Zirconium and its primary alloy, Zircaloy, are distinguished by their remarkable physical properties, including mechanical strength, corrosion resistance, and high-temperature stability. These characteristics make them ideal for various industries, such as biochemical, nuclear, aerospace, and petrochemicals. This case study explores the applications of Zirconium alloys, the preparation and techniques involved in welding them, and the stringent quality control measures required to ensure successful welds.
Applications of Zirconium Alloys
Zirconium and its principal alloy, Zircaloy, stand out among metallic materials for their exceptional physical properties. Their mechanical strength, corrosion resistance, and high-temperature stability make them ideal for various industries, including biochemical, nuclear, aerospace, and petrochemicals.
Zirconium alloys, particularly Zircaloy, are widely used in manufacturing pressure vessels and heat exchangers due to their excellent resistance to organic and inorganic acids, salt solutions, strong alkalis, and certain molten salts. These properties make Zircaloy suitable for critical applications like pumps, where strength and corrosion resistance are essential. Additionally, Zirconium alloys are biocompatible, making them ideal for body implants such as knee and hip replacements, where Zr-2.5Nb alloy is commonly used.
However, the most significant application of Zirconium alloys is in nuclear power plants. Zirconium alloys are extensively used in the manufacturing of fuel rods, especially in pressurised water reactors.
Preparation for Welding
Welding Zirconium requires meticulous preparation due to its high sensitivity to contamination from active gases like oxygen, nitrogen, and hydrogen. These contaminants can significantly affect the alloy's mechanical, chemical, and thermal properties. Therefore, the joint and filler wire must be thoroughly cleaned and free of foreign materials throughout welding. The metal surfaces must be protected with inert gas shielding until the weld metal cools from its 1,835°C melting point to below 315°C.
Electron beam welding (EBW) and gas tungsten arc welding (GTAW) are commonly used to welding Zirconium. While EBW is performed under a vacuum and doesn't require environmental protection, GTAW requires welding-grade argon (99.999% purity) for primary, secondary, backup shielding, and purging. Argon provides excellent arc stability and, being heavier than air effectively blankets the weld. Argon/helium mixtures can also be used for backup shielding and purging, particularly in blind spaces where helium’s low density is advantageous.
Ensuring Weld Quality
Although the technical aspects of welding Zirconium alloys are well understood, maintaining weld quality requires strict control of the welding environment to prevent contamination. Zirconium alloys are particularly susceptible to cracking and porosity if not properly handled. Therefore, ensuring that the welding area is sufficiently protected is essential, especially during site welding applications.
Pre-weld cleaning is critical; surfaces must be machined or vigorously brushed with stainless steel wire, followed by thorough degreasing with a suitable solvent. To minimise contamination risks, welding should commence within about eight hours of cleaning.
Controlling Purge Gas Coverage
A wide range of ancillary equipment is available to optimise the inert gas coverage of the weld zone. Expandable plugs and fully integrated inflatable devices can accommodate pipe sizes from 10 to 2,500 mm, ensuring efficient purging. Expandable plugs, available with nylon, steel, or aluminium bodies, provide an effective solution for purging pipework with various openings.
Inflatable Weld Purging Systems, such as the PurgElite® range and the QuickPurge® Family of Inflatable Tube, Pipe, and Pipeline Systems, have been developed to speed up the welding process. These systems offer significant time and inert gas savings, with high-temperature options for pre- and post-weld operations up to 300°C.
Weld Trailing Shields®
Trailing shields are available for the topside to protect the weld root during tube and pipe welding. These shields provide additional inert gas shielding to supplement the coverage from the basic welding torch. They come in various forms to match different diameters, including internal and external models.
Flexible Welding Enclosures®
For component welding, Flexible Welding Enclosures® offer an effective alternative to glove boxes and vacuum systems, providing comprehensive inert gas coverage at a fraction of the cost. These enclosures occupy less floor space and ensure that all welded components finish bright and shiny without oxidation or discolouration.
Controlling Purge Gas Quality
Using specialised weld purging equipment alone does not guarantee defect-free welds. Controlling the oxygen content in the purge gas is crucial for success. A reliable monitor that measures residual oxygen content down to 10 ppm is necessary to ensure that the purge gas quality meets the stringent requirements for welding Zirconium alloys.
