Nickel alloys are widely used in industries where high-temperature strength and excellent corrosion resistance are paramount. Common nickel alloys, such as those in the Hastelloy series, which contain chromium and molybdenum, find applications in aerospace, power generation, petrochemical, offshore, and automotive sectors. However, welding these alloys presents unique challenges, particularly in preventing porosity and contamination, which can compromise the integrity of the welds.
Nickel alloys are generally straightforward to weld but susceptible to porosity if not correctly managed. As little as 0.025% nitrogen in the weld area can lead to pore formation in the solidifying weld metal. Also, draughts can disrupt the gas shield, leading to atmospheric contamination and increasing the risk of porosity. This is especially problematic in site welding applications with less controlled environmental conditions.
Ensuring the weld area is sufficiently protected to prevent these issues is crucial. This involves controlling the gas shield's purity and efficiency, regularly checking gas hoses for damage and leaks, and using appropriate equipment, such as large ceramic shrouds and gas lenses, in the GTAW process. Gas purging of the root is essential when depositing a GTAW root pass to prevent contamination from the underside of the weld.
Pre-Weld Cleaning
Both weld metal and heat-affected zone (HAZ) cracking are often the result of contamination, such as grease, oil, and dirt left on the surface due to inadequate cleaning. While excess sulphur in the parent or filler metals is a rare cause of problems, proper surface preparation is critical. This typically involves machining or vigorously brushing stainless steel wire and thoroughly degreasing it with an appropriate solvent. Welding should commence within eight hours of cleaning to minimise the risk of recontamination.
Inert Gas Purging
Effective inert gas purging prevents oxidation and contamination during welding, especially in multi-pass welds. If contaminants are not removed before subsequent passes, poor purge gas quality can lead to the formation of "grapes" on the weld and reduce corrosion resistance. This necessitates time-consuming post-weld cleaning, significantly extending total welding times and increasing costs.
Choosing a Weld Purge Method for Tubes and Pipes:
The first requirement for pipe purging is to establish gas entry and exit points. Argon, the most widely used inert gas for purging due to its low cost and availability, is typically fed through one end seal, with an exit point for oxygen measurement using a Weld Purge Monitor®. The gas inlet should be positioned lower than the bleed end to ensure effective expulsion of air from the pipe bore, especially when using heavier gases like argon.
Integrated Inflatable Pipe Purging Systems:
Professionally designed pipe purging systems, such as the PurgElite® range, have revolutionised the welding process by speeding up purging and reducing gas consumption. These systems use inflatable dams placed at calculated distances from the joint line to minimise the weld purge space while avoiding overheating from the weld heat input. The PurgElite® systems incorporate advanced engineering developments, including the IntaCal® automatic gas flow regulation device and the optional PurgeGate® anti-burst system, ensuring efficient and safe purging.
Single Inflatable Dams and High-Speed Purging Systems
For situations where a double-ended pipe purging system is impractical, single inflatable end dams are available to seal and purge the pipe. These dams are available for diameters ranging from 152 mm to 2235 mm and are made from low-outgassing materials to maintain purge integrity. For more demanding applications, QuickPurge® systems are designed for faster purging and higher heat resistance, making them ideal for welding high-strength steels requiring pre- and post-weld heat treatment cycles up to 300ºC (572ºF).
Water Soluble Weld Purge Film®
In applications where post-weld access is impossible, water-soluble purge materials, such as Weld Purge Film®, are available. Unlike water-soluble paper, which has a high water vapour content, the plastic film version is impervious to water vapour and dissolves entirely upon contact with water, leaving no residue.
Trailing Shields®
Argweld® Trailing Shields® are designed to provide additional inert gas coverage during GTAW welding, particularly for reactive metals like nickel alloys. These shields are available in various forms, from flat models to those suitable for circumferential welding on pipes and tubes. They ensure high protection by maintaining an inert gas environment over the weld as it cools, reducing the risk of oxidation and ensuring a high-quality weld.
Flexible Welding Enclosures®
For batch processing of components, flexible welding enclosures offer a low-cost alternative to traditional vacuum systems and glove boxes. These enclosures, made from ultra-violet-stabilised engineering polymers, provide a controlled environment for welding, ensuring that residual oxygen levels are kept low. They are lightweight, easy to set up, and available in various sizes, making them suitable for a wide range of applications.
Controlling Purge Gas Oxygen Levels
Using specialised weld purging equipment does not automatically guarantee defect-free welds. Accurate control of the oxygen content in the purge gas is critical. Standard oxygen monitors are typically calibrated for atmospheric oxygen levels and are not sensitive enough for welding applications. Weld Purge Monitors®, such as the PurgEye® series, are specifically designed for the welding industry and can measure oxygen levels as low as 10 ppm, ensuring the necessary control for high-quality nickel alloy welds.
Conclusion
Welding nickel alloys requires meticulous attention to detail in both preparation and execution. From pre-weld cleaning to effective gas purging and oxygen level monitoring, each step plays a crucial role in ensuring the quality and integrity of the final weld. By employing advanced tools like PurgElite® purging systems, Argweld® Trailing Shields®, and PurgEye® Weld Purge Monitors®, fabricators can achieve consistent, high-quality welds in even the most challenging environments.
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