2. Further treatments
page 8

Photo of cold rolls
Picture 2.2 Cold rolling steel. You can see the reels on the ground and loaded above the heavy rolls.
Cold working
Metals can be forced into new shapes at room temperature by cold working. There are a number of methods including:
cold rolling the metal strip is passed between two rolls that have only a narrow gap between them
drawing the metal is pulled through a small hole in a die
deep drawing a punch forces the metal sheet into a die. This is the process used to make seamless tubes and some cans (sometimes cans are formed from a flat sheet and welded)
pressing a metal sheet is pressed onto a shaped die or mould to form an intricate shape such as a car body panel
Video clips

Use the links below to see video clips of the processes - they will open in a new window.

Cold rolling (higher quality - 536k)
Cold rolling (lower quality - 184k)

Work Hardening
Cold working produces additional dislocations within the metal structure. When two or more dislocations meet, the movement of one tends to interfere with the movement of the other. The more dislocations there are, the more they will hinder each other's movement.

Initially the dislocations produced by cold working can move through the metal structure and the shape of the material will change. As the working continues, however, the movement of the dislocations becomes more difficult. This increases the strength of the metal and also makes it stiffer. Therefore it becomes less malleable and ductile. That is, it is more difficult to change its shape. We say that the metal has become work hardened.

To make it more malleable and ductile again, the metal can be annealed (see page 9).

Question 2-2.
a) What do we make from metals treated with the cold working process?

b) What does cold working produce within a metal structure that leads to it becoming work hardened?

Summary                   Close
  • cold working produces extra dislocations
  • initially, the metal changes shape
  • later the metal becomes stronger and less malleable and ductile
  • the metal can be made more malleable and ductile again by annealing it