Due to the cold working of metals, there arise a large number of dislocation and distortions of planes. Cold working of metals or alloys increases the hardness, yield strength, ultimate strength and electrical resistance whereas the ductility and plasticity are decreased. Following are some important changes in properties in metals due to plastic deformation.
Work (Strain) Hardening
Strain hardening or work hardening is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. The temperature at which deformation takes place is cold, relative to the absolute melting temperature of the metal, is a type of cold working operation. Most metals strain hardens at room temperature. This increase in hardening is accompanied by an increase in both tensile and yield strength.
Work hardening reduces the ductility and plasticity. Sometimes work hardening is expressed as percent cold work or percentage reduction in cross sectional area rather than as strain. Percent cold work is defined as:
where, Ao = Original cross-sectional area of the cross-section
Ad = Area after deformation
Uses. Work hardening is used in rolling of bars, drawing of tubes, pre-stretching of hoisting, chains and cables, initial pressurisation of pressure vessels, cylinders of hydraulic presses and guns.
If properties are same in every plane and in every direction, such crystals are called isotropic. If properties change according to the plane or direction, such crystals are called anisotropic. It has been observed that preferred orientation in metals or alloys results in an anisotropy in mechanical properties. If the crystals are randomly oriented, the elastic properties will be same in all direction. By various cold working processes, it is possible for crystals to assume identical orientation which is called preferred orientation of crystal as shown in Fig. 4 below. So, because of this preferred orientation, properties are not same in every direction (anisotropy).
Cold working causes grains to rotate with respect to the direction of flow. This rotation in polycrystalline alloys causes sections or bands within the grains to align with the direction of cold working. The slip planes do not necessarily tend to become parallel to the flow direction. So if the deformation is extensive, then every crystal present in the specimen should ultimately approach the same orientation relative to the axis of the principal strain. The above phenomenon is called preferred orientation or texture. Preferred orientation can be detected by different methods such as X-ray diffraction techniques, etching test and other metallographic methods.
In cold rolled sheets, the preferred orientation is developed which results in lower tensile strength, lower yield strength and lower ductility across the grain i.e., transverse to the rolling direction, than parallel to it. If more strength and ductility is required in transverse direction also, then break up preferred orientation. This can be done by cross rolling the sheets (for deep drawing purposes) i.e., rolling alternatively in two directions at right angles to each other.
Application. The application of preferred orientation is in the:
- Manufacturing of iron sheets used for transformer.
- Cast metals having columnar growth.
- Strengthening of magnetization in silicon steels etc.
If an alloy is over-strained in order to remove the yield point and is allowed to rest after plastic deformation, it is found that the yield point returns with a higher stress when the alloy is reworked. This is called strain ageing. It has a hardening effect on the alloy.
In the Fig 6 shown below, initially the material is loaded in X region. Then the material (specimen) is unloaded. This corresponds to region Y. Let the specimen be strained up to point B and unloaded again. If it is reloaded again after some hours, the yield point will reappear at a higher value than the initial yield point of region X. This is because of strain ageing.
Internal stresses are there in the components or materials which are produced from cold working. The effect of this internal stress is very high on copper alloys mainly brass. Brass with internal stresses becomes more susceptible to inter-crystalline corrosion, if it is stored for a long period. Because of this inter-crystalline corrosion, cracks may develop and these cracks are called season cracking. However this defect can be minimized by annealing the cold worked components at low temperatures of the order of 200°C to 300°C.