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Strain hardening and annealing

cold working - is thestrengthening of a metal byplastic deformation.This

strengthening occurs because of dislocation movements and dislocation generation

within thecrystal structure of the material.

Hot working - is defined as plastically deforming the metallic material at a

temperature above the recrystallization temperature. During hot working, only the

shape of the metal changes; the strength remains relatively unchanged because no

strain hardening occurs.

Annealing - is aheat treatment that alters the physical and sometimes chemical

properties of a material to increase itsductility and to make it more workable.

Elastic Strain - is observed as a distortion of the crystal lattice

Plastic Strain - is observed by the presence of dislocationsthe displacement of partof the crystal lattice.

Yield Strength - is defined as thestress at which a material begins todeformplastically.

Tensile strength - is the strength of the material against pulling or stretching stress.

Flow stress - as the stress that is needed to initiate plastic flow in previously deformed

material.

Deformation Processes: rolling. Forging extrusion

Necking - is a mode of tensiledeformation where relatively large amounts

of strain localize disproportionately in a small region of the material.

Porosity - is a measure of the void (i.e., "empty") spaces in a material, and is a

fraction of the volume of voids over the total volume, between 0 and 1, or asapercentagebetween 0 and 100%.

Plastic Deformation - This type of deformation is irreversible. However, an object in

the plastic deformation range will first have undergone elastic deformation, which is

reversible, so the object will return part way to its original shape.

Elastic Deformation - A reversible change in shape. Once the forces are no longerapplied, the object returns to its original shape.

Drawing - A deformation processing technique in which a material is pulled throughan opening in a die

Extrusion - A deformation processing technique in which a material is pushed through

an opening in a die. Used for metallic and polymeric materials.

Formability - The ability of a material to stretch and bend without breaking. Forming

diagrams describe the ability to stretch and bend materials.

Frank Read Source (FRS) - A pinned dislocation that, under an applied stress,

produces additional dislocations. This mechanism is at least partly responsible forstrain hardening.

Heat-affected zone (HAZ) -The volume of material adjacent to a weld that is heated

during the welding process above some critical temperature at which a change in thestructure, such as grain growth or recrystallization, occurs.

Recrystallization - A medium-temperature annealing heat treatment designed toeliminate all of the effects o f the strain hardening produced during cold working.

Recrystallization temperature - A temperature above which essentially dislocation-

free and new grains emerge from a material that was previously cold worked. This

depends upon the extent of cold work, time of heat treatment, etc., and is not a fixedtemperature.

Residual stresses - Stresses introduced in a material during processing. These can

originate as a result of cold work ing or differential thermal expansion and contraction.

A stress-relief anneal in metallic materials and the annealing of glasses minimize

residual stresses. Compressive residual stresses deliberately introduced on the surface

by the tempering of glasses or shot peening of metallic materials improve theirmechanical properties.

Strain hardening - Strengthening of a material by increasing the number of

dislocations by deformation. Also known as work hardening.

Strain-hardening exponent (n) - A parameter that describes the susceptibility of a

material to cold working. It descr ibes the effect that strain has on the resulting

strength of the material. A material with a high strain-hardening coefficient obtainshigh strength with only small amounts of deformation or strain.

Strain rate - The rate at which a material is deformed.Strain-rate sensitivity (m) - The rate at which stress changes as a function of strain

rate. A material may behave much di fferently if it is s lowly pressed into a shaperather than smashed rapidly into a shape by an impact blow.

Stress-relief anneal - The recovery stage of the annealing heat treatment during which

residual stresses are relieved without altering the strength and ductility of the material.

Thermomechanical processing - Processes involved in the manufacturing of metallic

components using mechanical deformation and various heat treatments.

Thermoplastics - A class of polymers that consist of large, long spaghetti-like

molecules that are intertwined (e.g.,polyethylene, nylon, PET, etc.).

Dislocations - Defined as line defects in a material's crystal structure.

Advantages:

No heating required

Better surface finish

Superior dimensional control

Better reproducibility and interchangeability

Directional properties can be imparted into the metal

Contamination problems are minimized

http://en.wikipedia.org/wiki/Strength_of_materialshttp://en.wikipedia.org/wiki/Plastic_deformationhttp://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Deformation_(engineering)http://en.wikipedia.org/wiki/Deformation_(mechanics)http://en.wikipedia.org/wiki/Percentagehttp://en.wikipedia.org/wiki/Percentagehttp://en.wikipedia.org/wiki/Deformation_(mechanics)http://en.wikipedia.org/wiki/Deformation_(engineering)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Plasticity_(physics)http://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Ductilityhttp://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Dislocationhttp://en.wikipedia.org/wiki/Plastic_deformationhttp://en.wikipedia.org/wiki/Strength_of_materials

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Disadvantages:

Greater forces are required

Heavier and more powerful equipment and stronger tooling are required

Metal is less ductile

Metal surfaces must be clean and scale-free

Intermediate anneals may be required to compensate for loss of ductility that

accompanies strain hardening

The imparted directional properties may be detrimental

Undesirable residual stress may be produced

Stages - The three stages of the annealing process that proceed as the temperature of

the material is increased are: recovery, recrystallization, and grain growth.