Technology & Engineering
Work Hardening
Work hardening, also known as strain hardening, is a process in which a material becomes stronger and more resistant to deformation as a result of plastic deformation. This occurs when a material is subjected to repeated plastic deformation, causing dislocations to accumulate and hinder further deformation. Work hardening can improve the mechanical properties of materials, making them more suitable for certain applications.
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9 Key excerpts on "Work Hardening"
eBook - ePubGamma Titanium Aluminide Alloys
Science and Technology
- Fritz Appel, Jonathan David Heaton Paul, Michael Oehring(Authors)
- 2011(Publication Date)
- Wiley-VCH(Publisher)
Compared with the large amount of mechanical data, little information is available about the Work Hardening of TiAl alloys. This is certainly in part due to the brittleness of the material, which persists up to relatively high temperatures. However, in compression it is possible to produce Work Hardening over large plastic strains, which provides a significant potential to strengthen the material. The competitive processes to Work Hardening are recovery and recrystallization. Thus, the rate at which the material hardens depends on the imparted mechanical strain energy and its release by diffusion-assisted processes. Taken together, these factors lead to a complex dependence of the Work Hardening on temperature. From the technical point of view Work Hardening is important because it is involved in various metallurgical processes, such as forming, shaping, diffusion bonding, and surface hardening. For all these processes detailed knowledge about the deformation-induced defect structures and their thermal stability is required. The available information about this subject will be assessed in the following sections.7.2.1 Work-Hardening PhenomenaWork-hardening mechanisms will be characterized in terms of glide obstacles controlling the velocity and the slip path of the dislocations, analogous to the procedure that has been described in Section 6.4. Within this approach the flow stress σ (ε) beyond yielding may be described as [25](7.3)ΔG is the Gibbs free energy of activation, k the Boltzmann constant and M T the Taylor factor. σ 0 represents a stress contribution from dislocation mechanisms operating at the onset of yielding and is considered to be independent of strain ε. Micromechanisms associated with σ0 have been discussed in Section 6.4.σμ(ε) is an athermal stress contribution to Work Hardening representing long-range dislocation interactions. σ *(ε) is an effective or thermal stress component due to thermally assisted overcoming of deformation induced short-range glide obstacles. V D (ε) and ΔF D
eBook - PDF- Marc André Meyers, Krishan Kumar Chawla(Authors)
- 2008(Publication Date)
- Cambridge University Press(Publisher)
Chapter 6 Geometry of Deformation and Work-Hardening 6.1 Introduction The relaxation times for the molecular processes in gases and in a majority of liquids are so short, that molecules/atoms are almost always in a well-defined state of complete equilibrium. Consequently, the structure of a gas or liquid does not depend on its past history. In contrast, the relaxation times for some of the significant atomic pro-cesses in crystals are so long, that a state of equilibrium is rarely, if ever, achieved. It is for this reason that metals in general (and ceram-ics and polymers, under special conditions) show the usually desirable characteristic of work-hardening with straining, or strain-hardening. In other words, plastic deformation distorts the atoms from their equilibrium positions, and this manifests itself subsequently in hard-ening. In fact, hardening by plastic deformation (rolling, drawing, etc.) is one of the most important methods of strengthening metals, in general. Figure 6.1 shows a few deformation-processing techniques in which metals are work-hardened. These industrial processes are used in the fabrication of parts and enable the shape of metals to be changed. The figure is self-explanatory. Rolling is used to produce flat products such as plates, sheets, and also more complicated shapes (with special rolling cylinders). In forging, the top hammer comes down, and the part is pushed into a die (closed-die forging) or is simply compressed. Extrusion uses a principle similar to that in the use of a tube of toothpaste. The material is squeezed through a die, and its diameter is reduced. In stamping, first the ends of a blank are held, and then the upper die comes down, punching the blank into the lower die. If deformation is carried out at low and moderate temperatures, the metal workhardens. However, if the temperature is sufficiently high, the dislocations generated in work-hardening are annealed out, and the final metal is in the annealed condition.
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Metallurgical and materials engineers are often called on to design alloys having high strengths yet some ductility and toughness; typically, ductility is sacrificed when an alloy is strengthened. Several hardening techniques are at the disposal of an engineer, and frequently alloy selection depends on the capacity of a material to be tailored with the mechanical characteristics required for a particular application. Important to the understanding of strengthening mechanisms is the relation be- tween dislocation motion and mechanical behavior of metals. Because macroscopic plastic deformation corresponds to the motion of large numbers of dislocations, the abil- ity of a metal to deform plastically depends on the ability of dislocations to move. Because hardness and strength (both yield and tensile) are related to the ease with which plastic deformation can be made to occur, by reducing the mobility of dislocations, the me- chanical strength may be enhanced—that is, greater mechanical forces are required to initiate plastic deformation. In contrast, the more unconstrained the dislocation motion, the greater is the facility with which a metal may deform, and the softer and weaker it becomes. Virtually all strengthening techniques rely on this simple principle: Restricting or hindering dislocation motion renders a material harder and stronger. The present discussion is confined to strengthening mechanisms for single-phase metals by grain size reduction, solid-solution alloying, and strain hardening. Deformation and strengthening of multiphase alloys are more complicated, involving concepts be- yond the scope of the present discussion; later chapters treat techniques that are used to strengthen multiphase alloys. Tutorial Video: How Do Defects Affect Metals? Mechanisms of Strengthening in Metals Figure 8.13 For a single crystal subjected to a shear stress τ, (a) deformation by slip, (b) deformation by twinning.
eBook - ePub- R. E. Smallman, A.H.W. Ngan(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
Of these many changes in properties, perhaps the most outstanding are those that occur in the mechanical properties; the yield stress of mild steel, for example, may be raised by cold work from 170 up to 1050 MN m −2. Such changes in mechanical properties are, of course, of interest theoretically, but they are also of great importance in industrial practice. This is because the rate at which the material hardens during deformation influences both the power required and the method of working in the various shaping operations, while the magnitude of the hardness introduced governs the frequency with which the component must be annealed (always an expensive operation) to enable further working to be continued. Since plastic flow occurs by a dislocation mechanism the fact that Work Hardening occurs means that it becomes difficult for dislocations to move as the strain increases. All theories of Work Hardening depend on this assumption, and the basic idea of hardening, put forward by Taylor in 1934, is that some dislocations become ‘stuck’ inside the crystal and act as sources of internal stress which oppose the motion of other gliding dislocations. One simple way in which two dislocations could become stuck is by elastic interaction. Thus, two parallel edge dislocations of opposite sign moving on parallel slip planes in any sub-grain may become stuck, as a result of the interaction discussed in Chapter 4. Taylor assumed that dislocations become stuck after travelling an average distance, L, while the density of dislocations reaches ρ, i.e. Work Hardening is due to the dislocations getting in each other’s way. The flow stress is then the stress necessary to move a dislocation in the stress field of those dislocations surrounding it
eBook - PDF- R. E. Smallman(Author)
- 2016(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 10 Work Hardening and annealing 10.1 Work Hardening 10.1.1 Introduction It is well known that the properties of a metal are altered by cold working, i.e. deformation of the metal at a low temperature relative to its melting point: such a working operation may be carried out at a slightly elevated temperature for some metals, e.g. up to 500 °C in the case of molybdenum or tungsten, and at sub-zero temperatures for others, e.g. lead. However, not all the properties are improved, for although the tensile strength, yield strength and hardness are increased the plasticity and general ability of a metal to deform decreases. Moreover, the physical properties such as electrical conductivity, density and others are all lowered. Of these many changes in properties, perhaps the most outstanding are those that occur in the mechanical properties, as is illustrated by the fact that the yield stress of mild steel, for example, may be raised by cold work from 172 up to 1050 M N / m ^ Such changes in mechanical properties are, of course, of interest theoretically, but they are also of great importance in industrial practice. This is because the rate at which a metal hardens during deformation influences both the power required and the method of working in the various shaping operations, while the magnitude of the hardness introduced governs the frequency with which metals must be annealed (always an expensive operation) to enable further working to be continued. Since plastic flow occurs by a dislocation mechanism the fact that Work Hardening occurs in metals means that it becomes difficult either to generate dislocations or to move them. The ease with which the muUiplication of the dislocations can occur from sources suggests that the hardening is not due to this cause but to the increased resistance these dislocations experience in moving through the lattice.
