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

2. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 2 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

3. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 3 Figure 8.1 Chapter 8: Strain Hardening and Annealing

4. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 4 Relationship of Cold Working to the Stress-Strain Curve Chapter 8: Strain Hardening and Annealing Strain hardeningStrengthening 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 RollingProcess used to produce metal plate, sheet, or foil ExtrusionProcess by which a material is pushed through a die to form products of uniform cross-sections

5. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 5 Figure 8.2 Chapter 8: Strain Hardening and Annealing

6. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 6 Chapter 8: Strain Hardening and Annealing Figure 8.3

7. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 7 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

8. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 8 Chapter 8: Strain Hardening and Annealing Figure 8.4

9. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 9 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.

10. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 10 Chapter 8: Strain Hardening and Annealing Figure 8.5 - The Frank-Read Source Can Generate Dislocations

11. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 11 Chapter 8: Strain Hardening and Annealing Figure 8.6

12. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 12 Chapter 8: Strain Hardening and Annealing Properties versus Percent Cold Work where A 0 original cross-sectional area of the metal A f final cross-sectional area after deformation where t 0 initial sheet thickness t f final thickness

13. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 13 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.

14. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 14 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.

15. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 15 Chapter 8: Strain Hardening and Annealing Figure 8.11

16. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 16 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 10 13 Poise) and then cooled slowly to minimize or eliminate residual stresses.

17. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 17 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).

18. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 18 Chapter 8: Strain Hardening and Annealing Figure 8.15

19. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 19 Chapter 8: Strain Hardening and Annealing Figure 8.16

20. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 20 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.

21. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 21 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.

22. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 22 Chapter 8: Strain Hardening and Annealing Figure 8.18

23. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 23 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 T m 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.

24. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 24 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.

25. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 25 Chapter 8: Strain Hardening and Annealing Figure 8.19

26. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 26 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.

27. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 27 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.

28. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 28 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

29. © 2011 Cengage Learning Engineering. All Rights Reserved. 8 - 29 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