Backing gas / purging
Purge means flooding the back side or inside of a weld joint with shielding gas so the root stays clean and oxide-free. For pipes and cooling channels, the inside of the part is flushed with argon or purge gas.
What Purge Is
Purge is flooding the back side of the weld joint or a cavity with shielding gas. While the weld pool is protected from the front by the nozzle, the purge protects the root from the back—otherwise it oxidizes and forms scale or discoloration.
When It's Needed
Everywhere the weld root must stay clean: on pipes, vessels, and cooling channels. Especially with stainless steels and copper, purging is a must because otherwise the root tins.
The Cooling Channel Trick
When repairing cooling channels, seal the channel exit with tape and poke fine holes with the electrode tip—a sieve principle. Argon enters at the inlet and floods the channel gently, without stirring up the weld pool at the surface.