When welding stainless steel pipes, the pipe should be purged or shielded beforehand to ensure the corrosion resistance of the materials. The purpose of forming is to create or maintain a high-quality surface condition of the root with regard to oxidation and root shaping. If the purging process of stainless steel is carried out correctly, rework can become superfluous.
If welding is carried out without prior forming, the root can oxidize and the material of the workpiece becomes porous or rusty. In the case of materials such as titanium or magnesium, it is even necessary to form them in advance, as these materials are sensitive to gases.
Displacing the air and flushing the root area is called pipe purging. During cooling, flushing with the backing gas must be carried out until no further reaction with the air can take place.
When purging stainless steel pipes, you should follow a few guidelines. This is the only way to ensure your safety and achieve the best possible result.
When purging pipes, particular attention must be paid to the following:
Lighter gases from above
Introduce heavy gases from below
The selection of the gas also depends on the position of the weld seam on the component
The formation process must include:
Displacing the air in the root area before welding
Maintaining during welding
Flushing on cooling after welding until no more oxidation occurs.
In the case of stainless steels, this temperature is approx. 250 °C
The three steps must also be carried out during tacking, since the oxides are not removed when the tacking points are welded over.