In industrial settings where pipe welds must endure significant stress, achieving high-quality weld roots is critical. Proper weld reinforcement, free from atmospheric contamination, ensures welded joints' mechanical strength and longevity. This case study explores the importance of gas purging in optimising root welds, highlighting techniques and tools that enhance weld quality in demanding environments.
Effective gas purging is crucial for removing oxygen and nitrogen from the weld zone, which can otherwise lead to defects such as discolouration, oxidation, nitrogen contamination, and cracking. Traditional methods like total purging with simple barriers are often inadequate for larger or more complex pipe systems. Advanced solutions, such as inflatable pipe purging systems, offer superior control over the welding environment, reducing the risk of weld defects and ensuring consistent, high-quality results.
The Argweld® product range, including HotPurge™, Aluminium Tape, PurgElite® Systems, and QuickPurge® II Systems, provides reliable and reusable options for creating an inert atmosphere within the pipe, allowing for smooth, defect-free weld reinforcement. These tools are precious in applications where weld integrity is non-negotiable, such as in power generation, petrochemical, and high-performance manufacturing industries.
By employing proper purging techniques and utilising advanced purging systems, welders can significantly improve the quality of root welds, ensuring solid and durable joints that meet the rigorous standards required in modern industrial applications.
Importance of Good Welding Practice
In situations where welds must endure significant stress during service, it is crucial to carefully consider both the metallurgy and the profiles of the welds. The shape and composition of the weld can significantly influence the mechanical properties of welds, particularly their fatigue resistance. The quality of the weld root is essential; a positive reinforcement combined with a smooth transition from weld to base metal is essential for achieving optimal mechanical strength.
When welding cylindrical sections such as tubes and pipes, it is essential to eliminate atmospheric gases and provide positive, smooth weld reinforcement. The presence of oxygen and, to a lesser extent, nitrogen around the molten weld can lead to a range of defects. Discolouration, while often just unsightly, can sometimes reflect a metallurgical imbalance, particularly in stainless steel. More critically, gross oxidation can result in reduced mechanical properties and a significant loss of corrosion resistance. Nitrogen contamination can cause brittleness, and gases trapped in the weld may lead to cracking during or after cooling.
Moreover, any reduction in the weld section at the root, as evidenced by a concave geometry, will reduce joint strength. Additionally, the presence of notches or cracks at the weld/base metal interface can propagate during service and ultimately cause failure.
Basic Principles of Gas Purging
Ensuring high weld root quality in tubular joints requires removing air from the fusion zone by providing an inert gas, a process known as gas purging. The general principles of gas purging involve feeding inert gas into the welding zone to displace atmospheric gases, as illustrated in Figure 1.
Selection of Purging Gases
Commercial-quality argon is the most used purging gas in Europe. At the same time, helium is most widely used in the U.S. Purging techniques have been developed for specialised applications using argon-hydrogen and helium-argon mixtures and nitrogen. The materials being welded, and the welding process are the primary factors determining the optimal gas or gas mixture.
Purging Procedure
The first gas purging step is establishing gas entry and exit points. The gas is fed through one end seal, with an exit hole at the other end to prevent undesirable pressure buildup. Because argon is denser than air, the gas inlet should be positioned lower than the exhaust to expel air from the pipe bore effectively.
Total Purging
The cost of continuous total purging may not be significant for small pipes and tubes with a small internal volume. Simple methods like using wooden or plastic discs taped to the tube ends may suffice in such cases. Plastic caps, commonly used to protect pipe ends and threads during transit, are often employed. It is crucial to eliminate potential leak paths and to vent any branch pipes to ensure the complete removal of air.
When total purging is impractical due to large pipe volumes or difficult access, alternative containment techniques must be employed.
Water-Soluble Films
Water-soluble films are a cost-effective and efficient solution for providing gas coverage. Discs made from water-soluble film can be fixed inside the pipes and joined using water-soluble adhesive. These discs should be placed after any pre-weld heat treatment and positioned far enough apart, typically 500 mm, to avoid thermal damage during welding. After welding, the film can be dissolved by passing water through the pipe.
Thermally Disposable Barriers
In cases where water-soluble products are not acceptable, thermally disposable barriers made from cardboard discs can be used. These discs, cut to fit the pipe's internal diameter and taped in position, if necessary, are a convenient solution. The cardboard is incinerated during post-weld heat treatment, effectively removing the barrier.
Advanced Purging Solutions: Inflatable Pipe Purging Systems
Inflatable pipe purging systems like the Argweld® system offer a reusable and cost-effective solution for more efficient and reliable gas containment. These systems are easy to use and economical, mainly when producing multiple similar joints. The systems are designed to seal the pipe's interior while providing controlled gas flow to eliminate atmospheric contamination effectively.
Pre-Purge Process
The pre-purge process involves displacing the air in the pipework system or dam volume. Several factors influence the pre-purge time, including pipe diameter, purge volume, and maximum permissible oxygen level. Contrary to common belief, increasing the purge flow rate does not necessarily reduce purge time; instead, it can increase turbulence and mix the purge gas with air, extending the purge time. A typical gas flow rate for purging is around 20 litres per minute.
The Tube and Pipe Weld Purge Process
Once the desired gas quality is achieved in the dammed volume, the gas flow can be reduced to about 5 litres per minute for the welding operation. Excessive flow can increase internal pipe pressure, resulting in concavity in the weld root geometry or, in extreme cases, the complete ejection of the molten weld pool.
For weld joints with a root opening or poor fit-up, the joint can be sealed with tape, which may provide an unwanted leak path for the purge gas. However, not all tapes are suitable for this purpose. Most tapes contain water or adhesive that can outgas and contaminate the weld pool. Argweld® Aluminium Tape is recommended for sealing the joint gap, as it is stable up to 550°C and does not outgas, ensuring a clean and stable weld environment.
Conclusion
Proper gas purging techniques are essential for producing high-quality welds in tubular joints. By eliminating atmospheric gases from the weld zone and ensuring smooth, positive weld reinforcement, welders can achieve solid and defect-free joints that meet the demanding standards of modern industry.
