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Answers to common questions about TIG trailing shields, purge systems, torch cable covers and torch holders.
A trailing shield is an attachment fitted behind the torch nozzle that keeps inert gas over the weld and the hot metal beside it as it cools. The weld stays covered until it has dropped below the temperature at which it would oxidise.This matters because metals such as titanium can pick up oxygen and nitrogen from the air while still hot, which can make the weld brittle. A trailing shield extends the protection beyond what the torch nozzle alone can provide, particularly at higher travel speeds. Check the shield's instructions for how it is fitted and how it is supplied with gas, and that the gas flow suits your torch.
Trailing shields are mainly used on metals that react with air while hot, such as titanium, zirconium and cobalt alloys. They are also used on stainless and duplex steels, nickel alloys and some other alloy steels where a bright, oxide-free surface or good corrosion resistance is wanted.Ordinary carbon steel in normal fabrication usually does not need one. Whether you need a trailing shield depends on the material, the specification you weld to and how much oxidation is acceptable, so check the procedure for your job.
The shape needs to follow the surface you are welding. Flat shields suit sheet and plate, and curved shields are made to match the diameter of pipe, tube or a vessel. Internal-radius versions exist for welding inside pipes and vessels.A shield that does not sit close to the work lets air in at the sides, so choose the one that matches your joint and size. Check the shield fits the nozzle on your torch and that the length suits your travel speed and the heat of the weld, as longer shields are used when more cooling cover is needed.
Often yes. A trailing shield protects the face of the weld, but the root side of a full-penetration weld on stainless steel is exposed to air unless it is purged. An effective back purge minimises heat tint, and the oxygen in the purge space should be controlled and kept low before welding starts.Heat tint reduces corrosion resistance and has to be cleaned off if it forms. Whether a back purge is needed depends on the material, the joint and your welding specification, so follow your procedure and the maker's instructions.
Do not rely on guesswork or on timing alone. The oxygen content in the purge space should be controlled and checked before welding starts, and a purpose-made oxygen monitor reading from the purge space is the usual way to do this.The oxygen level you need depends on the material and the specification, and the time to purge depends on the pipe size, volume and flow rate. Do not set the flow so high that pressure builds up in the pipe, as this can distort the root bead. Follow your procedure and the maker's instructions for the purge equipment.
Yes. Argon and other inert gases displace the air we breathe, can collect in low areas such as pits and sumps, and are colourless and odourless. People have lost consciousness within seconds, and would-be rescuers have been harmed too.Avoid entering enclosed spaces where purge gas may be present. If entry is unavoidable, the space needs to be tested for oxygen from outside with a calibrated meter, and suitable respiratory protection, a trained standby person and rescue arrangements are needed. Do your own risk assessment and follow the HSE guidance.