The cold drawn steel pipe is work hardened. The metal deforms plastically during cold drawing, and multiple slip systems start inside the crystal. The dislocation movements intercept each other. Many dislocations are pinned, causing dislocations to accumulate and at the same time the dislocation source stops. The above-mentioned series of processes lead to a decrease in the dislocation mobility, and the density of dislocations in the crystal increases significantly. When the plastic deformation further occurs, the stress increases enough to start the movement of the pinned dislocations, the screw dislocations intersect and slip, and the blade dislocations cannot intersect and slip, so that the dislocations intersect and the fixed order increases.
Therefore, through the cold drawing process, the internal dislocation density of the metal is increased, and the dislocation mobility is reduced. It is difficult to generate dislocations and it is difficult to move the dislocations, so the hardness and strength of the metal material are improved. This is the metallurgical principle of cold drawing.