How Cold Working Strengthens Metal

Stacked coils of aluminum sheet at the Novelis plant in Oswego, NY
••• Novelis, Inc.

In most cases, metal is cast or forged into the desired shape after it is made malleable through the application of heat. Cold working refers to the process of strengthening metal by changing its shape without the use of heat. Subjecting the metal to this mechanical stress causes a permanent change to the metal's crystalline structure, causing an increase in strength. 

Metal is rolled between two rollers, or drawn through (pushed or pulled) smaller holes. As the metal is compressed, the grain size can be reduced, increasing strength (within grain size tolerances). Metal can also be sheared to form it into the desired shape.

How Cold Working Strengthens Metal

The process gets its name because it is conducted at temperatures below the metal's recrystallization point. Mechanical stress is used instead of heat to affect change. The most common applications for this process are steel, aluminum, and copper

When these metals are cold worked, permanent defects change their crystalline makeup. These defects reduce the ability of crystals to move within the metal structure and the metal becomes more resistant to further deformation.

The resulting metal product has improved tensile strength and hardness, but less ductility (the ability to change shape without losing strength or breaking). Cold rolling and cold drawing of steel also improve surface finish.

Types of Cold Working

The major cold-working methods can be classified as squeezing or rolling, bending, shearing and drawing. See the table below for a summary of the various methods for cold working metal.

Squeezing

Bending

Shearing

Drawing

Rolling

Angle

Shearing

Bar wire and tube drawing

Swaging

Roll

Slitting

Wire drawing

Cold Forging

Roll forming

Blanking

Spinning

Sizing

Drawing

Piercing

Embossing

Extrusion

Seaming

Lancing

Stretch forming

Riveting

Flanging

Perforating

Shell drawing

Staking

Straightening

Notching

Ironing

Coining

 

Nibbling

High-energy rate forming

Peening

 

Shaving

 

Burnishing

 

Trimming

 

Die hobbing

 

Cutoff

 

Thread rolling

 

Dinking

 

Most Common Methods of Work Hardening

With so many options for work hardening, how do manufacturers decide which to use? It depends on the use to which the metal will be put. Three of the most common types of work hardening are cold rolling, bending, and drawing.

Cold rolling is the most common method of work hardening. This involves the metal being passed through pairs of rollers to reduce its thickness or to make the thickness uniform. As it moves through the rollers and is compressed, the metal grains are deformed. Examples of cold-rolled products include steel sheets, strips, bars, and rods.

Bending of sheet metal is another process for cold working, which involves deforming metal over a work axis, thereby creating a change in the geometry of the metal. In this method, the shape changes, but the volume of the metal remains constant.

An example of this bending process is simply bending of steel or aluminum parts to meet the desired curvature. Many car parts, for example, have to be bent in order to fit manufacture dimensions.

Drawing essentially involves pulling the metal through a small hole or die. This reduces the diameter of a metal rod or wire while increasing the product's length. The raw metal is pushed into the die via compression force to ensure that recrystallization occurs as the metal changes shape. Products made via this process include steel bars and aluminum rods.