Information about Work Hardening
Work hardening, or strain hardening, is an increase in the strength of a material due to plastic deformation. In metallic solids permanent change of shape is usually carried out on a microscopic scale by defects called dislocations which are created by stress and rearrange the material by moving through it. At normal temperatures these defects do not anneal out of the material but build up as the material is worked, interfering with one another's motion and thus increasing strength and decreasing ductility.
Any material with a reasonably high melting point can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at normal ambient temperatures; for example indium, which has a low melting point. This makes indium suitable for manufacturing gaskets, which deform to fill gaps, for high-vacuum use.
Work hardening is often produced by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have been designed to maintain the general shape of the workpiece during work hardening, including shot peening and constant channel angular pressing. A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing hardness tests before and after a process.
Cold forming is a type of cold working that involves forging operations, such as extrusion, drawing or coining, performed at low temperatures. Cold working may also refer to the process through which a material is given this quality. Such deformation increases the concentration of dislocations which may subsequently form low-angle grain boundaries surrounding sub-grains. Cold working generally results in a higher yield strength as a result of the increased number of dislocations and the Hall-Petch effect of the sub-grains, and a decrease in ductility. The effects of cold working may be reversed by annealing the material at high temperatures where recovery and recrystallization reduce the dislocation density.
A material deforms elastically for small deforming forces: it returns to its original shape when the deforming force is removed. As the deforming force increases beyond the elastic limit the material does not return to its original shape when the force is removed: it deforms plastically. For example, if we stretch a coil spring up to a certain point, it will return to its original shape; stretch it too hard, and it will not.
Elastic deformation stretches atomic bonds in the material away from their equilibrium radius of separation of a bond without applying enough energy to break the inter-atomic bond. Plastic deformation breaks atomic bonds. Before this happens, if the force is increased gradually, there is a permanent movement of dislocations.
The strained bonds around a dislocation are described with the terminology of lattice strain fields. For example, there are compressively strained bonds directly next to an edge dislocation and tensilely strained bonds beyond the end of an edge dislocation. These form compressive strain fields and tensile strain fields respectively. Strain fields are like electric fields in some ways. Dislocations both form their own strain fields and are affected by the fields from other dislocations. In short, opposite fields attract and like fields repulse.
The visible (macroscopic) results of plastic deformation are the result of microscopic dislocation motion. Such as the stretching of a steel rod in a tensile tester.
Yield strength is increased in a cold-worked material. Using lattice strain fields, it can be shown that an environment filled with dislocations will hinder the movement of any one dislocation. Because dislocation motion is hindered, plastic deformation cannot occur at normal stresses. Upon application of stresses just beyond the yield strength of the non-cold-worked material, a cold-worked material will continue to deform using the only mechanism available: elastic deformation. The regular scheme of stretching or compressing of electrical bonds (without dislocation motion) continues to occur, and the modulus of elasticity is unchanged. Eventually the stress is great enough to overcome the strain-field interactions and plastic deformation resumes.
However, ductility of a work-hardened material is decreased. Ductility is the extent to which a material can undergo plastic deformation, that is, it is how far a material can be plastically deformed before fracture. A cold-worked material is, in effect, a normal material that has already been extended through part of its allowed plastic deformation. If dislocation motion and plastic deformation have been hindered enough by dislocation accumulation, and stretching of electronic bonds and elastic deformation have reached their limit, a third mode of deformation occurs: fracture.
The work-hardened steel bar has a large enough number of dislocations that the strain field interaction prevents all plastic deformation. Subsequent deformation requires a stress that varies linearly with the strain observed, the slope of the graph of stress vs. strain is the modulus of elasticity, as usual.
The work-hardened steel bar fractures when the applied stress exceeds the usual fracture stress and the strain exceeds usual fracture strain. This may be considered to be the elastic limit and the yield stress is now equal to the fracture stress, which is of course, much higher than a non-work-hardened-steel yield stress.
The amount of plastic deformation possible is zero, which is obviously less than the amount of plastic deformation possible for a non-work-hardened material. Thus, the ductility of the cold-worked bar is drastically reduced.
When a material under substantial and prolonged cavitation situation, the material also gets strain hardened.
where K is the strength index and n is the strain hardening index.
If a material has been subjected to prior deformation (at low temperature) then the yield stress will be increased by a factor depending on the amount of prior plastic strain ε0
The constant K is structure dependent and is influenced by processing while n is a material property normally lying in the range 0.2-0.5. The strain hardening index can be described by:
This equation can be evaluated from the slope of a log(σ) - log(ε) plot. Rearranging allows a determination of the rate of strain hardening at a given stress and strain
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Any material with a reasonably high melting point can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon steel, are often work-hardened. Some materials cannot be work-hardened at normal ambient temperatures; for example indium, which has a low melting point. This makes indium suitable for manufacturing gaskets, which deform to fill gaps, for high-vacuum use.
Work hardening is often produced by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have been designed to maintain the general shape of the workpiece during work hardening, including shot peening and constant channel angular pressing. A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing hardness tests before and after a process.
