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Working with Metals
Chapter 10
Working with Metals Chapter Outline Elastic Limit Plastic Deformation Fracture Polycrystalline Materials Cold Working Stored Energy
83 84 84 84 84 85
Restoring the Lattice Structure of Metal after Cold Work – Annealing Grain Growth Hot Working
85 85 86
Working with metals involves understanding the limits of their mechanical properties. Several aspects of these properties can be used to get the best results from the metal. There cannot be a limit on what and how much information to know about any given material. As they can be used in very different environmental conditions, the impact of the environment on the metal must be studied in order to determine which properties of the material would be affected. A combination of information, including a material’s mechanical properties and corrosion behavior in the specific service environment must be studied for the proper selection of a material for any project. Some of the mechanical properties that have a significant impact on the workability of metals are discussed in this chapter.
ELASTIC LIMIT When a material is stressed below its elastic limit, the resulting deformation or strain is temporary. Removal of an elastic stress allows the object to return to its original dimensions. When a material is stressed beyond its elastic limit, plastic or permanent deformation takes place and it will not return to its original dimensions when the stress is removed. All shaping operations, such as stamping, pressing, spinning, rolling, forging, drawing, and extruding involve plastic deformation. Pressure testing, with few exceptions, is done within the elastic limits of the material. Applied Welding Engineering: Processes, Codes and Standards. Copyright © 2012 Elsevier Inc. All rights reserved.
83
84
SECTION | 1 Introduction to Basic Metallurgy
PLASTIC DEFORMATION Plastic deformation may occur by slip, twinning, or a combination of the two. Slip occurs when a crystal is stressed in tension beyond its elastic limit. It elongates slightly and a step appears on the surface, indicating the displacement of one part of the crystal. Increasing the load will cause movement on a parallel plane, resulting in another step. Each successive elongation requires a higher stress and results in the appearance of another step. Progressively increasing the load eventually causes the material to fracture. Twinning is a movement of planes of atoms such that the lattice becomes divided into two symmetrical parts that are differently oriented. Deformation twins are most prevalent in close-packed hexagonal metals, such as magnesium and zinc, and body-centered cubic metals, such as tungsten and α iron. Annealing twins can occur as a result of reheating previously worked facecentered cubic metals such as aluminum and copper.
FRACTURE Fracture is the separation of a body under stress into two or more parts. Brittle fracture involves rapid propagation of a crack with minimal energy absorption and plastic deformation. It occurs by cleavage along particular crystallographic planes and shows a granular appearance. Ductile fracture occurs when considerable plastic deformation takes place prior to failure. Fracture begins by the formation of cavities at nonmetallic inclusions. Under continued applied stress, the cavities coalesce to form a crack. This process is seen as micro-void coalescence on the fracture surface.
POLYCRYSTALLINE MATERIALS Commercial materials are made up of polycrystalline grains whose crystal axes are oriented at random. Therefore, depending on their orientation to the applied stress, the deformation processes occur differently in the grains. A fine-grained material in which the grains are randomly oriented will possess identical properties in all directions and is called isotropic. A metal with controlled grain orientation will have directional properties (anistropic) which may be either troublesome or advantageous, depending on the direction of loading. When a crystal deforms, there is distortion of the lattice which increases with increasing deformation. As a result, there is an increase in resistance to further deformation known as strain hardening or work hardening.
COLD WORKING A material is considered to be cold worked when its grains are in a distorted condition after plastic deformation has occurred. All of the properties
Chapter | 10 Working with Metals
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of a metal that are dependent on the lattice structure are affected by plastic deformation. Cold working increases the tensile strength, yield strength, and hardness of the material. Hardness increases most rapidly in the first 10% reduction by cold work and tensile strength increases linearly; conversely most of the ductility is lost due to cold work in the first 10% reduction; thereafter the reduction in ductility happens more slowly. Yield strength increases more rapidly than tensile strength. Cold work also reduces electrical conductivity.
STORED ENERGY Although most of the energy used to cold-work metal is dissipated in heat, a finite amount is stored in the crystal structure as internal energy that is associated with the lattice defects created by the deformation. This increase in internal energy is often concentrated in the grain boundaries, resulting in localized increased susceptibility to energy-driven reactions such as corrosion.
