As mentioned in our last article, inerting is a procedure that consists of replacing the ambient oxygen around a welding zone with a shielding gas such as argon in order to avoid any corrosion on the surface heated at high temperature. But what are the risks for the welded assembly in case of bad inerting and how do the different metals react?
Inerting and stainless steel
When welding stainless steel tubes and fittings, it is important to be aware of its thin chrome layer protecting the metal from corrosion. Stainless steel consists of several alloying elements, you can even say that it is an alloy of iron and carbon with chromium (more than 10.5%). The latter is responsible for the formation of this protective chromium oxide layer, which makes stainless steel durable and almost unsusceptible to corrosion.
When two stainless steel components are heated to high temperatures during welding, it is crucial to ensure that they are inert. Without any shielding gas or in the case of poor inerting, root porosity will develop and the metal will be at risk for corrosion. The fused section without protection will react the same way conventional steel would and the metal will no longer be rust-free.
Inerting and titanium: A particularity
A similar problem arises for titanium. Titanium and its alloys are relatively expensive but very light metals. They also stand out because of their long service life and excellent resistance to acids, chlorides, salt and negative as well as positive temperatures. Titanium has the best strength-to-weight ratio among metals.
When talking about this very sensitive alloy, one will not speak about root porosity but about discoloration in case of bad inerting. As soon as the temperature rises during the melting of the material, it must be ensured that all the parts to be welded are in a protected atmosphere throughout the manufacturing process until the final processing of the workpiece, whether for welding or other handling reasons. In the case of inerting defects, the coloration will first look golden. As soon as it turns blue, the piece can already be considered damaged. Some welders claim that titanium is difficult to weld, but in reality it’s the opposite: titanium can be welded even more easily than stainless steel, provided the inerting is executed properly. All titanium alloys are subject to the same rules.
Applications: When is inerting really necessary?
Before welding elements, it makes sense to ask the question if inerting is really necessary because when you speak of inerting, you’re talking about inerting tools and a gas cylinder. If your intention is to weld a gardening shovel, a decorative piece or even a bumper for a car, this step is not necessarily required.
On the other hand, as soon as the assembly is intended for transporting food products or fluids at high pressure, it is recommended to use stainless steel and in this case, inerting is obligatory for hygienic reasons. This is especially the case in "high purity" industrial sectors such as food and beverage, pharmaceutical, nuclear, semiconductor manufacturing and many others. The most common welding procedure in these industrial sectors is TIG welding (Tungsten Inert Gas welding) because it offers excellent melt bath control and is therefore very suitable for precision welding.
The different shapes and dimensions of the workpieces also need to be taken into consideration. Of course, tube welding is a very common application in industrial piping, but differently shaped work pieces such as sheet metal or beams also require inerting done the same way. Let's take the example of sheet metal boat hulls when they are joined together. They are not made of stainless steel but the principle is identical. Same goes for large tanks: they cannot be filled entirely with Argon. In these specific cases, it makes sense to use a U-shaped flange that will be placed at the back of the weld. This flange will then be filled with gas and therewith creating the inerting chamber. The aim is to perfectly fill the back side of the weld, which will also be subjected to high temperatures. This will be done as confined as possible for economic reasons and the desire to be as quick as possible. Let's look at the example of a 1000 liter tank. If the welder has to fill the entire tank with gas, he will need much more than 1000 liters of gas to avoid any risk of pollution. For tasks like this one, it is better to use an inverted trailing torch with a big nozzle spraying a lot of argon which follows the welder to protect the heated zone.
Would you like to learn more about the different shielding gases and their impact on the melt bath during welding?
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