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Transcript of © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 1 Chapter 8: Strain Hardening and...
© 2011 Cengage Learning Engineering. All Rights Reserved.© 2011 Cengage Learning Engineering. All Rights Reserved.8 - 8 - 11
Chapter 8:Chapter 8:
Strain Strain Hardening and Hardening and
AnnealingAnnealing
Chapter 8: Strain Hardening and Annealing
© 2011 Cengage Learning Engineering. All Rights Reserved.© 2011 Cengage Learning Engineering. All Rights Reserved.8 - 8 - 22
Learning Objectives
1. Relationship of cold working to the stress-strain curve
2. Strain-hardening mechanisms
3. Properties versus percent cold work
4. Microstructure, texture strengthening, and residual stresses
5. Characteristics of cold working
6. The three stages of annealing
7. Control of annealing
8. Annealing and materials processing
9. Hot working
Chapter 8: Strain Hardening and Annealing
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Figure 8.1
Chapter 8: Strain Hardening and Annealing
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Relationship of Cold Working to the Stress-Strain Curve
Chapter 8: Strain Hardening and Annealing
Strain hardening Strengthening of a material by increasing the number of dislocations by deformation. Also known as “work hardening”
Deformation processing
Techniques used for the manufacturing of metallic and other materials
Rolling Process used to produce metal plate, sheet, or foil
Extrusion Process by which a material is pushed through a die to form products of uniform cross-sections
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Figure 8.2Chapter 8: Strain Hardening and Annealing
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Chapter 8: Strain Hardening and Annealing
Figure 8.3
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Chapter 8: Strain Hardening and Annealing
Relationship of Cold Working to the Stress-Strain Curve
Strain-rate sensitivity (m)
For deep drawing, the plastic strain ratio r is
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Chapter 8: Strain Hardening and Annealing
Figure 8.4
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Chapter 8: Strain Hardening and Annealing
Relationship of Cold Working to the Stress-Strain Curve
Springback Elastic strain that is recovered after a material has been
plastically deformed. Bauschinger effect
An effect in which a material subjected to tension shows a reduction in compressive strength.
© 2011 Cengage Learning Engineering. All Rights Reserved.© 2011 Cengage Learning Engineering. All Rights Reserved.8 - 8 - 1010
Chapter 8: Strain Hardening and Annealing
Figure 8.5 - The Frank-Read Source Can Generate Dislocations
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Chapter 8: Strain Hardening and Annealing
Figure 8.6
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Chapter 8: Strain Hardening and Annealing
Properties versus Percent Cold Work
where
A0 original cross-sectional area of the metal
Af final cross-sectional area after deformation
where
t0 initial sheet thickness
tf final thickness
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Microstructure, Texture Strengthening,and Residual Stresses
Chapter 8: Strain Hardening and Annealing
Anisotropic behavior Fiber texture
A preferred orientation of grains obtained during the wire drawing process.
Sheet texture A preferred orientation of grains obtained during the rolling
process.
Drawing Process by which a metallic rod is pulled through a die to
produce a wire or fiber.
Texture strengthening Increase in the yield strength of a material as a result of
preferred crystallographic texture.
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Microstructure, Texture Strengthening,and Residual Stresses
Chapter 8: Strain Hardening and Annealing
Texture development in thin films Pole figure analysis
A specialized technique based on x-ray diffraction, used for the determination of preferred orientation of thin films, sheets, or single crystals.
Orientation microscopy A specialized technique, often based on scanning electron
microscopy, used to determine the crystallographic orientation of different grains in a polycrystalline sample.
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Chapter 8: Strain Hardening and Annealing
Figure 8.11
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Microstructure, Texture Strengthening,and Residual Stresses
Chapter 8: Strain Hardening and Annealing
Tempering and annealing of glasses Annealing
For glasses, annealing is a heat treatment that removes thermally induced stresses.
Annealed glass Glass that has been treated by heating above the annealing point
temperature (where the viscosity of glass becomes 1013 Poise) and then cooled slowly to minimize or eliminate residual stresses.
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Microstructure, Texture Strengthening,and Residual Stresses
Chapter 8: Strain Hardening and Annealing
Tempered glass A glass obtained by either heat treatment and quenching or by
the chemical exchange of ions. Laminated safety glass
Two pieces of annealed glass held together by a plastic such as polyvinyl butyral (PVB).
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Chapter 8: Strain Hardening and Annealing
Figure 8.15
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Chapter 8: Strain Hardening and Annealing
Figure 8.16
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Chapter 8: Strain Hardening and Annealing
The Three Stages of Annealing
Recovery A low-temperature annealing heat treatment designed to
eliminate residual stresses introduced during deformation without reducing the strength of the cold-worked material.
Polygonized subgrain structure: A subgrain structure produced in the early stages of annealing. The subgrain boundaries are a network of dislocations rearranged during heating.
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Chapter 8: Strain Hardening and Annealing
The Three Stages of Annealing
Recrystallization The process of forming new dislocation-free grains by heat
treating a cold-worked material. A fine recrystallized grain structure is formed.
Grain growth Grains grow larger at elevated temperatures with favored
grains consuming smaller grains.
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Chapter 8: Strain Hardening and Annealing
Figure 8.18
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Chapter 8: Strain Hardening and Annealing
Control of Annealing
Recrystallization temperature The temperature at which grains in the cold-worked
microstructure begin to transform into new, equiaxed, and dislocation-free grains.
Processes conducted below 0.3 times the melting temperature are considered cold working and processes conducted between 0.3 and 0.6 times Tm are considered warm working.
A smaller initial cold-worked grain size reduces the recrystallization temperature by providing more nucleation sites.
Pure metals recrystallize at lower temperatures than alloys. Increasing the annealing time reduces the recrystallization
temperature.
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Chapter 8: Strain Hardening and Annealing
Annealing and Materials Processing
Deformation processing By taking advantage of the annealing heat treatment, the
total amount of deformation can be increased. High temperature service
When the cold-worked metal is placed into service at a high temperature, recrystallization immediately causes a catastrophic decrease in strength.
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Chapter 8: Strain Hardening and Annealing
Figure 8.19
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Chapter 8: Strain Hardening and Annealing
Hot Working
Hot working is defined as plastically deforming the metallic material at a temperature above the recrystallization temperature.
Lack of strengthening Strengthening does not occur during deformation by hot
working. The amount of plastic deformation is almost unlimited.
Elimination of imperfections Imperfections may be eliminated or minimized during hot
working.
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Chapter 8: Strain Hardening and Annealing
Hot Working
Anisotropic behavior The final properties in hot-worked parts are not isotropic.
Surface finish and dimensional accuracy The surface finish formed during hot working is usually
poorer than that obtained by cold working.
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Key Terms
Strain hardening or cold working
Deformation processing
Rolling Forging Drawing Extrusion Strain-hardening
exponent Strain-rate sensitivity Formability Bauschinger effect Frank-Read source Thermoplastics
Fiber texture Sheet texture Texture strengthening Pole figure analysis Orientation microscopy Residual stresses Stress-relief anneal Annealing Annealed glass Tempering Tempered glass
Chapter 8: Strain Hardening and Annealing
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Key Terms
Laminated safety glass Polygonized subgrain
structure Recovery Recrystallization
temperature Recrystallization Warm working Hot working Heat-affected zone
Chapter 8: Strain Hardening and Annealing