RESTORING THE LATTICE STRUCTURE OF METAL AFTER COLD WORK – ANNEALING Full annealing is the process by which the distorted, cold-worked, lattice structure is changed back to one which is strain-free through the application of heat. This is a solid-state process and is usually followed by slow cooling in the furnace. Recovery is the first stage of annealing. This is a low temperature process and does not involve significant changes in the microstructure. The principal effect is relief of internal stresses. Recovery is a time and temperature dependent process. There is little change in mechanical properties and the principal application of recovery is stress relief to prevent stress corrosion cracking or to minimize distortion produced by residual stresses. Recrystallization occurs at higher temperatures, as minute new crystals appear in the microstructure. They usually appear in the regions of highest deformation such as at grain boundaries or slip planes. Recrystallization takes place by the process of nucleation of strain-free grains and the growth of these nuclei to absorb the cold-worked material. The recrystallization temperature refers to the approximate temperature at which a highly cold-worked material completely recrystallizes in one hour. It may be noted that the greater the amount of deformation, the lower the recrystallization temperature. Zinc, lead, and tin have recrystallization temperatures below room temperature and so cannot be cold-worked.
GRAIN GROWTH Large grains have lower free energy than small grains, since there is less grain boundary volume. Grain growth is driven by a single crystal’s lowest energy state. The rigidity of the lattice opposes grain growth.
86
SECTION | 1 Introduction to Basic Metallurgy
As temperature is increased, the rigidity of the lattice decreases and the rate of grain growth is more rapid. Holding a specimen for a long time in the grain-growth temperature region (slightly below the melting point) can grow very large grains. The final recrystallized grain size is controlled by factors that influence nucleation and growth rate.
HOT WORKING When a material is plastically deformed, it tends to become harder. But the rate of work hardening decreases as the working temperature is increased. Two opposing effects take place – hardening due to plastic deformation and softening due to recrystallization. For a given material, there is a temperature at which these two effects balance. Material worked above this temperature is said to be hot-worked and material that is worked below this temperature is said to be cold-worked. Lead and tin may be hot-worked at room temperature, whereas steel is cold-worked at 538°C (1,000°F). Hot-worked material cannot be manufactured to an exact size because of the dimensional changes which take place during cooling. Cold-worked materials can be held to close tolerances, but require more power for deformation and so are more expensive to produce. Normally, initial reductions are carried out at an elevated temperature by hot working and the final reductions are done cold to take advantage of both processes. The finishing temperature for hot working determines the grain size that is available for further cold working. Careful control of these processes is known as thermo-mechanical processing.
Working with Metals Chapter Outline Elastic Limit Plastic Deformation Fracture Polycrystalline Materials Cold Working Stored Energy
83 84 84 84 84 85
Restoring the Lattice Structure of Metal after Cold Work – Annealing Grain Growth Hot Working
85 85 86
Working with metals involves understanding the limits of their mechanical properties. Several aspects of these properties can be used to get the best results from the metal. There cannot be a limit on what and how much information to know about any given material. As they can be used in very different environmental conditions, the impact of the environment on the metal must be studied in order to determine which properties of the material would be affected. A combination of information, including a material’s mechanical properties and corrosion behavior in the specific service environment must be studied for the proper selection of a material for any project. Some of the mechanical properties that have a significant impact on the workability of metals are discussed in this chapter.
ELASTIC LIMIT When a material is stressed below its elastic limit, the resulting deformation or strain is temporary. Removal of an elastic stress allows the object to return to its original dimensions. When a material is stressed beyond its elastic limit, plastic or permanent deformation takes place and it will not return to its original dimensions when the stress is removed. All shaping operations, such as stamping, pressing, spinning, rolling, forging, drawing, and extruding involve plastic deformation. Pressure testing, with few exceptions, is done within the elastic limits of the material. Applied Welding Engineering: Processes, Codes and Standards. Copyright © 2012 Elsevier Inc. All rights reserved.
83
84
SECTION | 1 Introduction to Basic Metallurgy
PLASTIC DEFORMATION Plastic deformation may occur by slip, twinning, or a combination of the two. Slip occurs when a crystal is stressed in tension beyond its elastic limit. It elongates slightly and a step appears on the surface, indicating the displacement of one part of the crystal. Increasing the load will cause movement on a parallel plane, resulting in another step. Each successive elongation requires a higher stress and results in the appearance of another step. Progressively increasing the load eventually causes the material to fracture. Twinning is a movement of planes of atoms such that the lattice becomes divided into two symmetrical parts that are differently oriented. Deformation twins are most prevalent in close-packed hexagonal metals, such as magnesium and zinc, and body-centered cubic metals, such as tungsten and α iron. Annealing twins can occur as a result of reheating previously worked facecentered cubic metals such as aluminum and copper.
