Everything about Work Hardening totally explained
Work hardening,
strain hardening, or
cold work is the
strengthening of a material by increasing the material's
dislocation density. In metallic crystals, irreversible deformation is usually carried out on a microscopic scale by defects called
dislocations,which are created by fluctuations in local
stress fields within the material culminating in a lattice rearrangement as the dislocations propagate through the lattice. At normal temperatures the dislocations are not annihilated by
annealing. Instead, the dislocations accumulate, interact with one another, and serve as
pinning points or obstacles that significantly impede their motion. This leads to an increase in the
yield strength of the material and a subsequent decrease in
ductility.
Any material with a reasonably high
melting point such as metals and alloys can be strengthened in this fashion. Alloys not amenable to
heat treatment, including low-carbon
steel, are often work-hardened. Some materials can't 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
Main article: Deformation
Work hardening is a consequence of
plastic deformation, or a permanent deformity to a material. This is distinct from
elastic deformation, which is a
reversible deformity. Most materials don't exhibit one or the other, but rather a combination of the two. The following discussion mostly applies to
metals, especially
steels, which are well studied. Work hardening occurs most notably for ductile materials such as metals.
Ductility is the ability of a material to undergo large plastic deformations before
fracture (for example, bending a steel rod until it finally breaks).
The
tensile test is widely used to study deformation mechanisms. This is because under compression, most materials will experience trivial (lattice mismatch) and non-trivial (buckling) events before plastic deformation or fracture occur. Hence the intermediate processes that occur to the material under uniaxial compression before the incidence of plastic deformation make the compressive test fraught with difficulties.
A material generally deforms elastically if it's under the influence of small
forces, allowing the material to readily return to its original shape when the deforming force is removed. This behavior in materials is governed by
Hooke's Law. Materials behave elastically until the deforming force increases beyond the
elastic limit, also known as the yield stress. At this point, the material is rendered permanently deformed and fails to return to its original shape when the force is removed. This phenomenon is called
plastic deformation. For example, if one stretches a
coil spring up to a certain point, it'll return to its original shape, but once it's stretched beyond the elastic limit, it'll remain deformed and won't return to its original state.
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 bonds. Plastic deformation, on the other hand, breaks interatomic bonds, and involves the rearrangement of atoms in a solid material.
Dislocations and lattice strain fields
Main article: Dislocation
In materials science parlance,
dislocations are defined as line defects in a material's
crystallographic structure. They are surrounded by relatively strained (and weaker) bonds than the bonds between the constituents of the regular crystal
lattice. This explains why these bonds break first during plastic deformation. Like any thermodynamic system, the crystals tend to lower their energy through bond formation between constituents of the crystal. Thus the dislocations interact with one another and the atoms of the crystal. The results in a lower but energetically favorable energy conformation of the crystal. Dislocations are a "negative-entity" in that they don't exist: they're merely vacancies in the host medium which does exist. As such, the material itself doesn't 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 characterized by 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 analogous to
electric fields in certain ways. Additionally, the strain fields of dislocations, obey the laws of attraction and repulsion.
The visible (
macroscopic) results of plastic deformation are the result of
microscopic dislocation motion. For example, the stretching of a steel rod in a tensile tester is accommodated through dislocation motion on the atomic scale.
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 can't 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's 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.
Governing Equations
The stress,
, of dislocation is dependent on the shear modulus, G, the
lattice constant, b, and the
dislocation density,
:
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