Inert Gas Purging with Orbital Welding - World Pipelines & Global Energy
It is often difficult to meet requirements for quality using manual welding. Orbital welding has become an essential process with many tube and pipe fabrications since modern equipment can be fully automatic and reliable and offers repeatable results.
Certain manual 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 systems a welding procedure can run independently with very little need for intervention from the operator.
Orbital welding is usually carried out using the tungsten inert gas (GTAW) technique with additional cold-wire feed where necessary. Virtually all the metal alloys employed in the pipeline fabrication sector 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.
Range of Pipe Sizes and Application Areas for Orbital Welding
Diameters up to 170 mm with wall thicknesses up to 3.5 mm can be routinely joined using closed chamber systems. These weld heads allow the torch to be positioned precisely and ensure that the pipe is held securely. The inert gas atmosphere in the closed chamber prevents heat from tinting the upper weld bead, even with the most sensitive of materials. For larger tube diameters it is possible to use more manageable open welding heads. A flexible hose system is used to supply the welding head with power, inert gas, cooling water and filler wire where required. The need for filler wire during the welding process depends on the type of welding task; thicker tube walls and difficult-to-control parent materials require the use of additional material, whereas thin-walled tubes can be welded autogenously.
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 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.
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, can not only affect appearance, but in addition corrosion resistance and mechanical properties. Properly developed welding procedures thus address the issues by specifying weld purging and, in most applications, even defining the purging equipment.
The use of chromium nickel stainless steels in pipeline fabrication provides an illustration of the loss of corrosion resistance due to oxidation. Published information from BOC1 indicates that oxygen levels in the purge gas of 50 ppm and above can reduce corrosion resistance. Other materials such as titanium alloys are more sensitive and reducing purge gas oxygen content to below 20 ppm is desirable.
It may well be that the discolouration per se, even if the reduction in corrosion resistance is not significant, is unacceptable cosmetically or because it may be a source of contamination, such as in the food and semiconductor sectors.
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 plugs in use during welding of valve into pipework
Soluble plastic has been specially developed for use during weld purging to create a seal. The sheet is cut oversize on the pipe diameter and the excess bonded to the internal diameter using water soluble adhesive.
Expandable pipe plugs can be very effective. The sealing area is large and the time involved in preparation is relatively small. Removal after welding however presents problems if access from both sides of the joint cannot be assured since the plug diameter can only be reduced by a very limited amount and may not pass the internal weld bead without damage to the rubber seal.
Soluble plastic discs cut to pipe size and glued to the internal diameter provide some degree of protection and the discs can be removed by flushing with water after use. The bond to the pipe is prone to leakage however and the time and skill involved in preparation and bonding can be considerable.
Rubber and silicone discs connected by a flexible tube have been developed specifically for use as seals in weld purging. They can be deployed quickly and removal post-joining is easy since the assembly can be withdrawn past the weld. Reliability is suspect however because disc to pipe sealing depends upon a very small contact area. Some have a semi-rigid connecting tube and this may be unsuitable where the system needs to be used either side of a pipe bend.
The only totally reliable purging systems are those based on inflatable seals. Considerable design effort has been applied by the manufacturers to these solutions over the past decade or so and currently available systems 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 have also been resolved. They provide a large pipe contact area and therefore 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. Some limit the use of metallic materials to control valves and these can be placed away from the welding zone—this allows for post weld radiographic inspection whilst the purging system remains in place.
The alternative to weld purging is post weld removal of oxidation products but this can be very expensive, tedious and time consuming. Pickling through treatment with a mixture of acids is effective but can often not be employed for reasons of availability and environmental protection. Electrochemical removal of discolouration require manual operation and is slow; it is generally only practical for use with smaller products. Grit blasting and the use of mechanical techniques such as grinding contains the risk of leaving unwanted residual materials behind.
Classification of purge gases and suitability for various materials
|Purge Gas||ISO 14175||Materials|
|Argon-hydrogen mixtures||R1 or R2||Austenitic Cr-Ni steels
Nickel based alloys
|Nitrogen-hydrogen mixtures||R5||Austenitic Cr-Ni steels|
All metallic materials suitable for fusion welding, eg: austenitic Cr-Ni steels, austenitic-ferritic steels (duplex), ferritic Cr steels, gas-sensitive materials (e.g. Ti, Zr), hydrogen-sensitive materials (highstrength fine-grain structural steels, copper and copper alloys, aluminium and aluminium alloys, other nonferrous metals)
|Nitrogen||N1||Austenitic Cr-Ni steels, austenitic-ferritic steels (duplex)|
Argon is the most commonly used purge gas, but nitrogen and hydrogen also offer protection. Nitrogen is said to offer improved pitting corrosion resistance, whilst hydrogen has two essential properties: firstly, it is able to bond any residual oxygen still present, and, assuming sufficiently high temperatures, it can return oxides already present to their constituent elements. Secondly, hydrogen affects the surface tension of the molten material at the weld root, which in turn provides a very good transition to the base material.
The reducing effect of hydrogen has been demonstrated by BOC1. A series of welds was made with the same residual oxygen level on the root side, but the hydrogen level varied from 0 to 20%. The work illustrates that, in spite of identical residual oxygen levels, the welds where the backing gas also contained hydrogen exhibit less temper colour formation.
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. Current models should be robust enough for site application and offer a calibration function.
Orbital welding offers opportunities to produce high quality joints that are capable of meeting the exacting standards of the most sensitive sectors of the pipe welding industry. The pharmaceutical, food, beverage, nuclear and semiconductor sectors are particularly demanding of weld quality where discolouration is unacceptable and where reduction of corrosion resistance and mechanical properties cannot be tolerated. Coupled with orbital welding, however, is the need to use weld purging to protect the underbead.
Advances in technology have allowed the development of sophisticated, robust and reliable orbital welding and inert gas purging equipment and these have been proven to be effective across the globe under the often extreme conditions of on-site fabrication.
- Purging While Welding Thomas Ammann. BOC Document 2010
- Welding consumables. Gases and gas mixtures for fusion welding and allied processes
- BS EN ISO 14175:2008
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