Strain hardening, also known as work hardening, is a process used primarily with metals to make them stronger and harder through physical manipulation. This strengthening occurs when a material is permanently deformed at a temperature below its recrystallization point. The result is an increase in the material’s ability to resist further deformation, which is a desirable characteristic in many manufactured components. The underlying mechanism involves permanent changes to the material’s internal atomic structure, which is accomplished by pushing the material past its natural elastic limit.
How Materials Are Strained
Strain hardening is achieved by applying mechanical forces that permanently change the shape of a material. This process is commonly called “cold working” because the shaping occurs below the material’s recrystallization temperature, often at room temperature. Working the material at this lower temperature prevents the atoms from quickly rearranging themselves back into their original, softer state.
To induce strain hardening, the material must be pushed beyond its elastic limit, meaning the force applied causes a permanent change in shape rather than one that springs back once the force is removed. Common methods of cold working include rolling, where a metal sheet is passed through rollers to reduce its thickness, or drawing, where a metal rod is pulled through a die to reduce its diameter. Other techniques like bending and shearing also cause this permanent deformation, which is the necessary action to trigger the internal strengthening.
The Microscopic Cause of Hardening
The physical change in strength is caused by changes to the material’s internal crystalline structure. Metals are composed of atoms arranged in an orderly grid, but this structure contains imperfections called dislocations. These dislocations are line defects within the crystal lattice that allow the metal to deform relatively easily under stress.
When a metal is plastically deformed, an external force causes these dislocations to move and slide along specific atomic planes. As the cold working continues, the movement of these defects causes them to multiply, intersect, and eventually become entangled with one another. This increasing density of tangled dislocations creates a kind of microscopic traffic jam inside the metal.
The entangled network of defects acts as an internal barrier that restricts the movement of any new or existing dislocations. Since the ability of the material to permanently deform is dependent on the motion of these defects, restricting their movement makes the material more resistant to further stress.
The Tradeoff in Material Strength
The increase in strength and hardness from strain hardening comes with a corresponding change in other physical properties. While the material can resist higher forces, its ductility, or ability to stretch and deform without fracturing, decreases significantly. The material becomes less able to absorb energy through plastic deformation, making it more brittle.
The strengthening process also affects the material’s yield strength, which is the point at which permanent deformation begins. Strain hardening raises this yield point, meaning a greater force is required to start the permanent shape change in the newly hardened material. The internal friction and rapid movement of dislocations during the cold working process also generate heat.
Where Strain Hardening Is Used
Strain hardening is intentionally employed across many manufacturing sectors to tailor material properties without using high-temperature heat treatments. One common application is in the production of wire, where the process of drawing the wire through progressively smaller dies results in a thinner and much stronger product.
Cold-rolled steel sheets are another widespread example, used in applications like automobile body panels where increased strength is beneficial for structural integrity. Fasteners, such as bolts and screws, are often cold-formed to increase their yield strength, ensuring they can handle higher tightening torques and operational loads.