TPT July 2018
AR T I C L E
Huntingdon Fusion Techniques Ltd
White paper 301 – Welding of zirconium and its alloys By Huntingdon Fusion Techniques Limited, UK
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 1,835°C melting point to below 315°C. Electron beam (EBW) and gas tungsten arc (GTAW) process- es are both used for zirconium welding. EBW is undertaken under vacuum so the requirement for environmental protection is not necessary. Welding-grade argon – ie ten parts per million (ppm) of other gases (99.999 per cent 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. 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, zircaloy 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 significant applications however are in nuclear power plants. Zirconium alloys are widely used in the manufacture of fuel rods, especially in pressurised water reactors 1 .
Figure 1: Zirconium alloy welded with effective inert gas protection showing no discolouration
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 -51°C. 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
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