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The Science and Engineering of Materials, 4th edDonald R. Askeland – Pradeep P. Phulé

Chapter 7 – Strain Hardening and Annealing

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Objectives of Chapter 7

To learn how the strength of metals and alloys is

influenced by mechanical processing and heat treatments.

To learn how to enhance the strength of metals and alloys

using cold working.

To learn how to enhance ductility using annealing heat

treatment.

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Chapter 7 Outline

7.1 Relationship of Cold Working to the Stress-Strain Curve

7.2 Properties versus Percent Cold Work

7.3 Characteristics of Cold Working

7.4 Annealing

7.5 Control of Annealing

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Development of strain hardening from the engineering stress-strain diagram. (a) A

specimen is stressed beyond the yield strength Sy before the stress is removed. (b) Now

the specimen has a higher yield strength and tensile strength, but lower ductility. (c) By

repeating the procedure, the strength continues to increase and the ductility continues to

decrease until the alloy becomes very brittle.

Section 7.1

Relationship of Cold Working to the Stress-Strain Curve

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(d) The total strain is the sum of the elastic and plastic components. When the stress is

removed, the elastic strain is recovered, but the plastic strain is not. (e) Illustration of

springback.

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Manufacturing processes that make use

of cold working as well as hot working.

Common metalworking methods.

(a) Rolling.

(b) Forging (open and closed die).

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(c) Extrusion (direct and indirect).

(d) Wire drawing.

(e) Stamping.

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Section 7.2

Properties versus Percent Cold Work

The effect of cold work on the mechanical properties of copper

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Section 7.3

Characteristics of Cold Working

Advantages and limitations:

We can simultaneously strengthen the metallic material and produce the

desired final shape.

We can obtain excellent dimensional tolerances and surface finishes by

the cold working process.

The cold-working process can be an inexpensive method for producing

large numbers of small parts.

Some metals, such as HCP magnesium, have a limited number of slip

systems and are rather brittle at room temperature; thus, only a small

degree of cold working can be accomplished.

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Advantages and limitations:

Since the effect of cold working is decreased or eliminated at higher

temperatures, we cannot use cold working as a strengthening

mechanism for components that will be subjected to high temperatures

during service.

Ductility, electrical conductivity, and corrosion resistance are impaired

by cold working.

Since the extent to which electrical conductivity is reduced by cold

working is less than that for other strengthening processes, such as

introducing alloying elements, cold working is a satisfactory way to

strengthen conductor materials, such as the copper wires used for

transmission of electrical power.

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A comparison of strengthening copper by (a) cold working and (b) alloying with zinc.

Note that cold working produces greater strengthening, yet has little effect on electrical

conductivity

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Section 7.4

Annealing

Annealing - A heat treatment used to eliminate some or all of the effects of

cold working.

Annealing at a low temperature may be used to eliminate the residual

stresses produced during cold working without affecting the mechanical

properties of the finished part.

Annealing may be used to completely eliminate the strain hardening

achieved during cold working. In this case, the final part is soft and

ductile but still has a good surface finish and dimensional accuracy.

After annealing, additional cold work can be done since the ductility is

restored; by combining repeated cycles of cold working and annealing,

large total deformations may be achieved.

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herein under license.

The effect of cold work on the properties of a Cu-35% Zn alloy and the effect of

annealing temperature on the properties of a Cu-35% Zn alloy that is cold-worked 75%.

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The Three Stages of Annealing

1. Recovery - A low-temperature annealing heat treatment designed to eliminate

residual stresses introduced during deformation without reducing the strength

of the cold-worked material.

When we first heat the metal, the additional thermal energy permits the dislocations

to move and form the boundaries of a polygonized subgrain structure.

The dislocation density is virtually unchanged.

This low temperature treatment removes the residual stresses due to cold working

without causing a change in dislocation density.

The mechanical properties of the metal are relatively unchanged because the number

of dislocations is not reduced during recovery.

Since residual stresses are reduced or even eliminated when the dislocations are

rearranged, recovery is often called a stress relief anneal.

In addition, recovery restores high electrical conductivity to the

metal, permitting us to manufacture copper or aluminum wire for

transmission of electrical power that is strong yet still has high

conductivity.

Recovery often improves the corrosion resistance of the material.

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The Three Stages of Annealing

2. Recrystallization - A medium-temperature annealing heat treatment designed to

eliminate all of the effects of the strain hardening produced during cold

working.

When a cold-worked metal is heated above a certain temperature, rapid recovery

eliminates residual stresses and produces the polygonized dislocation structure.

New small grains then nucleate at the cell boundaries of the polygonized structure,

eliminating most of the dislocations.

Because the number of dislocations is greatly reduced, the recrystallized metal has

low strength but high ductility.

The temperature at which a microstructure of new grains that have very low

dislocation density appears is known as the recrystallization temperature.

The recrystallization temperature depends on many variables and is not a fixed

temperature.

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The Three Stages of Annealing

3. Grain growth - Movement of grain boundaries by diffusion in order to reduce

the amount of grain boundary area.

At still higher annealing temperatures, both recovery and recrystallization occur

rapidly, producing a fine recrystallized grain structure.

If the temperature is high enough, the grains begin to grow, with favored grains

consuming the smaller grains.

Grain growth is almost always undesirable.

Grain growth will occur in most materials if they are subjected to a high enough

temperature and is not related to cold working.

Recrystallization or recovery are not needed for grain growth to occur.

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(a) cold-worked, (b) after recovery, (c) after recrystallization, and (d) after grain growth.

The effect of annealing temperature on the microstructure of cold-worked metals.

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Photomicrographs showing the effect of annealing temperature on grain size in brass.

Twin boundaries can also be observed in the structures. (a) Annealed at 400oC, (b)

annealed at 650oC, and (c) annealed at 800oC.

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Section 7.5

Control of Annealing

Recrystallization Temperature - A temperature above which essentially

dislocation-free and new grains emerge from a material that was previously

cold worked.

The recrystallization temperature decreases when the amount of cold work

increases.

A smaller initial cold-worked grain size reduces the recrystallization

temperature by providing more sites—the former grain boundaries—at which

new grains can nucleate.

Pure metals recrystallize at lower temperatures than alloys.

Increasing the annealing time reduces the recrystallization temperature, since

more time is available for nucleation and growth of the new recrystallized

grains.

Higher melting-point alloys have a higher recrystallization temperature. Since

recrystallization is a diffusion-controlled process, the recrystallization

temperature is roughly proportional to 0.4Tm (kelvin).

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Longer annealing times reduce the recrystallization temperature.

Note that the recrystallization temperature is not a fixed temperature.

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0.53

0.45

0.41

0.45

0.51

0.38

0.35

0.40

0.51

0.41

0.40

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Control of Annealing

Recrystallized Grain Size - A number of factors influence the size of the

recrystallized grains.

Reducing the annealing temperature, the time required to heat to the annealing

temperature, or the annealing time reduces grain size by minimizing the

opportunity for grain growth.

Increasing the initial cold work also reduces final grain size by providing a

greater number of nucleation sites for new grains.

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Hot working - Deformation of a metal above the recrystallization

temperature. During hot working, only the shape of the metal changes; the

strength remains relatively unchanged because no strain hardening occurs.

Cold working - Deformation of a metal below the recrystallization

temperature. During cold working, the number of dislocations increases,

causing the metal to be strengthened as its shape is changed.

Warm working - A term used to indicate the processing of metallic materials

in a temperature range that is between those that define cold and hot working

(usually a temperature between 0.3 to 0.6 of melting temperature in K).