Difficult to meet Requirements for Quality using Manual Welding
Orbital welding has become an essential process with many tube and pipe fabrications because modern equipment can be fully automatic and reliable and offers repeatable results.
Certain welding positions such as overhead and down-hand welds often lead to faulty welds because of restricted and uncomfortable operator access.
For full control over the weld pool, a good balance must be maintained between gravitational force and surface tension at every position of the torch.
With automated welding, a computer-controlled welding process runs completely independently, without the need for any intervention from the operator.
Although there are orbital MIG (GMAW) welding systems, most of the Orbital welding today is carried out using the tungsten inert gas (GTAW) technique with additional cold-wire feed where necessary.
Virtually all the metal alloys employed in the pipework fabrication and pipeline sectors can be welded and since the process is carried out in an inert atmosphere it produces results that are extremely clean, oxide free and without spatter.
The reliability of orbital welding led quickly to applications in the construction of pipework and equipment for diverse industries like food processing, pharmaceuticals, chemical engineering, biotechnology, semiconductors and shipbuilding. Weld defects such as notches, porosity and cracking must be avoided since these create weak points that lead to subsequent failure. In safety critical applications such failures can be catastrophic.
When orbital welding tube and pipe systems, there is often a variety of stubs, branches and other openings of all diameters that need sealing, to obtain that perfect internal Weld Purge.
The Significance of Weld Purging
Whilst the orbital welding process can thus be relied upon to produce high quality joints the need to provide protection of the weld underbead by using inert gas purging techniques is often overlooked.
The presence of oxygen, and also nitrogen and other contaminants, may not only affect appearance but in addition corrosion resistance and mechanical properties and it is often difficult to meet requirements for quality using manual welding.
In safety critical applications such effects can be catastrophic. Properly developed welding procedures thus address the issues by specifying weld purging and in most applications even defining the purging equipment.
Argon is the most commonly used purge gas but nitrogen and hydrogen also offer protection and act as inert gases during root protection.
On a cautionary note however there are limitations with regard to hydrogen and nitrogen containing backing gases. These are unsuitable for use with materials such as titanium that are sensitive to gas uptake, since this can lead to embrittlement and/or porosity formation. Nor should such mixtures be used with most fine-grain structural steels.
Measurement of Purge Gas Oxygen Content
The fact that even very small amounts of oxygen in the purge gas can cause discolouration around the weld underbead makes it desirable that sensitive instruments be employed to measure residual oxygen.
Resorting to 'do it yourself' solutions such as the use of a flame at the exhaust end of the purged volume are prone to serious errors. They may be unsafe and only provide information about exit gas—nothing at all about the oxygen level at the weld root.
Two essential characteristics of a suitable instrument are that it must have an adequate measuring range and it must sample the purge gas inside the purge volume. The sensitivity should be such that an oxygen level as low as 10 ppm can be detected. Instruments that only display down to 1000 ppm (0.1%) are totally unsuitable.
A typical high sensitivity instrument will include a sampling tube, gas extraction facility and sensing electronics that are reliable and repeatable. It should be robust enough for site application and offer an accurate, easy to use, push button type calibration function.
Tube and Pipe Weld Purging Systems
A wide variety of purging techniques have been developed to meet the requirements for protection of the weld underbead from oxidation.
Generally the simple and low-cost solutions may well serve to offer limited protection but they are far from being totally reliable.
It's hard to believe that the use of screwed-up newspaper or cardboard discs to block pipe on each side of the joint and rely on this as an effective seal is still considered by some to be adequate. Even if they don't burst into flames during the welding cycle the problem of removal after completion of the joint is rarely considered.
For small diameter tubes, say up to 10 mm, the use of continuous inert gas flow without seals is not uncommon but it overlooks the possibility of turbulence and thus entrapment of oxygen. Continuous gas flow can also be costly.
Expandable pipe plugs are very effective and particularly useful in use with orbital welding where pipe diameters are typically less than 150 mm. The sealing area is large and the time involved in preparation is relatively small.
HFT manufactures orbital welding kits that contain a number of each size of plug and
is contained in a handy briefcases size carry case to allow storage and transportation of plugs for use on the job site in the vicinity of the welder.
The expansion range of each plug overlapping the next, to cover the whole range of imperial and metric tube and pipe sizes up to 150 mm diameter.
Advanced purging equipment based on inflatable seals have been developed by Huntingdon Fusion Techniques Ltd. These are fully integrated systems that address the problems of controlled inert gas pressure and flow, the need for easy and rapid deployment and removal to limit overall welding time, thermal resistance and leak-tight access for oxygen monitoring equipment. They also provide a large surface contact area and therefore provide excellent and reliable sealing.
Coupled with these advantages comes flexibility to allow access and removal through pipe bends, abrasion resistance and the use of materials that meet nuclear compliance standards.
They limit the use of metallic materials to avoid scratching polished surfaces, thus causing loss of corrosion resistance. The Company's unique "IntaCal" weld purging device delivers purge gas at the correct flow and will not cause scratching.
Post-weld radiographic inspection can be carried out whilst the purging system remains in place.
See HFT Technical Note TN-10 'PurgElite® Inflatable Tube and Pipe Weld Purging Systems'.
Acknowledgement: Thanks to Magnatech Inc for allowing the publication of their Orbital Welder Photograph.
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