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Cold working is the simultaneous deformation and strengthening of an alloy.
Cold working (also known as cold forming) is metal forming performed at room temperature or slightly above.
Advantages
Significant advantages of cold forming compared to hot working are:
- better accuracy, meaning closer tolerances;
- better surface finish;
- strain hardening increases strength and hardness of the part;
- grain flow during deformation provides the opportunity for desirable directional properties to be obtained in the resulting product; and
- no heating of the work is required, which saves on furnace and fuel costs and permits higher production rates to be achieved.
Owing to this combination of advantages, many cold forming processes have developed into important mass-production operations. They provide close tolerances and good surfaces, minimizing the amount of machining required and permitting these operations to be classified as net shape or near net shape processes.
Disadvantages
There are certain disadvantages or limitations associated with cold forming operations:
- higher forces and power are required to perform the oepration;
- care musst be taken to ensure that the surfaces of the starting workpiece are free of scale and dirt; and
- ductility and strain hardening of the work metal limit the amount of forming that can be done to the part.
In some operations, the metal must be annealed in order to allow further deformation to be accomplished. In other cases, the metal is simply not ductile enough to be cold worked.
To overcome the strain hardening problem and reduce forces and power requirements, many forming operations are performed at elevated temperatures. There are two elevated temperature ranges involved, giving rise to the terms warm working and hot working.
Characteristics
There are a number of advantages and limitations to strengthening a metal by cold working or strain hardening:
- A metal can be strengthened while at the same time producing the desired final shape.
- Excellent dimensional tolerances and surface finishes can be obtained.
- The cold-working process is an inexpensive method for producing large numbers of small parts, since high forces and expensive forming equipment are not needed.
- 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.
- Ductility, electrical conductivity, and corrosion resistance are impaired by cold working. Because cold working reduces electrical conductivity less than 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.
- Properly controlled residual stresses and anisotropic behavior may be beneficial.
- Some deformation processing techniques can be accomplished only if cold working occurs. For example, wire drawing requires that a rod be pulled through a die to produce a smaller cross-sectional area. For a given draw force, a different stress is produced in the original and final wire. The stress on the initial wire must not exceed the yield strength of the metal to cause deformation. The stress on the final wire must be less than its yield strength to prevent failure. This is accomplished only if the wire strain hardens during drawing.