Strain Hardening | What is strain hardening in material?
Ever notice how it gets harder to bend a metal rod the more you bend it? That's strain hardening at work! This process strengthens a material, typically metals and polymers, through permanent deformation.
Strain hardening process
Strain hardening, also known as work hardening or cold working, is an important industrial process that increases the strength and hardness of a material that do not respond to heat treatment through plastic deformation. This deformation occurs when a material is subjected to mechanical stress, such as compression, tension, or torsion, causing its shape to change permanently.
The rate of strain hardening can be gaged from the slope of the flow curve. For alloys strengthened by solid-solution additions the rate of strain hardening may be either increased or decreased compared with the behavior for the pure metal. However, the final strength of a cold-worked solid-solution alloy is almost always greater than that of the pure metal cold-worked to the same extent.
Figure (1) shows the typical variation of strength and ductility parameters with increasing amount of cold-work. Since in most cold-working processes one or two dimensions of the metal are reduced at the expense of an increase in the other dimensions, cold-work produces elongation of the grains in the principal direction of working.
A high rate of strain hardening implies mutual obstruction of dislocations gliding on intersecting systems. This can come Through:
- Interaction of the stress fields of the dislocations.
- Interactions which produce sessile locks.
- The interpenetration of one slip system by another (like cutting trees in a forest) which results in the formation of dislocation jogs.
The basic equation relating flow stress (strain hardening) to structure is:
Note that:
- Generally, the rate of strain hardening is lower for hcp metals than for cubic metals.
- Increasing temperature also lowers the rate of strain hardening.
Strain hardening mechanism
Strain hardening typically begins when a material is subjected to external forces or stresses, such as compression, tension, or shear. These forces cause the material to undergo plastic deformation, where its shape changes permanently without fracturing. During this deformation, dislocations start to move within the material's crystal lattice. Dislocations are line defects where the atoms are out of their regular positions, and their movement plays a crucial role in plastic deformation.
As the material undergoes plastic deformation, new dislocations are generated at the points of stress concentration or at grain boundaries (in polycrystalline materials). These newly created dislocations contribute to the material's ability to deform plastically but also introduce additional obstacles to dislocation movement.
The movement of dislocations is impeded by obstacles such as other dislocations, grain boundaries, and impurities within the material. As plastic deformation continues, dislocations interact and tangle with each other, forming a complex network throughout the material. This tangled network of dislocations effectively hinders further dislocation movement, making it more difficult for the material to deform.
The accumulation of dislocations and the formation of dislocation tangles lead to an increase in dislocation density within the material. Dislocation density refers to the number of dislocations per unit volume of material. A higher dislocation density corresponds to a greater resistance to deformation because the dislocations act as barriers that impede the motion of other dislocations.
Effects of strain hardening
One of the primary effects of strain hardening is the enhancement of a material's strength. The resistance to deformation makes the material stronger and able to withstand higher loads. The hardness of the material also increases due to the rearrangement and entanglement of dislocations.
Strain hardening can also improve a material's toughness, which is its ability to absorb energy and deform plastically before fracturing.
While strain hardening improves strength and toughness, it often leads to reduced ductility, which is the ability of a material to undergo large plastic deformations without fracturing.
The increased strength and reduced ductility can affect the material's workability, making it more challenging to form or shape using conventional processes.
In addition to the changes in mechanical properties, strain hardening produces changes in other physical properties. There is usually a small decrease in density of the order of a few tenths of a percent, an appreciable decrease in electrical conductivity due to an increased number of scattering centers, and a small increase in the thermal coefficient of expansion. Because of the increased internal energy of the cold-worked state, chemical reactivity is increased. This leads to a general decrease in corrosion resistance and in certain alloys introduces the possibility of stress-corrosion cracking.
Strain hardening applications
Manufacturing of metal components:
Strain hardening is extensively used in the manufacturing of metal components such as sheets, plates, bars, and wires. Cold working processes like rolling, forging, drawing, and extrusion are employed to deform metal parts, increasing their strength and hardness.
These components are then used in industries like automotive, aerospace, machinery, and construction for their improved mechanical properties.
Structural steel and alloy production:
Strain hardening is crucial in producing high-strength structural steels and alloys. Cold rolling and cold drawing processes are used to deform steel sheets and rods, resulting in increased strength and hardness. These materials are widely used in building construction, bridges, pipelines, and infrastructure projects where strength and durability are paramount.
Wire and cable manufacturing:
Cold working processes are commonly employed in the production of wires and cables for electrical and telecommunications applications. Cold drawing techniques are used to reduce the diameter of metal wires while increasing their tensile strength and conductivity. Strain-hardened wires are used in power transmission lines, electrical wiring, and communication cables.
Automotive industry:
Strain-hardened metals are used extensively in the automotive industry for manufacturing components such as chassis, body panels, engine parts, and suspension systems. Cold working processes improve the strength and durability of automotive parts, leading to enhanced safety and performance of vehicles.
Aerospace and defense:
Aircraft and spacecraft components require high-strength materials that can withstand extreme conditions. Strain-hardened alloys are used in the aerospace and defense sectors for producing components like aircraft wings, fuselage structures, landing gear, and engine parts. Cold working techniques are employed to achieve the desired mechanical properties, including increased fatigue resistance and damage tolerance.
