Challenges in Weld Cladding and the Importance of Inert Gas Protection
Fusion cladding is a well-established technique used to enhance metallic components' durability by depositing a material layer to provide a corrosion- or erosion-resistant surface. This process, often using arc welding methods, is critical in power generation, aerospace, and subsea operations. By cladding components with materials like copper, nickel, cobalt-based alloys, and various stainless steels, significant cost savings can be achieved—up to 60% in some cases when comparing cladded carbon steel to components made entirely from solid nickel 625.
The cladding process's success hinges on reliably and repeatably depositing the specified alloy, carefully considering potential dilution with the substrate material. However, a frequently overlooked aspect is the need to exclude oxygen during welding to prevent the oxidation of crucial elements like chromium, cobalt, and nickel. Oxidation can severely diminish the cladding layer's effectiveness in resisting corrosion and erosion and impact its mechanical properties.
The primary challenge in weld cladding is protecting the molten and solidified weld from atmospheric contamination. Oxidation during welding can lead to discoloured welds, reduced corrosion resistance, and compromised mechanical strength. In many cases, oxidising stainless steels and superalloys containing elements like chromium and nickel can lead to significant performance degradation.
An inert atmosphere around the weld area must be created to prevent oxidation, typically using gases such as argon or helium. This becomes particularly crucial when dealing with high-temperature welding processes, where the weld deposit remains hot long after the torch has passed. For low-current processes, the inert gas from the torch might suffice. Still, higher thermal outputs necessitate additional protection to shield the weld and heat-affected zones until they cool below 400°C.
Argweld® Trailing Shields®: Ensuring Comprehensive Inert Gas Coverage
One effective solution to these challenges is the use of Argweld® Trailing Shields®. Developed by Huntingdon Fusion Techniques Ltd over 50 years ago, these shields have been continually refined to take advantage of advancements in materials and manufacturing techniques. Argweld® Trailing Shields® provide additional inert gas coverage beyond what the basic welding torch supplies, ensuring that the weld area is protected from oxidation until it has cooled sufficiently.
These trailing shields are designed for GTAW (TIG) or PAW (plasma) torches. They are available in models tailored for internal and external curved applications, accommodating diameters as small as 25 mm. The welding torch is mounted on the leading end of the shield, with inert gas fed through ports behind the fusion zone. A seal is maintained between the shield and the workpiece using flexible, pre-formed silicone skirts, while a fine mesh filter minimises turbulence within the shielded area.
Protection for Complex Three-Dimensional Products
Weld cladding is often used to protect complex, three-dimensional components, such as valve parts, where access to a welding torch can be limited. In these cases, flexible enclosures offer an ideal solution. These enclosures can house the entire component with a robot-controlled welding torch, ensuring complete protection from oxidation throughout the welding process.
Flexible enclosures represent a significant advancement over traditional metal glove boxes and vacuum systems initially developed for the aerospace industry. The modern, flexible alternatives are not only more cost-effective—often less than 10% of the cost of a metal glove box and only 2% of a vacuum system—but also offer other advantages, such as ease of storage and transport, quicker manufacturing times, and the ability to accommodate complex setups with multiple access points for operators and equipment.
These enclosures are made from ultra-violet stabilised engineering polymers, providing durability and flexibility. They are used globally across various industries, including aerospace, automotive, biochemical, medical, and nuclear sectors. The enclosures can be manufactured up to 9 cubic meters, with larger models available on request.
Monitoring and Controlling Oxygen Levels
Maintaining low oxygen levels inside the welding environment is crucial for preventing oxidation. Instruments like the PurgEye® Weld Purge Monitor® series from Huntingdon Fusion Techniques Ltd are specifically designed for this purpose. For instance, the PurgEye® 500 Desk model is equipped with a fast-response, long-life sensor capable of detecting oxygen levels as low as ten ppm. This monitor ensures consistent and accurate measurements, aided by an integral pump that delivers a steady flow of purge gas to the sensor.
The PurgEye® 500 also includes advanced software for computer interfacing, data acquisition, and quality management control. This software allows for storing and printing results and graphs for thorough documentation and analysis.
Conclusion
Preventing oxidation during weld cladding is critical, as any loss of protective elements like chromium, cobalt, or nickel can significantly affect the clad layer's corrosion resistance, erosion resistance, and mechanical properties. Equipment such as Argweld® Trailing Shields® and Flexible Welding Enclosures® from Huntingdon Fusion Techniques Ltd can eliminate oxidation risks by ensuring a protective inert gas environment throughout the welding process.
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