Weld purging is the act of removing, from the vicinity of the joint; oxygen, water vapour and any other gases or vapours that might oxidize or contaminate a welding joint as it is being welded and immediately after welding.
Stainless steels, duplex steels, titanium-, nickel- and zirconium- alloys are sensitive to the presence of air, oxygen, hydrogen, water vapour and other vapours and gases that may combine with the hot metal as it is being joined.
Such gases may combine with the metal to form undesirable compounds that may reduce corrosion resistance or may be instrumental in creating cracks or other structural defects in metals.
Weld Purging is generally necessary for the first weld run when joining two separate parts. This sealing weld will be called a “root run” when it takes more than one run or (pass) to fully seal the root area from above.
Once the root run has been completed, it is possible to stop the purging process unless the welding engineer has specified that purging should be continued for the second and third passes for example in case the root weld becomes hot enough to oxidise in the air that will have replaced the purge gas.
Purging methods
Unwanted gas is generally removed by flushing with an inert gas. Argon is generally used for this purpose but helium is an alternative depending on gas cost and availability.
Nitrogen has been used as a purge gas but is unsuitable for some stainless steels.
The most common way to remove gas from the weld zone is to flush it away with an inert gas. The weld zone can be contained to prevent fresh gas from entering once the contained volume has been purged.
Another method of purging is to enclose the metal parts completely in a vacuum chamber and evacuate it, prior to backfilling with inert gas for the welding process. When purging with inert gases, it is important to enter the gas very slowly.
When using Argon, which is heavier than air, the gas should enter from the bottom of the enclosed space. After filling the base area, be it the bottom of a pipe or tank, it will continue to displace air and move upwards. The displaced air is purged out of a release hole at the top. If process monitoring is desired, and oxygen sensor can be used - ideally placed near the release hole to ensure the weld region is appropriately purged.
Helium however, which is also an inert gas should be entered from the top of the enclosed space, where it will first flood across the upper limits and push the trapped air downwards and out through an exit where the oxygen level can be measured until it reaches the required level. The purpose of slow gas entry, typically 5 – 7 litres/m (….CFH), is to avoid turbulence, which would otherwise lengthen the purge process considerably.
Welding methods
Most weld purging is carried out on joints made by the TIG or GTAW arc welding process. When the laser welding process is chosen, joints or welds being made on reactive materials will need to be carried out and in the case of electron beam welding, this is carried out in a vacuum, in which case purging takes place by complete evacuation of all gas.
For TIG/GTAW welding, the top side (front side) of a weld is normally protected by inert gas flowing through the welding torch and it is the underside (back side) of the weld where atmospheric gases need to be purged.
When joining two parts together with a seal weld, it may be necessary to first hold them together, by making tack welds. This will be carried out by making very short welds at intervals varying according to the size and weight of the two parts. It is very important that the weld purging process should start for the tack welds, so the undersides of those tacks remain clean and shiny, without any oxidation or discolouration.
Gases
Unwanted gas is generally removed by flushing with an inert gas. Argon is generally used for this purpose but helium is an alternative depending on gas cost and availability. Nitrogen has been used as a purge gas but is unsuitable for some stainless steels. Purge gases must be of a certain quality, in order that the welds are made correctly. Because welds in Argon or Helium which have more than 0.05% (500 ppm) of oxygen will oxidise and discolour, welding engineers will include the mention of the correct gas quality in their welding procedures.
If process monitoring is desired, an oxygen sensor can be used to confirm the purge gas has appropriately filled the region of interest prior to starting the weld.