© 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 1 Chapter 8: Strain Hardening and Annealing Chapter 8: Strain Hardening and Annealing

© 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


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



Figure 8.1



Relationship of Cold Working to the Stress-Strain Curve


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


Figure 8.2Chapter 8: Strain Hardening and Annealing



Figure 8.3




Strain-rate sensitivity (m)

For deep drawing, the plastic strain ratio r is



Figure 8.4




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.



Figure 8.5 - The Frank-Read Source Can Generate Dislocations



Figure 8.6



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

© 2011 Cengage Learning Engineering. All Rights Reserved.© 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 8 - 1313

Microstructure, Texture Strengthening,and Residual Stresses


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.




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.



Figure 8.11




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.




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).



Figure 8.15



Figure 8.16



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.



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.

© 2011 Cengage Learning Engineering. All Rights Reserved.© 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 8 - 2222


Figure 8.18



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.



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.



Figure 8.19



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.



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.


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



Key Terms

Laminated safety glass Polygonized subgrain

structure Recovery Recrystallization

temperature Recrystallization Warm working Hot working Heat-affected zone


Documents

© 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 1 Chapter 8: Strain Hardening and Annealing Chapter 8: Strain Hardening and Annealing