Artem Komarov noted that zirconium is widely used in many industries for the manufacture of process equipment. Main applications include pressure vessels, heat exchangers, pipelines, tanks, mines, mixers, and other mechanical equipment; valves; pumps; sprinklers; trays; and cooling nozzle.
The material is often chosen for its resistance to corrosion from most organic and inorganic acids, saline solutions, strong alkalis, and some molten salts. Particularly suitable for many applications in sulfuric, nitric, hydrochloric, and acetic acids.
Zirconium is easily welded using practical inert gas fusion welding methods such as gas tungsten arc welding (GTAW) and plasma arc welding (PAW), as well as specialized processes such as electron beam and laser welding.
The material requires special attention to cleanliness and inert gas shielding to protect the weld area from foreign materials and atmospheric contaminants. Cleanliness and shielding are the two most important factors affecting the final quality of the weld.
Fundamentals of metallurgy affecting welding
The strength of unalloyed zirconium is determined primarily by the content of oxygen, nitrogen, carbon, hydrogen, and iron. The more these elements are present, the higher the strength and the lower the ductility.
The weld joint and welding wire must be thoroughly and completely cleaned and free of foreign material during the welding process. The weld face, weld base, and adjacent hot metal must also be shielded with shielding inert gas (using welding argon) during welding and until the weld metal has cooled from its melting point of 3334°F (1835°C) to temperatures less than 600°F (315°C).
Cutting and preparing the weld
Zirconium can be cut by machining, cold sawing, chopping, friction sawing, shear cutting, abrasive cutting, plasma cutting, oxyfuel cutting, laser cutting, and waterjet cutting. When machining, sawing, cutting and waterjet processing, the surface remains free from thermal contamination.
Although oxy-fuel or plasma cutting can be used for rough cutting, both methods require the removal of at least 1.5 mm of material from the bottom of the cut face (by machining or grinding) to ensure that all metal contaminated in the cut is removed. cutting process
Abrasive grinding of zirconium usually involves the use of special aluminum oxide or silicon carbide wheels. In such applications, be sure to protect adjacent zirconia surfaces and the environment from grinding sparks, which are hot enough to puncture gas hoses and can certainly cause a fire if contacted with flammable debris.
When abrasive grinding on sanded surfaces, rotary or draft filing should be used to remove possible abrasive particles from the joint surface. File or finish abrasive cut surfaces in the same manner to remove abrasive particles and areas of light soiling caused by localized heat. Another suitable process for zirconium is abrasive water jet cutting. Freeform cut surfaces allow the cost-effective production of parts with configurations that would be too expensive to machine. Lower cutting speeds provide a smooth finish and largely eliminate post-processing.
Abrasive can be trapped in excessive surface roughness, and abrasive particles can even adhere to smooth cut surfaces, so be sure to post-process, at least rotary or draw, any surfaces to be welded.
The most common process used to weld zirconium is GTAW. Manual GTAW allows you to weld in any position in any configuration that the torch has access to. Automatic GTAW is usually limited to flat or horizontal positions. For direct connections, welding machines with copper support rods and continuous pressure using copper rods are usually used. Butt welding of tube-to-tube sheet and small tubes using GTAW automatic orbital equipment is also common.
PAW is often used for single pass welds up to about 9 mm thick using automated equipment, copper support rods and square butt weld blanks. Typically, a GTAW through-pass is used to eliminate insufficient backfill, and a cosmetic reflow pass or mechanical removal of excess seepage may be required on the root side.
For repetitive welding of heavy profiles, a high current process such as keyhole GTAW should be considered. Electron beam and laser welding are also suitable.
Power supply, torch, and tungsten electrodes
Zirconium GTAW or PAW is typically done with a reduced dc power supply connected to a straight polarity torch that is equipped with high frequency arc start, manual current control, and a contactor to start and break the arc.
Tungsten is usually ground at an angle of approximately 20-30 degrees with a slightly blunt end. Larger tungsten allows for greater elongation, minimizing overheating and the risk of tungsten inclusions.
The large gas cap and gas lens are the most important elements for protecting the weld. They minimize turbulence and provide uniform gas shielding even when the tungsten electrode is extended far beyond the hood for visibility or access. Smaller burners, still fitted with oversized ceramic cups and a gas lens, are sometimes used for fine work or limited access applications.
Burners with transparent gas domes are also available. For unusual geometry, special protective screens are used. External fillet welds may require a baffle to restrict gas flow from the burner to maintain effective primary and final protection.
Automatic GTAW or PAW equipment uses similar large gas cartridges and gas lenses.
The filler metal is pure when it comes from the supplier, provided that the filler metal container remains sealed. Obviously, the storage of filler metal requires some attention. Keep the storage area clean and dry and store the zirconium filler packages in a tightly closed cabinet. Packages must remain sealed upon receipt and reseal and tape after use.
In case of wound wire, protect the spool with a closed cover. In the absence of commercial equipment, a plastic bag glued with tape is suitable. Wire feed tubes should be clean and, if possible, designed for zirconium and possibly other reactive metals.
Maintaining cleanliness when welding zirconium is essential to obtain high quality results. This is an ongoing process that includes:
— Material handling
— Cleaning of warehouses and work areas
— Technological procedures during preparation
— Cleaning of welded parts in general
— Thoroughly clean the local area where the weld is to be made
Zirconium requires protection of the molten weld from inert gases (using only argon, helium, or mixtures thereof) to prevent contamination of the weld metal with oxygen, nitrogen, and moisture from normal atmospheres. Zirconium also requires protection of the hardened weld and adjacent metal surfaces with an inert gas heated during welding to prevent surface oxidation and associated degradation.
Internal surfaces in the volumes to be purged must be free of dirt that could trap air or moisture. This requires cleaning the entire surface area to be purged almost as thoroughly as the weld preparation area itself.
Be aware of blind spaces where air can trap. Either provide a secondary vent or consider using argon/helium gas mixtures to make sure the traps are cleared. Cover the areas prepared for welding with painter’s tape until the quality of the purge has been verified.
A typical initial purge rate is 20 to 50 CFH (10 to 25 liters per minute [L/min]). After creating an inert atmosphere, they can be reduced to 5-20 CFH (2.5-10 l/min). Maintain gas flow at these low speeds to maintain a slight positive pressure and remove any volatilized contaminants from the purge space until the weld is completed, or the coating is thick enough to prevent color from forming on the surface during welding. If root control is not possible, reduce the thickness of the base metal or weld metal by at least 9 mm before removing the purge gas.
If the tape used for sealing is heated during welding, the materials contained in the adhesive will evaporate and may themselves contaminate the weld. During root pass welding, it may be necessary to reduce heat input, typically by welding a short distance, then stopping to cool before proceeding. For this reason, it may be necessary to manually weld the root pass in joints that can otherwise be fully welded automatically. To minimize (rather than eliminate) this problem, high-temperature fiberglass tapes with special adhesives are used, summed up Artem Komarov.