
of the oxide film
formed while tempering a well-polished blade.
Heat treatment
Heat treating is a group of industrial and metalworking processes 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...
technique for metals, alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
s and glass
Toughened glass
Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of...
. In steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
s, tempering is done to "toughen" the metal by transforming brittle martensite
Martensite
Martensite, named after the German metallurgist Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it can also refer to any crystal structure that is formed by displacive transformation. It includes a class of hard minerals occurring as lath- or...
or bainite
Bainite
Bainite is an acicular microstructure that forms in steels at temperatures from approximately 250-550°C . First described by E. S. Davenport and Edgar Bain, it is one of the decomposition products that may form when austenite is cooled past a critical temperature of 727 °C...
into a combination of ferrite
Ferrite (iron)
Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure. It is the component which gives steel and cast iron their magnetic properties, and is the classic example of a ferromagnetic material...
and cementite
Cementite
Cementite, also known as iron carbide, is a chemical compound of iron and carbon, with the formula Fe3C . By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more...
or sometimes Tempered martensite. Precipitation hardening alloys, like many grades of aluminum and superalloys, are tempered to precipitate intermetallic particles which strengthen the metal. Tempering is accomplished by a controlled reheating of the work piece to a temperature below its lower critical temperature.
The brittle martensite becomes tough and ductile after it is tempered. Carbon atoms were trapped in the austenite
Austenite
Austenite, also known as gamma phase iron, is a metallic non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of ; other alloys of steel have different eutectoid temperatures...
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 body-centred-tetragonal (BCT) structure. After the carbon diffuses, the result is nearly pure ferrite with body-centred structure.
In metallurgy and materials science, there is always a trade-off between strength
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...
and ductility
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
. 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.
Process characteristics
- Improves ductility and toughness
- Reduces cracking
- Improves machinability
- Increases impact resistance
- Improves malleability
- Decreases hardness
Tempering in steel
Typically steel is heat treated in a multi-step process. First it is heated to create a solid solutionSolid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
of iron and carbon in a process called austenizing. Austenizing is followed by quench
Quench
In materials science, quenching is the rapid cooling of a workpiece to obtain certain material properties. It prevents low-temperature processes, such as phase transformations, from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and...
ing to produce a martensitic microstructure. The steel is then tempered by heating between the ranges of 150–260 °C (302–500 F) and 370–650 °C (698–1,202 F). Tempering in the range of 260–370 °C (500–698 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). 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 bainitic steels, upper bainite or lower bainite may form depending on the duration and temperature of the tempering process. It is thermodynamically impossible that the martensite will be totally converted during tempering, so a mixture of martensite, bainite, ferrite and cementite is often formed.
Quench and self-temper
Modern reinforcing bar of 500MPa strength can be made from expensive microalloyed steelMicroalloyed steel
Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements . Standard alloying elements include: niobium, vanadium, titanium, molybdenum, zirconium, boron, and rare-earth metals...
or by a quench and self-temper (QST) process. After the bar exits the final rolling pass, where the final shape of the bar is applied, the bar is then sprayed with water which quenches the outer surface of the bar. The bar speed and the amount of water are carefully controlled in order to leave the core of the bar unquenched. The hot core then tempers the quenched skin, leaving a bar with high strength but with a degree of ductility.
Tempering in precipitation hardened alloys
Before a precipitation hardened alloy can be tempered, it must be placed in solution. During this process, 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 dissolving 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 aluminum 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 color of the workpiece. In this case, the blacksmithBlacksmith
A blacksmith is a person who creates objects from wrought iron or steel by forging the metal; that is, by using tools to hammer, bend, and cut...
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 color
Thin-film optics
Thin-film optics is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin-film optics, the thickness of the layers of material must be on the order of the wavelengths of visible light...
of the oxide film
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
formed while tempering a well-polished blade.