User:Weldpurging/sandbox

Weld Purging
'Weld Purging’

Weld Purging is the act of removing, from the vicinity of the joint being welded, oxygen, water vapour and any other gases or vapours that might be harmful to a weld joint as it is being welded and immediately after welding.

Stainless Steels, Duplex Steels, Titanium-, Nickel- ,zirconium- alloys and Chrome steels such as P11, P22, P91, P92, CMV, etc 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 oxides, nitrides or other compounds that may reduce corrosion resistance or may be instrumental in creating cracks, porosity or other structural defects in metals.

When welding with the “TIG” /“ GTAW” or Plasma/PAW weld processes localised weld purging is generally carried out at the rear of the part being welded. That is, inside pipes, tanks and other cylindrical vessels, that are being welded from the outside and at the backside of sheet metal welds or welds on rectilinear structures. The main purpose is to protect the “Root” of the weld from oxidation.

Additionally, extra weld purging can be carried out on the front side of the weld, by extending the purging distance behind the welding torch. See “Trailing Shields” below.

'Purging Methods’

The most common way to remove unwanted air or other gases and vapours from the weld zone is to flush it away with an inert gas. The weld zone can be contained to prevent fresh air or other gases and vapours from entering once the contained volume has been purged.

Other methods of purging for welding are:

1.0 enclose the metal parts completely in a vacuum chamber and evacuate it, prior to backfilling with inert gas, up to atmospheric pressure for the welding process.

2.0 enclose the metal parts to be welded in a leak tight metal, perspex or flexible plastic enclosure and flush out the enclosed gases and vapours with an inert gas.

'Welding Methods’

Most weld purging is done for joints that are being made by the TIG or GTAW welding process. In the case of vacuum purging, the electron beam or laser beam processes may be used.

In the case of TIG/GTAW welding, the front side of a weld is normally protected by inert gas flowing through the welding torch and it is the back side of the weld that requires the extra attention.

'Purge Gases’

Unwanted gas is generally removed by an inert gas, the most common being Argon. Helium is used occasionally for process reasons however it is a more limited resource than argon and is therefore more expensive.

Argon is commonly available from Gas suppliers and may be obtained in a range of purity levels for use as the welding activities require.

Titanium and Zirconium and their alloys, are very sensitive to atmospheric gases, particularly oxygen and hydrogen, so those responsible for joining this material might use the highest level of inert gas purity.

'Measurement of purge quality’

There are charts available that show the discolouration of some metals, caused by the presence of oxygen measured at different levels in the parts per million range.

It is possible to monitor oxygen levels by an instrument known as a “Weld PurgeMonitor®”.

There are different Weld Purge Monitors® available to measure oxygen levels accurately between ambient levels and and 10 parts per million (ppm) which is low enough to prevent contamination to the most sensitive materials.

Some of these instruments are hand held and powered by battery for ease of use around construction sites where the materials mentioned are being welded and others are powered by mains electricity due to the power requirements needed to run switching operations, collect data and operate internal pumps to draw the purge gas over the sensor for measurement.

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”"Weld Purging Dams”"

For welding purging of tubes and pipes, a variety of weld purging dams are available, see:

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Dr Fletcher concludes that the only totally reliable tube and pipe weld purging systems are the inflatable models, where the dams are inflated by the weld purge gas, prior to purging out the unwanted gases and vapours.

The inflatable systems have a large surface area in contact with the tube and pipe walls so that they do not allow atmospheric gases to leak back into the purged void.

“"Water Soluble Weld Purge Dams”"

1.0 Water Soluble Paper.

A water soluble paper is available which was invented by “Hans Boe” in 1952 or thereabouts.

The patent describes how the paper making process is changed to include water soluble binders that release the normal paper constituents into a pulpy mass, leaving any written matter on the paper, completely indecipherable.

“”Trailing Shields””

Trailing Shields are devices that can be attached to welding torches to follow the weld torch in the welding direction.

As the welding torch has joined the metal under the protection of the “Shield gas” that surrounds the welding arc, it moves further forward, thus exposing the cooling metal to ambient air.

In the case of most metals that are welded by the TIG/GTAW and Plasma/PAW processes, oxidation can still occur as the torch shield gas departs from the hot metal, because the temperature of the metal may not have dropped below it’s oxidation level.

By fitting a trailing shield to the welding torch, additional shielding gas will cover the metal for some minutes extra which is enough for the outside of the weld to remain bright and shiny and eliminate the cleaning time and the risk of defects in the metal.

Trailing Shields are manufactured in flat format for sheet metal and rectilinear structures. For manual welding they are small and light in weight so that the welder can easily move the shields with the welding torch without suffering extra fatigue.

When welding cylindrical components, the Trailing Shields are manufactured to the radius of the component, whether it will be small pipe or large vessel.

Where the processes are automated, the trailing shields are longer and wider, to allow for the higher speeds of automated welding equipment.

The additional weld purging gas, usually argon, is fed to the hoses attached to the trailing shields. From there the gas is dispersed by a series of fine meshes so that it drops or flows because of the slight pressure differential, over the hot weld metal keeping it protected.

The trailing shields are usually held a short distance above the weld, by way of their attachment to the welding torch. The shields have flexible silicon rubber skirts on either side, that drag along the surface of the metal being welded, behind the welding torch, thus keeping the hot weld metal in a containment volume that prevents air from entering.

Ron Sewell - CEO, Huntingdon Fusion Techniques HFT® Stukeley Meadow BURRY PORT Carms SA16 OBU UK

'Trailing Shields’

‘Weld Backing Tape’

‘Argweld®’

Argweld® is a trademark owned and used by Huntingdon Fusion Techniques Ltd for their weld purging products

‘PurgEye®’

Purgeye is a registered trademark owned and used by Huntingdon Fusion Techniques limited for its family Weld Purge Monitors®

‘Purge Plugs'