Diameter enlargement is formed by using a blank smaller than the diameter of the large end of the pipe with an internal punching die along the inner diameter of the tube blank. The expanding process mainly solves the problem that it is not easy for the large diameter pipe to be formed by reducing. Sometimes, according to the need of material and product forming, the methods of expanding and reducing are combined.

In the process of reducing or expanding deformation and pressing, cold or hot pressing is adopted according to different materials and changing sizes. Usually, cold pressing should be used as far as possible, but hot pressing should be used for severe work hardening caused by multiple diameter changes, thick wall or alloy steel materials.