Backup Purge
Backup purge gas can also be used to protect the under side of the weld and its adjacent base metal surfaces from oxidation on the time of welding . Uniformity of root bead contour, freedom from undercut and the desired amount of root bead reinforcement are more likely to be achieved when using gas backup under controlled conditions. In most materials, gas backup reduces root cracking and porosity in the weld.
Argon and helium are satisfactory for the backup purge when welding all materials. Nitrogen may be used satisfactorily for backup welds in austenitic stainless steel, copper and copper alloys. In the rule of thumb, a relatively inert atmosphere will beobtained by flushing with four times the volume to be purged. After purging is completed, the flow of backup gas during welding should be reduced until only a slight positive pressure exists in the purged area. After the root and first filler passes are done, the backup purge may be discontinued. When using argon & nitrogen, the backup gas should preferably enter the system at a low point, to displace the atmosphere upwards and be vented at points beyond the joint to be welded.