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Practical answers on using trailing shields and cups to protect hot TIG welds from the air.
A trailing shield is a gas attachment that fits behind the torch and supplies extra argon over the weld and the hot metal as it cools. The normal gas cup only covers the arc and the molten pool, but some metals stay hot enough after the torch has passed to react with the air.The shield keeps an inert atmosphere over that cooling zone until the metal is cool enough not to pick up oxygen. It does this with its own gas supply, separate from the one feeding the torch.
Titanium is the main one. TWI explains that titanium absorbs oxygen and nitrogen when hot, which makes it brittle, so the cooling weld and heat-affected zone need extra protection from a trailing shield. Other reactive metals can be similar.Stainless steel is more forgiving, but oxide or heat tint on the surface damages corrosion resistance, so a trailing shield can help give brighter, cleaner welds where appearance or hygiene matters. Always check the welding procedure for your material and application.
Until it has cooled below the temperature at which the metal reacts with air. For titanium, TWI gives this as roughly 520 degrees C, above which it absorbs oxygen and nitrogen. Do not stop the gas, or lift the shield, while the weld is still glowing or heavily coloured.The shield length and your travel speed decide how long the metal is covered, so slow down if you see colour appearing behind the shield. Your procedure or specification takes priority over general figures.
No. A trailing shield protects the face of the weld as it cools, but the back (root) side of a full-penetration joint also needs protecting. TWI notes that the back face of a titanium weld needs a gas purge as well, and the HSE describes back purging with inert gas for stainless steel and other corrosion-resistant metals.Use the shield for the top side and a purge for the underside, with a vent so pressure cannot build up. Remember that purge gas is also an asphyxiation risk.
Colour shows how much air reached the hot metal. TWI describes perfect shielding as bright and silvery, followed by light straw and dark straw, then dark blue, light blue, grey and finally powdery white as contamination increases.Light and dark straw are normally acceptable, dark blue may be acceptable depending on service conditions, and light blue, grey and white are not. Colour alone is not a reliable guide on multi-pass welds, because contamination in earlier runs affects later ones. Your specification decides what you can accept.
Very clean. TWI calls for scrupulous cleanliness of the parent metal and the filler wire, because oil, dirt and fingerprints can add oxygen and hydrogen and cause porosity, discolouration and brittleness.A typical routine is to degrease, remove oxide with a clean stainless steel brush kept for the purpose, and degrease again. Handle cleaned parts and filler with clean gloves. Keep solvents and rags away from ignition sources. Follow your procedure and the solvent maker's safety instructions.