What is purging?
“Purging” is the process of removing air that contains oxygen. Welding is a high-temperature process, so it is very important to keep oxygen and any other gases away from hot metal. We do this to avoid any kind of reaction between the metal and the atmospheric gases.
Oxygen has a tendency to react with metals, forming metal oxides that can lead to discoloration or leave pores on the welds. Hydrogen has a tendency to cause embrittlement (loss of ductility) of the metal, especially in the presence of titanium and some steels. Therefore, it is very important to purge before your welds are tacked.
Why purge when welding AL6XN?
- AL-6XN is selected for its high corrosion resistance. This corrosion resistance is obtained by the formation of a very thin, stable, oxide film that is rich in chromium. This film forms at normal temperature and pressure. To maintain the corrosion resistance, it is very important to protect the chromium oxide film from contamination during the fabrication process. This is why we must purge the backside of the weld in AL-6XN material.
- Purging prevents the formation of oxides and, by doing so, protects the material from losing its corrosion resistance.
How to purge?
- Apply an Argon shielding gas to the ID of the assembly. To do this, both ends of the tube should be plugged. The gas purging time will vary depending on the volume of atmosphere that needs to be evacuated. We typically allow enough gas-flow time to exchange 4 times the original held volume. For high purity applications, there are other methods that can be used to ensure that the ID of the assembly has been properly purged. In some cases you may want to monitor the escaping gas with an oxygen meter.
- Purging time should also continue until the weld seam cools down to avoid any other oxidation.
- Skin tacks should be very light to avoid penetrating the wall of the tubing. Full penetration tacks requires a high purity of purging gas and should be avoided.
What has your experience been with purging? Do you recommend it, or do you think it’s a waste of time? Join the discussion!
