Challenges in Small Diameter Tube Welding
Welding small-diameter metal tubes, particularly those with diameters less than 15 mm, poses unique challenges not typically encountered in larger welding applications. Maintaining consistent weld quality is difficult, even with mechanized processes. Additionally, achieving effective internal weld purging with inert gas is essential to prevent oxidation at the weld root, which can severely compromise the tube's corrosion resistance. This is especially critical in aerospace, food and beverage industries, where even minor contamination can lead to serious issues.
As tube diameters increase, manual welding becomes more feasible. However, proper purging to avoid oxidation remains crucial, mainly when critical elements like chromium are present in the metal. Inadequate purging can result in a significant loss of corrosion resistance, which is unacceptable in many high-stakes applications.
Effective Purging Solutions
To address the purging challenges, the tube must be sealed on both sides of the joint, with a small entry hole for the inert gas at one end and an exit hole at the other for the displaced gas. The interspace is then filled with inert gas. Various methods have been tried over the years, such as using paper, plastic foam, and cardboard as sealants. However, these materials often leak or even catch fire during welding. The most effective solution is to use thermally resistant expandable plugs, which allow inert gas to pass through and effectively purge the weld area. To ensure proper purging, it is advisable to use oxygen monitoring instruments to detect and control any residual oxygen content near the weld, maintaining levels well below 100 ppm.
Welding Techniques for Small-Diameter Tubes
Given the need for precise control over the weld pool, Gas Tungsten Arc Welding (GTAW) is almost exclusively used for small-diameter tubes. This process is well-suited for welding a wide range of metal alloys and, when performed in an inert atmosphere, produces clean, oxide-free results without spatter.
High-quality applications, such as those in the aerospace, food, electronics, and petrochemical industries, orbital welding equipment is often integrated to create fusion welds. This automated process is the most reliable method for ensuring consistency and eliminating imperfections, which is critical in applications with stringent quality demands. However, orbital welding equipment can be expensive, and manual welding may be necessary for small-batch production. In such cases, skilled operators are essential to maintain weld quality.
Argon is the most used purge gas, but helium, nitrogen, and hydrogen can also be used as inert gases for root protection. However, caution must be exercised when using hydrogen- and nitrogen-containing gases, as they are unsuitable for materials like titanium sensitive to gas uptake, leading to embrittlement or porosity. These gases are also not recommended for most fine-grain structural steels.
Advanced Purging Systems for Small Diameter Tubes
For tube diameters as small as 15 mm, expandable purging systems like the Argweld® Weld Purge Plugs™ are effective. These plugs are manufactured by Huntingdon Fusion Techniques (HFT) and can accommodate pipe diameters from 15 mm, with flexibility for slightly larger or smaller sizes. HFT is also developing purging systems for diameters as small as 5 mm.
For tube diameters between 25 mm and 40 mm, fully integrated inflatable systems such as the Argweld® PurgElite® range are available. These systems feature a pair of inflatable dams connected by an armoured, heat-resistant spinal tube. The system is inserted into the bore with one dam on either side of the weld joint. The spinal tube carries the gas to inflate the purge dams and gas flow is automatically controlled to prevent turbulence. Only one gas connection is needed for both dam inflation and purging. Heat-resistant covers are available as accessories to protect the inflatable dams, with options for use up to 300ºC.
Advanced purging systems may include features such as:
Monitoring Purge Gas Oxygen Content
Even trace amounts of oxygen in the purge gas can cause discolouration around the weld, making it essential to use sensitive instruments to measure residual oxygen. DIY methods, such as using a flame at the exhaust end of the purged volume, are unreliable and unsafe. They only provide information about the exit gas and do not measure oxygen levels at the weld root.
An effective oxygen monitoring instrument must have an adequate measuring range and the capability to sample gas inside the purge volume. The sensitivity should allow for the detection of oxygen levels as low as 10 ppm. Instruments that only display down to 1000 ppm (0.1%) are unsuitable for high-quality welding.
Advanced instruments, such as the PurgEye® range from Huntingdon Fusion Techniques, offer continuous monitoring of oxygen levels. These devices can interrupt the welding process if oxygen levels exceed a preset limit, ensuring the weld remains uncontaminated. They are also valuable for quality control, with the ability to log and store data using optional software interfaces.
Conclusion
Small-diameter tube welding presents unique challenges, particularly in achieving consistent weld quality and effective purging. Advanced welding techniques and filler materials have been developed to meet these challenges. Modern inert gas purging systems and continuous monitoring of oxygen levels offer excellent protection against oxidation, even in tubes as small as 25 mm in diameter. Ongoing research aims to develop even smaller purging devices for diameters down to 5 mm. For quality control, it is recommended that the entire welding process, especially the purge gas oxygen level, be recorded and monitored.
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