eBook - PDFFundamentals of Materials Science and Engineering
An Integrated Approach
- William D. Callister, Jr., David G. Rethwisch(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
292 • Chapter 8 / Deformation and Strengthening Mechanisms Metallurgical and materials engineers are often called on to design alloys having high strengths yet some ductility and toughness; typically, ductility is sacrificed when an alloy is strengthened. Several hardening techniques are at the disposal of an engineer, and frequently alloy selection depends on the capacity of a material to be tailored with the mechanical characteristics required for a particular application. Important to the understanding of strengthening mechanisms is the relation be- tween dislocation motion and mechanical behavior of metals. Because macroscopic plastic deformation corresponds to the motion of large numbers of dislocations, the abil- ity of a metal to deform plastically depends on the ability of dislocations to move. Because hardness and strength (both yield and tensile) are related to the ease with which plastic deformation can be made to occur, by reducing the mobility of dislocations, the me- chanical strength may be enhanced—that is, greater mechanical forces are required to initiate plastic deformation. In contrast, the more unconstrained the dislocation motion, the greater is the facility with which a metal may deform, and the softer and weaker it becomes. Virtually all strengthening techniques rely on this simple principle: Restricting or hindering dislocation motion renders a material harder and stronger. The present discussion is confined to strengthening mechanisms for single-phase metals by grain size reduction, solid-solution alloying, and strain hardening. Deformation and strengthening of multiphase alloys are more complicated, involving concepts be- yond the scope of the present discussion; later chapters treat techniques that are used to strengthen multiphase alloys. Tutorial Video: How Do Defects Affect Metals? Mechanisms of Strengthening in Metals Figure 8.13 For a single crystal subjected to a shear stress τ, (a) deformation by slip, (b) deformation by twinning.
- Donald Askeland, Wendelin Wright(Authors)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
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. Deformation processing Techniques for the manufacturing of metallic and other materials using such processes as rolling, extrusion, drawing, etc. Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Chapter 8 Strain Hardening and Annealing 294 Drawing A deformation processing technique in which a material is pulled through an opening in a die (e.g., wire drawing). Extrusion A deformation processing technique in which a material is pushed through an opening in a die. Used for metallic and polymeric materials. Fiber texture A preferred orientation of grains obtained during the wire drawing process. Certain crystallographic directions in each elongated grain line up with the drawing direction, causing anisotropic behavior. Frank-Read source A pinned dislocation that, under an applied stress, produces additional dislocations. This mechanism is at least partly responsible for strain hardening. Heat-affected zone The volume of material adjacent to a weld that is heated during the welding process above some critical temperature at which a change in the structure, such as grain growth or recrystallization, occurs. 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.
- Donald Askeland, Wendelin Wright, Donald Askeland(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
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. Deformation processing Techniques for the manufacturing of metallic and other materials using such processes as rolling, extrusion, drawing, etc. Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. Chapter 8 Strain Hardening and Annealing 294 Drawing A deformation processing technique in which a material is pulled through an opening in a die (e.g., wire drawing). Extrusion A deformation processing technique in which a material is pushed through an opening in a die. Used for metallic and polymeric materials. Fiber texture A preferred orientation of grains obtained during the wire drawing process. Certain crystallographic directions in each elongated grain line up with the drawing direction, causing anisotropic behavior. Frank-Read source A pinned dislocation that, under an applied stress, produces additional dislocations. This mechanism is at least partly responsible for strain hardening. Heat-affected zone The volume of material adjacent to a weld that is heated during the welding process above some critical temperature at which a change in the structure, such as grain growth or recrystallization, occurs. 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.
eBook - PDF- J. Beddoes, M. Bibby(Authors)
- 1999(Publication Date)
- Butterworth-Heinemann(Publisher)
In contrast, a high n value is desirable for sheet formability, in which resistance to local necking, or reduction in sheet thickness, is necessary. When a high n value material begins to neck, the deforming region rapidly strain hardens, causing subse- quent plastic deformation to occur in the surrounding softer metal. This produces a long diffuse neck, as seen in Fig. 3.9. In contrast, necking in a material with a low n value occurs more locally, causing failure at a lower strain. It is noteworthy that many plastics strain harden rapidly, providing them with good sheet forming characteris- tics, but often causing poor cutting properties. By definition, hot working is deformation at temperatures above one-half the absolute melting temperature (the melting temperature in Kelvin). By contrast, cold working usually occurs at below 0.3 of the absolute melting temperature. During hot working several metallurgical mechanisms operate concurrently. Strain hardening (or work 92 Stressand strain during deformation b L L .=_ (- w Diffuse neck, high n Engineering strain (e) Fig. 3.9 Schematic engineering stress-strain curve for a low n and a high n material, to illustrate the effect of strain hardening on necking. hardening) can occur as a result of an increase in dislocation density. However, because of the elevated temperature, sufficient internal energy may be available to initiate dynamic recovery or dynamic recrystallization. These two processes are said to be dynamic, because they occur while the deformation is being applied. Both dynamic recovery and recrystallization serve to annihilate dislocations, causing soft- ening. The combined effect of dynamic recovery/recrystallization is to lower the stress required for deformation. Therefore, the stress required for hot deformation repre- sents a dynamic equilibrium between the hardening processes (dislocation generation) and the softening processes (dislocation annihilation).
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