Cold forming is a type of cold working that involves forging operations, such as extrusion, drawing or coining, performed at low temperatures. Cold working may also refer to the process through which a material is given this quality. Such deformation increases the concentration of dislocations which may subsequently form low-angle grain boundaries surrounding sub-grains. Cold working generally results in a higher yield strength as a result of the increased number of dislocations and the Hall-Petch effect of the sub-grains, and a decrease in ductility. The effects of cold working may be reversed by annealing the material at high temperatures where recovery and recrystallization reduce the dislocation density.
Theory
Elastic and plastic deformation
Work hardening occurs as a consequence of plastic deformation, as distinct from elastic deformation. The following discussion mostly applies to metals, especially steels, which are well studied. The tensile test is widely used to study deformation mechanisms.A material deforms elastically for small deforming forces: it returns to its original shape when the deforming force is removed. As the deforming force increases beyond the elastic limit the material does not return to its original shape when the force is removed: it deforms plastically. For example, if we stretch a coil spring up to a certain point, it will return to its original shape; stretch it too hard, and it will not.
Elastic deformation stretches atomic bonds in the material away from their equilibrium radius of separation of a bond without applying enough energy to break the inter-atomic bond. Plastic deformation breaks atomic bonds. Before this happens, if the force is increased gradually, there is a permanent movement of dislocations.
Dislocations and lattice strain fields
Dislocations are defined as irregularities in a crystal structure. They are surrounded by relatively strained bonds than in the regular crystal lattice. This is why these bonds break first in plastic deformation. The next step is the reformation of these bonds with alternate nearby atoms: to leave atoms unbonded altogether would be severely energetically unfavored. The end result is an alternate, but lower energy conformation within the applied strain. Dislocations are a "negative-entity" in that they do not exist: they are merely defects in the host medium which does exist. As such, the material itself does not move much. To a much greater extent visible "motion" is movement in a bonding pattern of largely stationary atoms. (Please see for further discussion: edge dislocation, screw dislocation)The strained bonds around a dislocation are described with the terminology of lattice strain fields. For example, there are compressively strained bonds directly next to an edge dislocation and tensilely strained bonds beyond the end of an edge dislocation. These form compressive strain fields and tensile strain fields respectively. Strain fields are like electric fields in some ways. Dislocations both form their own strain fields and are affected by the fields from other dislocations. In short, opposite fields attract and like fields repulse.
The visible (macroscopic) results of plastic deformation are the result of microscopic dislocation motion. Such as the stretching of a steel rod in a tensile tester.
Increase of dislocations and work hardening
Increase in the number of dislocations is a quantification of work hardening. Plastic deformation occurs as a consequence of work being done on a material; energy is added to the material. In addition, the energy is almost always applied fast enough and in large enough magnitude to not only move existing dislocations, but also to produce a great number of new dislocations by jarring or working the material sufficiently enough.Yield strength is increased in a cold-worked material. Using lattice strain fields, it can be shown that an environment filled with dislocations will hinder the movement of any one dislocation. Because dislocation motion is hindered, plastic deformation cannot occur at normal stresses. Upon application of stresses just beyond the yield strength of the non-cold-worked material, a cold-worked material will continue to deform using the only mechanism available: elastic deformation. The regular scheme of stretching or compressing of electrical bonds (without dislocation motion) continues to occur, and the modulus of elasticity is unchanged. Eventually the stress is great enough to overcome the strain-field interactions and plastic deformation resumes.
However, ductility of a work-hardened material is decreased. Ductility is the extent to which a material can undergo plastic deformation, that is, it is how far a material can be plastically deformed before fracture. A cold-worked material is, in effect, a normal material that has already been extended through part of its allowed plastic deformation. If dislocation motion and plastic deformation have been hindered enough by dislocation accumulation, and stretching of electronic bonds and elastic deformation have reached their limit, a third mode of deformation occurs: fracture.
Example
For an extreme example, in a tensile test a bar of steel is strained to just before the distance at which it usually fractures. The load is released smoothly and the material relieves some of its strain by decreasing in length. The decrease in length is called the elastic recovery, and the end result is a work-hardened steel bar. The fraction of length recovered (length recovered/original length) is equal to the yield-stress divided by the modulus of elasticity. (Here we discuss true stress in order to account for the drastic decrease in diameter in this tensile test.) The length recovered after removing a load from a material just before it breaks is equal to the length recovered after removing a load just before it enters plastic deformation.The work-hardened steel bar has a large enough number of dislocations that the strain field interaction prevents all plastic deformation. Subsequent deformation requires a stress that varies linearly with the strain observed, the slope of the graph of stress vs. strain is the modulus of elasticity, as usual.
The work-hardened steel bar fractures when the applied stress exceeds the usual fracture stress and the strain exceeds usual fracture strain. This may be considered to be the elastic limit and the yield stress is now equal to the fracture stress, which is of course, much higher than a non-work-hardened-steel yield stress.
The amount of plastic deformation possible is zero, which is obviously less than the amount of plastic deformation possible for a non-work-hardened material. Thus, the ductility of the cold-worked bar is drastically reduced.
