Chapter 8: Strain Hardening and Annealing

Transcription

1 Slide 1 Chapter 8: Strain Hardening and Annealing 8-1 Slide 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 8-2 Slide 3 Figure

2 Slide 4 Relationship of Cold Working to the Stress-Strain Curve Strain hardening Deformation processing Rolling Strengthening of a material by increasing the number of dislocations by deformation. Also known as work hardening Techniques used for the manufacturing of metallic and other materials 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 8-4 Slide 5 Figure Slide 6 Figure

3 Slide 7 Relationship of Cold Working to the Stress-Strain Curve Strain-rate sensitivity (m) For deep drawing, the plastic strain ratio r is 8-7 Slide 8 Figure Slide 9 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. 8-9

4 Slide 10 Figure The Frank-Read Source Can Generate Dislocations 8-10 Slide 11 Figure Slide 12 Properties versus Percent Cold Work where A 0 A f original cross-sectional area of the metal final cross-sectional area after deformation where t 0 t f initial sheet thickness final thickness 8-12

5 Slide 13 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 Slide 14 Microstructure, Texture Strengthening, and Residual Stresses 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 Slide 15 Figure

6 Slide 16 Microstructure, Texture Strengthening, and Residual Stresses 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 Poise) and then cooled slowly to minimize or eliminate residual stresses Slide 17 Microstructure, Texture Strengthening, and 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) Slide 18 Figure

7 Slide 19 Figure Slide 20 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 Slide 21 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. 8-21

8 Slide 22 Figure Slide 23 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 Slide 24 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. 8-24

9 Slide 25 Figure Slide 26 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 Slide 27 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. 8-27

10 Slide 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 8-28 Slide 29 Laminated safety glass Polygonized subgrain structure Recovery Recrystallization temperature Recrystallization Warm working Hot working Heat-affected zone Key Terms 8-29