Post on 01-Jun-2018
8/9/2019 Strain Hardening and Annealing
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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_materials8/9/2019 Strain Hardening and Annealing
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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.