| Root Protection and Backing Gases
A backing gas is sometimes used to protect the backside of a workpiece (or the root side of the weld) from oxidation and/or discoloration during welding. Oxidation (discoloration) can affect the corrosion properties of the backside of the weld and its visual appearance.
Backing gases are very common when welding stainless steel pipes and tubes, for instance in process and food industries. In addition, a backing gas should be used whereever the good corrosion properties of a stainless steel must be preserved.
The backing gas replaces air, thus avoiding oxidation on the hot surfaces of the backside of the weld. Typical root protection and/or backing gases are argon, nitrogen and nitrogen mixed with hydrogen (typically 10%). Other gases such as argon, helium, argon/helium and hydrogen mixtures may be suited to individual applications.
In order to reduce the oxygen concentration sufficiently (ideally 5 – 10 ppm) on the backside of the weld, special purging techniques and hardware must be used. Specialized purging hardware is available for tube welding, for instance. |
