Back Shielding (Forming)
When welding high-alloy steels the weld root must also be protected from contact with ambient oxygen. Root shielding is used with both TIG and MAG welding. There is usually a requirement of less than 20 ppm residual oxygen at the root. Which weld colours are acceptable depends on where each individual component will be used.
To protect the weld root, small pipes are flushed with shielding gas. Here it is important to have an aligned outlet. In the case of larger pipes, devices are used to directly lead the root shielding gas to the weld. It is important to have a sufficiently long pre-surge gas flow time.
Back Shielding (Forming) can also be used when welding non-alloyed steels or aluminium, or when soldering copper pipes.
As a rule, so-called forming gases, nitrogen/hydrogen mixtures, are used. The hydrogen component provides more security against residual oxygen. High-quality forming, however, requires only very low hydrogen content.
- Protects the root side
- No reaction of the melt with air components
- No changes in colour or slag on the root side
- Better corrosion resistance for CrNi-steels and nickel alloys