Understanding Back Purging
Back purging supplies inert gas to the back side of a weld joint to protect the root pass from atmospheric contamination. Without back purging, the root side of the weld oxidizes, creating a rough, discolored surface with compromised corrosion resistance and mechanical properties.
For materials like stainless steel, titanium, and nickel alloys, back purging is essential for quality.
Materials Requiring Back Purging
Stainless Steel: Back purging prevents chromium oxide formation on the root, maintaining corrosion resistance.
Titanium: Absolutely essential—titanium oxidizes instantly at welding temperatures. Without purging, the root is severely embrittled.
Nickel Alloys: Back purging maintains corrosion resistance and prevents oxide inclusions.
Back Purging Methods
Sealed Enclosure Purging
For small parts or short welds, the entire part can be sealed in a chamber or bag and filled with argon.
Local Dam Purging
For pipe and tubular joints, dams create a local purge chamber at the weld area. Dam materials include rubber or inflatable pipe plugs, cardboard or fiber discs, aluminum tape dams, and foam plugs.
Backing Bar Purging
Special backing bars have built-in gas channels that distribute argon behind the weld.
Argon Flow Rates
Small Pipe (under 2"): 5-15 CFH
Medium Pipe (2-6"): 15-30 CFH
Large Pipe (over 6"): 30-60+ CFH
Verifying Purge Quality
Oxygen Analyzers: Measure oxygen content in the purge gas. Stainless steel requires less than 500 ppm oxygen; titanium requires less than 50 ppm.
Match Test: Light a wooden match or butane lighter and insert into purge area. Flame should extinguish immediately in inert atmosphere.
Best Practices
Clean the joint before welding. Install dams properly. Position argon inlet to sweep air effectively. Provide vent for displaced air. Test before welding. Maintain flow during welding and post-flow.
Conclusion
Back purging is essential for producing quality root passes on reactive materials. The investment in proper purging equipment and technique pays dividends through improved weld quality, reduced rework, and longer service life.
