The Science and Engineering of Materials, 4 th ed Donald R. Askeland Pradeep P. Phulé. Chapter 7 Strain Hardening and Annealing

Transcription

1 The Science and Engineering of Materials, 4 th ed Donald R. Askeland Pradeep P. Phulé Chapter 7 Strain Hardening and Annealing 1

2 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. 2

3 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 3

4 Section 7.1 Relationship of Cold Working to the Stress-Strain Curve Development of strain hardening from the engineering stress-strain diagram. (a) A specimen is stressed beyond the yield strength S y 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. 4

5 (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. 5

6 Manufacturing processes that make use of cold working as well as hot working. Common metalworking methods. (a) Rolling. (b) Forging (open and closed die). 6

7 (c) Extrusion (direct and indirect). (d) Wire drawing. (e) Stamping. 7

8 Section 7.2 Properties versus Percent Cold Work The effect of cold work on the mechanical properties of copper 8

9 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. 9

10 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. 10

11 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 11

12 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. 12

13 2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used 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%. 13

14 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. 14

15 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. 15

16 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. 16

17 The effect of annealing temperature on the microstructure of cold-worked metals. (a) cold-worked, (b) after recovery, (c) after recrystallization, and (d) after grain growth. 17

18 Photomicrographs showing the effect of annealing temperature on grain size in brass. Twin boundaries can also be observed in the structures. (a) Annealed at 400 o C, (b) annealed at 650 o C, and (c) annealed at 800 o C. 18

19 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 0.4T m (kelvin). 19

20 Longer annealing times reduce the recrystallization temperature. Note that the recrystallization temperature is not a fixed temperature. 20

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22 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. 22

23 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). 23