Zirconium and its principal alloy zircaloy possess physical properties unmatched by most other metallic materials. The combination of mechanical strength, corrosion resistance and their high temperature stability make them attractive for use in sectors as diverse as biochemical, nuclear, aerospace and petrochemicals.
More specifically, zircalloy is used in the manufacture of pressure vessels and heat exchangers. The alloy has excellent resistance to most organic and inorganic acids, salt solutions, strong alkalis, and some molten salts and these properties makes it suitable for use in pumps where strength coupled with corrosion resistance is mandatory. Zirconium alloys are biocompatible, and therefore can be used for body implants: a Zr-2.5Nb alloy is used in knee and hip implants.
Preparation for welding
Zirconium is highly sensitive to contamination by active gases such as oxygen, nitrogen and hydrogen and absorption of these materials can have a significant effect on mechanical, chemical and thermal properties 2. The joint and filler wire must be carefully and completely cleaned and remain free of all foreign material throughout the welding process. The metal surfaces must be protected using inert gas shielding until the weld metal cools from its 1,835⁰C melting point to below 315⁰C.
Electron Beam (EBW) and Gas Tungsten Arc (GTAW) processes are both used for zirconium welding. EBW is undertaken under vacuum so the requirement for environmental protection is not necessary. Welding-grade argon ie 10 parts per million (ppm) other gases (99.999 percent argon) is essential for primary, secondary, and backup shielding during GTAW, as well as for purging. Argon provides excellent arc stability and because it is heavier than air, it blankets the weld and provides protection. Argon and argon/helium mixtures can also be employed for backup shielding and purging, in which helium's low density can effectively purge blind spaces. Gas dew point should be not more than -51⁰C.
In a high proportion of these application areas fusion welding is an essential requirement but care is necessary to ensure that reproducible weld quality is achieved.
All the conventional welding processes can be used and the basic technical aspects have been understood for many years. It is however essential to ensure that contamination does not occur—zirconium alloys can be particularly susceptible to cracking and porosity if the welding environment is not properly controlled.
Machining or vigorous stainless steel wire brushing followed by thorough degreasing with a suitable solvent is necessary prior to welding, with the welding taking place within about eight hours to reduce the risk of contamination.
The presence of nitrogen in the shielding gas can give rise to porosity so care must be taken to ensure that the weld area is sufficiently protected and this is particularly relevant in site welding applications. With the gas shielded processes, gas purity and the efficiency of the gas shield needs careful monitoring. Gas hoses should be checked for damage and leaks at regular intervals and, with the GTAW process, as large a ceramic shroud as is available should be used together with a gas lens.
It goes without saying that gas purging of the root is essential when depositing a GTAW root pass. Failure to control purging can result not only in the introduction of weld metal inclusions, but also reduce corrosion resistance if left on exposed surfaces. Post weld cleaning to remove these undesirable contaminants can be time-consuming and expensive.
Controlling Purge Gas Coverage
A wide range of ancillary equipment is available specifically to ensure optimised coverage of the weld zone with inert purge gas. From simple expandable plugs to fully integrated inflatable devices the products can accommodate pipe sizes from 10 to 2,500 mm.
Expandable Plugs are a popular choice and these are available with nylon, steel and aluminium bodies. Surrounding each body is a flexible seal that can be expanded by applying a radial force through a manually operated wing nut on the shaft.
These mechanical plugs can be used for purging pipework fabrications where a variety of openings are present and where it is easier to purge the complete assembly.
Inflatable Weld Purging Systems have been developed to help speed up the welding process for engineers involved in the fabrication of pipes and tubes.
The revolutionary PurgElite® range, manufactured by Huntingdon Fusion Techniques, is a robust, easy to use, welding ancillary that offers considerable savings in time and inert gas.
Two inflatable dams are connected by a synthetic flexible hose that will not scratch the inside of polished tube or pipe work. The hose is made of a self-sealing intumescent material that resists even hot metal being dropped on it and will not disturb the purge gas flow.
The QuickPurge® Family of Inflatable Tube, Pipe and Pipeline Systems have been designed for pipe welding between 6 and 88” (150 and 2,235 mm). Both PurgElite® and QuickPurge® products are multi-use systems and include high temperature options for pre- and post- weld operations up to 300oC.
Weld Trailing Shields®
Expandable plugs and inflatable purge systems meet the requirements for protection of the weld root during tube and pipe welding: trailing shields are available for the topside.
They can provide a high level of additional inert gas shielding to supplement that supplied by the basic welding torch. Shields are available to match a wide variety of forms from flat to diameters as small as 25 mm. Both internal and external models are available
Flexible Welding Enclosures®
For component welding, Flexible Enclosures overcome all the disadvantages of glove boxes and vacuum systems but at a fraction of the cost. They occupy considerably less floor space and all parts of the welded component finish bright and shiny with no oxidation or discolouration. Argon gas costs are reduced significantly and cleaning costs are eliminated.Figure 2. The most efficient method of ensuring high quality inert gas coverage is to use flexible enclosures. They are available in sizes approaching 30 cubic metres with residual oxygen content as low as 10 parts per million.
Controlling Purge Gas Quality
Using specialised weld purging equipment does not guarantee defect free welds. Control of the oxygen content of the purge gas is crucial to success and a monitor that measures residual oxygen content reliably and accurately at the low levels considered is necessary when welding zirconium alloys.
These instruments are capable of measuring oxygen content accurately as low as 10 ppm - more than adequate to satisfy the residual oxygen recommended for zirconium alloy welding.
Developments in End Closure Welding Technology for Zircaloy Clad Fuel
Elements. Amota I and Carena G, Energia Nucleari 17 (1970)
Welding zirconium and zirconium alloys