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Technical Papers
Huntingdon Fusion Techniques
Welding of Zirconium and its Alloys
Zirconium alloy welded with effec- tive inert gas protection showing no discolouration
Zirconium and its principal alloy zircaloy possess physical prop- erties unmatched by most other metallic materials. The combina- tion of mechanical strength, cor- rosion resistance and their high temperature stability make them attractive for use in sectors as diverse as biochemical, nuclear, aerospace and petrochemicals.
More speci cally, 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.
By far the most signi cant appli- cations however are in nuclear power plant. Zirconium alloys are widely used in the manufacture of fuel rods especially in pressurised water reactors
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 signi cant effect on mechanical, chemical and thermal properties 2. The joint and ller wire must be care- fully and completely cleaned and remain free of all foreign material throughout the welding process. The metal surfaces must be pro- tected using inert gas shielding until the weld metal cools from its 1,835°C melting point to below 315°C.
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, second- ary, and backup shielding during GTAW, as well as for purging. Argon provides excellent arc sta- bility 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 shield- ing and purging, in which heli- um’s low density can effectively purge blind spaces. Gas dew point should be not more than -51°C.
In a high proportion of these application areas fusion welding is an essential requirement but care is necessary to ensure that repro- ducible 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 particu- larly susceptible to cracking and porosity if the welding environ- ment is not properly controlled.
Machining or vigorous stainless steel wire brushing followed by thorough degreasing with a suit- able 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 suf ciently protected and this is particularly relevant in site welding applications. With the gas shielded processes, gas purity and the ef ciency of the gas shield needs careful monitoring. Gas hoses should be checked for damage and leaks at regular inter- vals 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 introduc- tion 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 equip- ment is available speci cally to ensure optimised coverage of the weld zone with inert purge gas.
ITAtube Journal No2/July 2018
32