Strain Hardening - Material Science for Engineers - Lecture Slides, Slides of Material Engineering

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The Science and Engineering of

Materials, 4

th

ed

Chapter 7 – Strain Hardening and

Annealing

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.

• Flow stress
• Strain hardening
• Strain hardening exponent ( n )
• Strain-rate sensitivity ( m )
• Bauschinger effect

Section 7.

Relationship of Cold Working to

the Stress-Strain Curve

Figure 7. Development of strain hardening from the stress- strain diagram

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning

is a trademark used herein under license.™ (^) Figure 7.3 The true

stress-true strain curves for metals with large and small strain-hardening exponents. Larger degrees of strengthening are obtained for a given strain for the metal with the larger n

Section 7.

Strain-Hardening Mechanisms

 Frank-Read source - A pinned dislocation that,

under an applied stress, produces additional

dislocations. This mechanism is at least partly

responsible for strain hardening.

 Thermoplastics - A class of polymers that

consist of large, long spaghetti-like molecules

that are intertwined (e.g., polyethylene, nylon,

PET, etc.).

Figure 7.5 The Frank-Read source can generate dislocations. (a) A dislocation is pinned at its ends by lattice defects. (b) As the dislocation continues to move, the dislocation bows, eventually bending back on itself. (c) finally the dislocation loop forms, and (d) a new dislocation is created. (e) Electron micrograph of a Frank-Read source (330,000). (Adapted from Brittain, J., ‘‘Climb Sources in Beta Prime- NiAl,’’ Metallurgical Transactions, Vol. 6A, April 1975.)

Section 7.

Properties versus Percent Cold Work

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learningtrademark used herein under license. ™ is a Figure 7.7 The effect of cold work on the mechanical properties of copper

Example 7.

Cold Working a Copper Plate

A 1-cm-thick copper plate is cold-reduced to 0.

cm, and later further reduced to 0.16 cm. Determine

the total percent cold work and the tensile strength

of the 0.16-cm plate. (See Figure 7.8.)

Example 7.1 SOLUTION

Example 7. Design of a Cold Working Process

Design a manufacturing process to produce a 0.1-cm-thick

copper plate having at least 65,000 psi tensile strength,

60,000 psi yield strength, and 5% elongation.

Example 7.2 SOLUTION

To produce the plate, a cold-rolling process would be

appropriate. The original thickness of the copper plate prior

to rolling can be calculated from Equation 7-4, assuming that

the width of the plate does not change. Because

there is a range of allowable cold work—between 40% and

45%—there is a range of initial plate thicknesses:

Section 7.

Microstructure, Texture Strengthening,

and Residual Stresses

 Fiber texture, Sheet texture

 Pole figure analysis, Orientation microscopy

 Residual stresses, Stress-relief anneal

 Annealing glass, Tempered glass

Figure 7.9 The fibrous grain structure of a low carbon steel produced by cold working: (a) 10% cold work, (b) 30% cold work, (c) 60% cold work, and (d) 90% cold work (250). (Source: From ASM Handbook Vol. 9, Metallography and Microstructure, (1985) ASM International, Materials Park, OH

44073. Used with permission.)