Ensuring strength, flow and no entrapment in welds

In many cases of TIG welding, the physical characteristics of welds are not of great significance.

In many cases of TIG welding, the physical characteristics of welds are not of great significance.

However, when joints are to withstand stress and be free from the risk of product entrapment, special consideration needs to be given to metallurgical aspects and weld profile, according to Huntingdon Fusion Techniques.

The mechanical properties of a weld can be considerably influenced by their shape and composition; a positive reinforcement at the weld root, combined with a smooth transition from weld to parent material are pre-requisites if optimum mechanical strength, flow characteristics and an absence of product entrapment are to be realised.

In this article, Huntingdon Fusion Techniques examines available solutions when making tubular ends, looking at good practice and the different types of purging apparatus available.

Joints of high quality should only be made between cylindrical sections such as tubes, vessels and pipes if atmospheric gases are limited.

That is because the presence of oxygen and nitrogen around the molten weld can lead to wide-ranging defects, unsightly discolouration and they might produce a metallurgical imbalance.

Also, gross oxidation can lead to significant loss of corrosion resistance and nitrogen contamination can result in brittleness and cracking during or after cooling.

In many applications of crucial importance, the presence of notches or cracks at the weld and parent material interface can propagate in service and cause failure.

The removal of air from the fusion zone and the provision of inert gas can ensure weld root quality when making tubular joints.

That is achieved by gas purging and the most commonly used purging gas in Europe is commercial quality argon.

In the USA, the cheaper helium is more generally used although argon-hydrogen and helium-argon mixtures have been developed for specialised applications.

Selection of the optimum gas will depend partly on the welding process used and the material being joined.

The first requirement for purging is to provide gas entry and exit points - gas is fed through one end and then released through an exit hole to prevent an undesirable build-up of pressure.

Variation in purge gas quality may arise during welding and it is desirable to continuously monitor the gas with oxygen analysers or dewpoint meters in order to control oxygen and moisture content.

On small pipes, total purging is often performed.

Commonly, wooden or plastic discs are taped to the tube ends, or foam bungs inserted in the pipe.

Such materials may outgas during the weld cycle and cause air or moisture to enter the weld spaces, causing porosity, coking and oxidation so expanding nylon purge plugs should be used.

When total purging is impractical or costly, damming techniques that limit the purge volume are available.

Water-soluble film is a low cost and effective solution providing gas coverage.

Discs from water-soluble film are glued inside the pipes using water-soluble adhesive.

These should only be placed after any pre-weld treatment and be equidistant from the joint (at least 500mm apart to avoid thermal damage).

Purge gas is then introduced into the area by means of a hypodermic tube through the weld joint line.

On completion, the film is removed by passing water into the pipe.

Thermally disposable barriers usually consist of cardboard discs, which are cut to fit the internal diameter of the pipe and taped into position to provide a gas seal (distance between discs is typically 500mm to avoid thermal damage).

This method is particularly convenient if followed by a post-weld heat treatment cycle since the card is removed by incineration.

These methods are expedient solutions when access to the tube or pipe bore is impractical after welding, although some materials may outgas.

If access can be gained, there are several alternative purge gas damming techniques such as collapsible disc barriers.

They are made from any readily available rigid sheet material, plywood being a good medium if in-house manufacture is planned.

The discs are split across the diameter and hinged, and a sealing pad of synthetic foam is bonded to the periphery.

Cords are attached to collapse the dams and remove the discs.

Once again, there is a risk that such materials will outgas.

Rubber gasket discs, where a rubber disc is sandwiched between wooden or metal discs and where adjustments to diameters can be made by applying axial pressure, are not collapsible and need to be pulled out past the weld root, which may cause difficulties.

Also, discs need to be available for every tube or pipe internal diameter.

Inflatable bladder dams represent by far the most efficient and versatile purge gas containment method, specifically when several similar joints are to be made.

The bladders, which have sufficient length to ensure sound sealing, are manufactured from rubber with a protective canvas cover.

One is placed on each side of the joint and inflated using the purge gas itself, although single bladders can be used for closed-end joints.

Purge inlet and outlet pipes are incorporated into the bladder to allow the full circumference to seal against the pipe wall.

Longer or shorter spinal connecting tubes are available.

A pre-purge process is performed to displace the air present in the pipe work system at a rate of approximately 20 litres per minute.

Weld joints with bad end matching, or which require a root gap, provide leak paths for the purge gas that should be sealed with tape.

Oxygen and moisture levels in the purge gas should be checked using appropriate equipment at the outlet point.

Once the quality of the gas in the dammed volume has reached the required level, gas flow can be reduced to about 15 litres per minute for the welding operation.

On joints that are not fully sealed, high flow rate will be necessary to avoid contamination.

As the joint becomes permanently sealed, the gas flow rate will need to be reduced to avoid over pressurisation: excessive flow can cause concavity in the weld root geometry or even the complete ejection of the molten weld pool.

However, provision can be made for continuous alteration in gas flow rate.

One should constantly monitor purge gas quality to control oxygen and moisture content here as well.

For pipes that need to be pre-heated, heat resistant inflatable bladders can be used for temperatures up to 300 degrees Celsius.

Correct installation and use of this system should ensure clean and even penetration beads on tubular pipe joints with a smooth transition from weld to parent material.

Where several welds have to be made on similar diameters, there can be genuine cost savings when using inflatable bladders as the sealing medium as well as a reduction of working time due to the localisation of the volume.

Add to this ease of use and the technical advantages of reliable sealing and the inflatable purge bladder concept offers significant attractions.

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