Inconel 740H, a newly developed nickel-based superalloy, is gaining prominence in fossil fuel-fired boiler manufacturing due to its exceptional high-temperature corrosion resistance and mechanical properties. However, welding this advanced material presents significant challenges, particularly in preventing oxidation during welding, which can compromise its integrity and performance.
The study highlights the effectiveness of integrated inflatable purging systems, such as the QuickPurge® range. These systems create an oxygen-free environment inside pipes, reducing oxygen levels to as low as 50 ppm. This level of control is crucial for ensuring that the weld root remains free of oxidation, thereby preserving the alloy’s corrosion resistance.
Advanced oxygen monitoring tools, like the PurgEye® 600 Weld Purge Monitor®, are also critical in maintaining low oxygen levels during welding. These monitors offer precise measurement and continuous recording, ensuring the welding environment remains within stringent limits, ultimately leading to high-quality, corrosion-resistant welds.
By implementing these advanced purging techniques and monitoring systems, fabricators can overcome the challenges of welding Inconel 740H and ensure the production of durable, high-performance components in demanding industrial applications.
Challenges in Welding Inconel 740H
Recent advancements in material science have led to the development of new nickel-based superalloys that significantly improve mechanical properties, particularly in high-temperature environments. Among these is Inconel 740H, a nickel alloy that has garnered considerable interest in fossil fuel-fired boiler manufacturing due to its enhanced resistance to coal ash and superior mechanical properties at elevated temperatures.
Inconel 740H has a higher chromium content than other alloys, contributing to its excellent high-temperature corrosion resistance. However, during fusion welding, this alloy is prone to chromium loss through oxidation, a problem that can severely impact its corrosion resistance and mechanical integrity. Strict control over the welding environment is crucial to preserve these desirable characteristics.
When welding Inconel 740H, particularly in applications involving pipework and boilers, failing to purge the weld area properly can result in a heavily oxidised weld root inside the cavity, significantly reducing corrosion resistance. Even after completing the root weld, continuing the weld purge for several passes is essential to avoid heat tint or oxidation inside the tube or pipe.
The Importance of Inert Gas Purging
In gas tungsten arc welding (GTAW), the upper weld deposit can be directly protected using an inert gas shroud and a trailing shield. However, the weld root remains exposed to the atmosphere, making it susceptible to oxidation. To prevent this, an interior inert gas purge, typically using argon or helium, is necessary when making butt joints.
Continuous gas flow can sometimes provide adequate protection for small tubes, but this method is prone to turbulence and may result in inconsistent coverage. A more reliable approach involves isolating the section to be welded by inserting inflatable dams on either side of the joint and filling the volume with inert gas. This technique ensures consistent protection and minimises the risk of oxidation.
Inflatable Purging Systems: The Global Standard
While some fabricators have attempted to use makeshift dams made of paper, cardboard, timber, or polystyrene for economic reasons, these methods often result in poor sealing, contamination, and difficulties in removing debris after welding. The superior solution, now widely adopted by leading fabricators worldwide, uses commercially available integrated inflatable purging systems, such as the QuickPurge® range.
These systems are designed to create an oxygen-free environment inside the pipe, ensuring that the internal purge atmosphere has an oxygen level reduced to 50 ppm (0.005%) or less. This level of control is crucial for producing a weld root surface with minimal or no oxidation, preserving the alloy's corrosion resistance.
Monitoring Oxygen Levels During Welding
Controlling the oxygen content of the purge gas is a critical aspect of welding nickel alloys. Recent advancements in monitoring instruments have led to the development of equipment that measures oxygen levels with high precision and provides continuous recording capabilities. Some systems can even override the welding process if oxygen levels exceed the operator's preset limits.
While techniques for measuring oxygen content have existed for decades, instruments have recently been developed specifically for welding applications. These instruments are designed to meet the increasingly stringent demands for welds that are free from discolouration and have no loss of corrosion resistance, requiring purge gas oxygen content to be as low as 20 ppm (0.002%). Few oxygen purge monitors can achieve this level of sensitivity, but the PurgEye® series from Huntingdon Fusion Techniques, including the PurgEye® 600, meets these rigorous standards.
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