This case study explores the critical role of weld purging in orbital welding, a process increasingly used across industries requiring high-quality, repeatable welds, particularly in challenging positions such as overhead or down-hand. While orbital welding offers significant advantages over manual welding by eliminating variability and ensuring consistent results, maintaining weld quality depends on effective purging techniques to remove atmospheric gases that could compromise the weld.
The study compares various purging methods, emphasising the superiority of advanced systems like inflatable seals over outdated, unreliable methods such as using makeshift materials. These advanced systems, including those offered by Huntingdon Fusion Techniques (HFT), ensure effective sealing, proper inert gas flow, and accurate oxygen level monitoring, all crucial for achieving defect-free welds.
HFT's solutions, such as the IntaCal Weld Purging Device and PurgEye® Weld Purge Monitors®, are highlighted for their ability to maintain low oxygen levels, prevent contamination, and ensure the mechanical integrity of welds. These systems are particularly beneficial in industries where weld integrity is paramount, including aerospace, pharmaceuticals, and petrochemicals.
The case study concludes that adopting advanced weld purging techniques is essential for maximising the benefits of orbital welding, ensuring high-quality, durable welds in critical industrial applications.
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The Challenges of Manual Welding
Orbital welding has become crucial in many industries due to its ability to deliver high-quality, repeatable results, particularly in tube and pipe fabrications. This technique is precious when manual welding is challenging due to difficult positions, such as overhead or down-hand welds. In such cases, maintaining control over the weld pool becomes difficult, often leading to faulty welds. Orbital welding, which can be fully automated and controlled by computer, eliminates the variability associated with manual welding, ensuring consistent, high-quality welds without operator intervention.
Manual welding requires the operator to maintain a delicate balance between gravitational force and surface tension at every position of the torch. This is particularly challenging in overhead and down-hand positions where access is restricted, and the operator's comfort is compromised. As a result, the quality of manual welds can be inconsistent, leading to potential defects such as porosity, notches, and cracking. These defects not only compromise the integrity of the weld but can lead to catastrophic failures, particularly in safety-critical applications.
The Advantages of Orbital Welding
Orbital welding, mainly using the Gas Tungsten Arc Welding (GTAW) process with additional cold-wire feed, offers a solution to these challenges. The process can weld virtually all metal alloys used in pipework fabrication and pipeline sectors. Since the welding is carried out in an inert atmosphere, the results are immaculate, oxide-free, and free from spatter. The reliability and precision of orbital welding make it ideal for industries such as food processing, pharmaceuticals, chemical engineering, biotechnology, semiconductors, and shipbuilding. These industries require welds that are free from defects to prevent contamination and ensure the safety and longevity of their products.
The Importance of Weld Purging
Despite the precision of orbital welding, atmospheric gases like oxygen and nitrogen can still compromise the weld quality. These gases can lead to oxidation, affecting the weld's appearance, corrosion resistance, and mechanical properties. Oxidation can be catastrophic in industries where weld integrity is crucial, such as aerospace, nuclear, and petrochemical. Proper weld purging techniques involve removing these gases from the weld area and are essential to achieving the desired weld quality. Argon is the most used purge gas due to its inert nature, although nitrogen and hydrogen can also be used in specific applications. However, caution must be exercised when using hydrogen and nitrogen, as they are unsuitable for materials like titanium and fine-grain structural steels, which are sensitive to gas uptake.
Measuring Purge Gas Oxygen Content
Even small amounts of oxygen in the purge gas can cause discolouration and compromise the weld. To prevent this, it is essential to use sensitive instruments to measure residual oxygen levels. Relying on makeshift solutions, such as using a flame at the exhaust end of the purged volume, can lead to severe errors. These methods are unsafe and only provide information about the exit gas, not the oxygen level at the weld root. A suitable oxygen monitoring instrument must have an adequate measuring range and be able to detect oxygen levels as low as ten ppm. Instruments that only display down to 1000 ppm (0.1%) are inadequate for ensuring the low oxygen levels required for high-quality welds.
Tube and Pipe Weld Purging Systems
Various purging techniques have been developed to protect the weld from oxidation. Some still employ simple, low-cost solutions, such as screwed-up newspaper or cardboard discs, but these methods are far from reliable. They often fail to seal effectively, leading to potential weld contamination. Continuous inert gas flow without seals is also used for small-diameter tubes, but this approach overlooks the risk of turbulence, which can trap oxygen and compromise the weld quality.
Expandable Pipe Plugs
Expandable pipe plugs are a more effective solution, particularly in orbital welding, where pipe diameters are typically less than 150 mm. These plugs provide a large sealing area, making them easy to prepare and use. Huntingdon Fusion Techniques (HFT) manufactures orbital welding kits that include a variety of plug sizes stored in a convenient carry case for use on job sites. These plugs cover the full range of imperial and metric tube and pipe sizes up to 150 mm in diameter.
Advanced Purging Equipment
Advanced purging equipment based on inflatable seals is recommended for larger or more complex jobs. These systems address the challenges of controlling inert gas pressure and flow, providing easy and rapid deployment and removal, and ensuring thermal resistance and leak-tight access for oxygen monitoring. They also offer a large surface contact area, ensuring excellent and reliable sealing.
HFT's inflatable systems are particularly advantageous because they limit the use of metallic materials, preventing scratching of polished surfaces and thus maintaining corrosion resistance. These systems are also compatible with post-weld radiographic inspection, which can be carried out while the purging system remains in place. HFT's unique "IntaCal" weld purging device ensures that purge gas is delivered at the correct flow rate without causing scratching.
Conclusion
Orbital welding is a reliable and precise process that offers significant advantages over manual welding, particularly in challenging positions. However, to achieve the highest quality welds, it is essential to use proper weld purging techniques to remove atmospheric gases that can compromise the weld. Advanced purging systems, such as those offered by HFT, provide the necessary control, reliability, and ease of use to ensure the weld is free from defects. By incorporating sensitive oxygen monitoring instruments, fabricators can ensure optimal purge gas, resulting in clean, strong, and durable welds.