FRACTURE Fracture is the separation of a body under stress into two or more parts. Brittle fracture involves rapid propagation of a crack with minimal energy absorption and plastic deformation. It occurs by cleavage along particular crystallographic planes and shows a granular appearance. Ductile fracture occurs when considerable plastic deformation takes place prior to failure. Fracture begins by the formation of cavities at nonmetallic inclusions. Under continued applied stress, the cavities coalesce to form a crack. This process is seen as micro-void coalescence on the fracture surface.
POLYCRYSTALLINE MATERIALS Commercial materials are made up of polycrystalline grains whose crystal axes are oriented at random. Therefore, depending on their orientation to the applied stress, the deformation processes occur differently in the grains. A fine-grained material in which the grains are randomly oriented will possess identical properties in all directions and is called isotropic. A metal with controlled grain orientation will have directional properties (anistropic) which may be either troublesome or advantageous, depending on the direction of loading. When a crystal deforms, there is distortion of the lattice which increases with increasing deformation. As a result, there is an increase in resistance to further deformation known as strain hardening or work hardening.
COLD WORKING A material is considered to be cold worked when its grains are in a distorted condition after plastic deformation has occurred. All of the properties
Chapter | 10 Working with Metals
85
of a metal that are dependent on the lattice structure are affected by plastic deformation. Cold working increases the tensile strength, yield strength, and hardness of the material. Hardness increases most rapidly in the first 10% reduction by cold work and tensile strength increases linearly; conversely most of the ductility is lost due to cold work in the first 10% reduction; thereafter the reduction in ductility happens more slowly. Yield strength increases more rapidly than tensile strength. Cold work also reduces electrical conductivity.
STORED ENERGY Although most of the energy used to cold-work metal is dissipated in heat, a finite amount is stored in the crystal structure as internal energy that is associated with the lattice defects created by the deformation. This increase in internal energy is often concentrated in the grain boundaries, resulting in localized increased susceptibility to energy-driven reactions such as corrosion.
RESTORING THE LATTICE STRUCTURE OF METAL AFTER COLD WORK – ANNEALING Full annealing is the process by which the distorted, cold-worked, lattice structure is changed back to one which is strain-free through the application of heat. This is a solid-state process and is usually followed by slow cooling in the furnace. Recovery is the first stage of annealing. This is a low temperature process and does not involve significant changes in the microstructure. The principal effect is relief of internal stresses. Recovery is a time and temperature dependent process. There is little change in mechanical properties and the principal application of recovery is stress relief to prevent stress corrosion cracking or to minimize distortion produced by residual stresses. Recrystallization occurs at higher temperatures, as minute new crystals appear in the microstructure. They usually appear in the regions of highest deformation such as at grain boundaries or slip planes. Recrystallization takes place by the process of nucleation of strain-free grains and the growth of these nuclei to absorb the cold-worked material. The recrystallization temperature refers to the approximate temperature at which a highly cold-worked material completely recrystallizes in one hour. It may be noted that the greater the amount of deformation, the lower the recrystallization temperature. Zinc, lead, and tin have recrystallization temperatures below room temperature and so cannot be cold-worked.
GRAIN GROWTH Large grains have lower free energy than small grains, since there is less grain boundary volume. Grain growth is driven by a single crystal’s lowest energy state. The rigidity of the lattice opposes grain growth.
86
SECTION | 1 Introduction to Basic Metallurgy
As temperature is increased, the rigidity of the lattice decreases and the rate of grain growth is more rapid. Holding a specimen for a long time in the grain-growth temperature region (slightly below the melting point) can grow very large grains. The final recrystallized grain size is controlled by factors that influence nucleation and growth rate.
HOT WORKING When a material is plastically deformed, it tends to become harder. But the rate of work hardening decreases as the working temperature is increased. Two opposing effects take place – hardening due to plastic deformation and softening due to recrystallization. For a given material, there is a temperature at which these two effects balance. Material worked above this temperature is said to be hot-worked and material that is worked below this temperature is said to be cold-worked. Lead and tin may be hot-worked at room temperature, whereas steel is cold-worked at 538°C (1,000°F). Hot-worked material cannot be manufactured to an exact size because of the dimensional changes which take place during cooling. Cold-worked materials can be held to close tolerances, but require more power for deformation and so are more expensive to produce. Normally, initial reductions are carried out at an elevated temperature by hot working and the final reductions are done cold to take advantage of both processes. The finishing temperature for hot working determines the grain size that is available for further cold working. Careful control of these processes is known as thermo-mechanical processing.