In order to obtain a strong and resistant connection when welding, it is important to ensure that the tubes and pipes to be welded are inerted. Before starting a weld, it is therefore necessary to make sure that the ambient oxygen around a welding area has been removed and replaced by a shielding gas. That way, a discoloration or imbalance in the composition of the metal after welding can be avoided and at the same time, it ensures the quality of the weld and especially of the root.
But how to shield properly? What are effective tools for purging?
The use of shielding gases differs in many aspects depending on the application. With an open head (as used for automatic or manual welding) the shield gas is broadly distributed through the nozzle and a diffuser. The diffuser is located in the torch below the ceramic nozzle and is pierced with a multitude of small holes. If you are thinking of acquiring a manual TIG torch, AXXAIR strongly advises you not to pinch pennies when it comes to choosing the diffuser as it is the tool responsible for the even distribution of the shielding gas.
Another very interesting tool for manual or automated welding with an open head is the trailing torch which can also be used for back gas. It is a very easy way of working and the shield gas is directed straight onto the part to be welded. If this option is chosen for manual welding, the maneuver must be carried out with great care and the quantity of gas to be used must be large enough to obtain a satisfying result.
...What about closed heads?
Let’s take a weld that is done with a closed head for orbital welding for example: In this case the closed chamber of the welding head is filled with shield gas, thus expelling the atmospheric air including oxygen. The weld can then be carried out without any problems as long as the pre-gas time is respected. The pre-gas time for the closed chamber is calculated automatically by the orbital power supply. The time for purging the tube can vary depending on the size of the tube to be welded and the length of the inerting zone.
For tubes that can be several kilometers long, it makes sense to try and confine the inerting zone in order to create a shielding chamber and to use silicone flanges or accessories in the form of a balloon, positioned at 10 cm on each side of the weld, which will seal the shielding chamber. After the welding is completed, it is very easy to remove these balloons by sliding them with a special cable, manufactured just for this purpose.
What are possible pitfalls to avoid?
First and foremost: the gas
In TIG welding, opening the gas cylinder during the welding process is crucial and must not be forgotten at all cost – that is something every welder has in the back of their head at all times. If the cylinder is not opened, the tube will be burnt during priming due to the lack of shielding gas. At the same time, it will also have burnt the tungsten electrode which itself is very sensitive to the inert atmosphere.
Draughts and airflow
Draughts are to be avoided as well. When welding operations are performed outside, oxygen intrusions may occur at random. It is therefore recommended to establish a protected surrounding to better control the welding.
Gasflow and pressure inside the tube
During the entire welding process, a steady gas flow must be maintained inside the parts to be welded and the workpiece must be kept under control to prevent pressure build-up. If, due to an overflow of gas and too little exhaust, there is a difference in pressure between the inside and outside of the workpiece, this will have an immediate effect on the look of the weld.
The idea is simple:
- If the pressure inside the workpiece is too high, the weld seam will be pushed outwards and the result will look hollow.
- If the pressure on the inside is the same as on the outside, the result will not be influenced by this parameter.
In some areas the differences in pressure are voluntary and calibrated in order to change the appearance of the weld bead. But be careful to control these variations to ensure a repeatable result.
The important role of gas density
Gas density is an important factor to take into account. Argon is heavier than air. When filling a tank, the gas inlet hole must be placed at the bottom of the tank and the outlet hole at the top. When filling a pipe with nitrogen – which is quite common – the inlet hole should be placed at the top and the outlet hole at the bottom, since nitrogen is lighter than air. This way, all the oxygen will be expelled towards the bottom.
Safety in confined spaces
On another note, all safety rules must be followed precisely. A welder working inside a tank risks serious or even fatal accidents. Shield gases are odorless and the welder is often unaware of the influence they can have on the body. As soon as you start to feel tired and feel the urge to yawn for example, dizziness and therewith discomfort may occur a few moments later. Soon after the first feelings of discomfort, one is in respiratory insufficiency and risks serious consequences. If the brain is deprived of oxygen, it can lead to death in the worst case. This all happens within a few minutes and explains why all safety rules must be followed to the full extent when working in a tank. It is also because of this danger that the welder must work in pairs with a person situated on the outside of the tank. This applies to all confined spaces. The hazard analysis of these spaces must be carried out as accurately and carefully as possible beforehand.
Choosing the proper gas and the elements to be welded
Another important aspect is the good quality of the shielding. You should choose the purest gas possible according to the desired welding result. If, for example, a coloration within a strict tolerance is desired, the purest gas possible should be selected. But this too can present a pitfall if you consider the whole path the gas will travel. If the gas passes through porous, old or soiled pipes, the welding result will be of poorer quality. For this reason, rigid tubes should be chosen over flexible pipes. Many welders prefer to use flexible pipes since they are more manageable, but this flexible tube that will be put under pressure or vacuum will present a "sponge" effect by trapping gas. The next time it is pressurized, this gas will be released again. When this happens, one might permanently have to battle with impurities in the gas.
Of course, this kind of problem occurs predominantly in applications requiring high quality welds. In other cases, a flexible pipe may be perfectly suitable. The same applies to all fittings: you will never find rubber sealings on pipes that are thought to transport pure gases.
Shielding – balloons or flanges?
Another aspect that is not to be neglected when you shield tubes: Balloons are more fragile than silicone flanges. If the balloon is pierced or if it is placed too close to the weld and overheats, it will have to be replaced. On the other hand, if for certain applications it is possible to shield the tubes with a "U" shaped flange, the latter is much more robust and can be used infinitely. However, it should be noted that its implementation is more difficult and the set-up is rather tedious. Another solution are water-soluble adhesive tape: A plastic membrane used to plug the piping and confine the shielding zone. After the welding is complete, you will simply have to fill the pipe with water and the plug will dissolve.
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