Displacement of air from the root side of a weld with argon, or sometimes nitrogen for suitable alloys, so the backside of stainless, nickel alloy and titanium welds stays clean and corrosion resistant. Dams or water-soluble paper block the pipe, and oxygen is brought down to a low level, often below 0.1 percent, checked with an oxygen meter before welding. Without it, the root oxidizes into sugar.
Purge meter reads 40 ppm, start the root, and keep the flow low so you don't blow it concave.
Get the dams in and purge well before the first tack, then turn the flow down once welding starts; high purge pressure pushes the root concave.
Also heard as
- back purge
- backpurge
- purging
- purge gas
Related terms
Sugaring
Heavy oxidation on the back of a stainless or titanium weld that leaves a rough, grainy crust like sugar.
GTAW
Gas tungsten arc welding: an arc from a non-consumable tungsten electrode under inert gas, known as TIG.
Root pass
The first weld pass in a joint, laid at the root, which sets the quality of everything above it.
Shielding gas
Gas that protects the arc and molten weld from the surrounding air in GMAW, GTAW and gas-shielded FCAW.
Suck-back
Shop name for a concave root on pipe, where the inside of the weld sits below the pipe wall.
Same word, different worlds
To pump fresh grease into a bearing until the old grease is pushed out of the relief port or seal.
AdhesivesTo run adhesive through a mixer or dispense valve and discard it, clearing old, unmixed or off-ratio material.
Boilers & BurnersBlowing air through the furnace and flue before ignition or after shutdown to clear out unburned fuel.