When a material under substantial and prolonged cavitation situation, the material also gets strain hardened.
Mathematical descriptions
There are two common mathematical descriptions of the work hardening phenomenon. Hollomon's equation is a power law relationship between the stress and the amount of plastic strain εp. Ludwik's equation is similar but includes the yield stress σy-
(Hollomon's)
-
(Ludwik's)
where K is the strength index and n is the strain hardening index.
If a material has been subjected to prior deformation (at low temperature) then the yield stress will be increased by a factor depending on the amount of prior plastic strain ε0
The constant K is structure dependent and is influenced by processing while n is a material property normally lying in the range 0.2-0.5. The strain hardening index can be described by:
This equation can be evaluated from the slope of a log(σ) - log(ε) plot. Rearranging allows a determination of the rate of strain hardening at a given stress and strain
See also
- Taxonomy of manufacturing processes for various cold work processes
- Cold rolling
- Swaging, a process that involves work hardening
Strength of materials is materials science applied to the study of engineering materials and their mechanical behavior in general (such as stress, deformation, strain and stress-strain relations).
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dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of real materials. The theory was originally developed by Vito Volterra in 1905.
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Stress is a measure of force per unit area within a body. It is a body's internal distribution of force per area that reacts to external applied loads. Stress is often broken down into its shear and normal components as these have unique physical significance.
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Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating and maintaining at a suitable temperature, and then cooling.
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The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds
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Heat Treatment
(1976) The Pink Parker (EP)
(1977)
Howlin' Wind
(1976) Heat Treatment
(1976) Stick to Me
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Second album by rock and roll singer-songwriter near-legend Graham Parker.
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(1976) The Pink Parker (EP)
(1977)
Howlin' Wind
(1976) Heat Treatment
(1976) Stick to Me
(1977)
Second album by rock and roll singer-songwriter near-legend Graham Parker.
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Indium (IPA: /ˈɪndiəm/) is a chemical element with chemical symbol In and atomic number 49. This rare, soft, malleable and easily fusible poor metal is chemically similar to aluminium or gallium but more
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Cold rolling is a metal working process in which metal is deformed by passing it through rollers at a temperature below its recrystallization temperature. Cold rolling increases the yield strength and hardness of a metal by introducing defects into the metal's crystal structure.
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The metallurgical process of Hot rolling, used mainly to produce sheet metal or simple cross sections from billets describes the method of when industrial metal is passed or deformed between a set of work rolls and the temperature of the metal is generally above its
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Extrusion is a manufacturing process used to create long objects of a fixed cross-sectional profile. A material, often in the form of a billet, is pushed and/or drawn through a die of the desired profile shape.
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Shot peening is a process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass or ceramic particles) with force sufficient to create plastic deformation.
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Hardness refers to various properties of matter in the solid phase that give it high resistance to various kinds of shape change when force is applied. Hard matter is contrasted with soft matter.
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forge or smithy is the workplace of a smith or a blacksmith. Forging is the term for shaping metal by plastic deformation. Cold forging is done at low temperatures, while conventional forging is done at high temperatures, which makes metal easier to shape and
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Extrusion is a manufacturing process used to create long objects of a fixed cross-sectional profile. A material, often in the form of a billet, is pushed and/or drawn through a die of the desired profile shape.
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Drawing is a manufacturing process for producing a wire, bar or tube by pulling on a material until it increases in length. This is typically accompanied by a thinning out of the material, usually through a reduction in the girth.
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dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of real materials. The theory was originally developed by Vito Volterra in 1905.
..... Click the link for more information.
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yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to plastically deform. Prior to the yield point the material will deform elastically and will return to its original shape when the applied
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The Hall-Petch relationship or Hall-Petch Law is a relation in materials science that deals with the connection between the grain size, or crystallite size, and the yield point of a material.
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Ductility is the mechanical property of being capable of sustaining large plastic deformations due to tensile stress without fracture (in metals, such as being drawn into a wire). It is characterized by the material flowing under shear stress. It is contrasted with brittleness.
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Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating and maintaining at a suitable temperature, and then cooling.
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Recovery is a process by which deformed grains can reduce their stored energy by the removal or rearrangement of defects in their crystal structure. These defects, primarily dislocations, are introduced by plastic deformation of the material and act to increase the yield strength
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Recrystallization (see also crystallization) is an essentially physical process that has meanings in chemistry, metallurgy and geology.
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Chemistry
In chemistry, recrystallization is a procedure for purifying compounds...... Click the link for more information.
In engineering mechanics, deformation is a change in shape due to an applied force. This can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described in terms of strain.
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In engineering mechanics, deformation is a change in shape due to an applied force. This can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described in terms of strain.
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Steel is an alloy consisting mostly of iron, with a carbon content between 0.02% and 1.7 or 2.04% by weight (C:1000–10,8.67Fe), depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese and
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Tensile stress (or tension) is the stress state leading to expansion; that is, the tensile stress may be increased until the reach of tensile strength, namely the limit state of stress.
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