Theory of Butt Fusion: approaching the butt welding world

Basic concepts

The principle of butt fusion is to heat up the surfaces of two pipes in order to fuse them and then press them together at a designed force so that the two pipes become one only thing.

The heating of the material makes the material melt, while the force applied causes molecular mixing and fusion.

Once the joint cools down, the molecules go back to their initial form, and the two pipes turn into one united piece.

The connection is stronger than the original single pipe, because the fusion bead caused by the heating increases the thickness in the welding area.

Butt fusion

The butt welding process takes place thanks to the aid of specific machines. The fabrication of these machine is regulated by rules set by expert entities both national and international.

These construction standards normally divide the butt welding machines in three main types:

1. Manual butt-welding: machines where every phase of the job is performed manually by employed welder.
   Manual butt welding is nowadays used only for drainage pipelines, where no fluid under pressure is involved.
2. Semi-automatic butt fusion: the welding is performed with the aid of more evolved machines capable of automating some functions of the process like pre-setting of the pressure to be applied for the welding.
3. Automatic butt fusion: the machines used automate most of the process. This includes regulation and control of temperature and pressure, ejection of the heating mirror and in some cases movemente of the trolleys.

Butt welding basic principles

Good quality welds can be obtained following some core principles that affect quality of the welds:

• The two pipes must be of the same material, diameter and wall thickness
• particular care must be taken on the cleanliness
• temperature must be appropriate and stable
• pressure control in all the phases
• respect of the phases time
• the welder must be trained and competent

The welding process (in brief)

The butt welding process is performed following these steps:

1. Cleaning: The pipe ends are cleaned to remove dust, dirt and other contamination deposited during transport, handling and storage.
2. Clamping: the two pipes ends are laid and clamped so that they are firmly held and perfectly aligned one to the other.
3. Facing: the two faces of the paipes are faced both to remove the surface oxidated layer and to ensure perfect mating between the faces, without any light.
4. Aligning: the pipe ends are taken close in order to check and refine the alignment.
5. Heating: The pipe ends are pressed in perfect contact against a hot plate at a given temperature.
   In this way the pipe walls start to melt and absorb the heating necessary for the fusion of the material.
6. Fusion: the melted surfaces are joined together using a specified force, which is constant around the pipe interfacial area.
7. Cooling: The fusion joint is held down under pressure until it is properly cooled.
8. Inspecting: the welded joint in its entire circumference is visually inspected.

Butt fusion standards

Many national and international organizations have been providing reference guides for proven welding procedures and welding tests, here are the main: