Shielding is a procedure that consists of replacing a reactive atmosphere (oxidizing, flammable, explosive) or simply the ambient air with an inert gas, also called shielding gas.
The inert gas most often used in welding is argon but others such as nitrogen or CO2 are also used. Shielding gases are intended to flush out the ambient oxygen around a welding area, mainly to avoid any corrosion on the surface heated to a high temperature.
Where does the term shielding gas come from?
A shielding gas does not contain any active material that could influence the final result of a weld. However, one must be very careful about the correct definition of the term shielding gas because some welders add very small quantities of active gases. These active gases have a direct effect on the weld.
There are two types of active gases: Oxidizing gases and reducing gases.
- Oxidizing gases are used in manual and automatic MAG welding (Metal Active Gas welding) as well as in robotic welding.
- Reducing gases such as hydrogen, which has a high avidity for oxygen and thus improves the look of the weld bead but also limits the discoloration of the surface to a minimum are used in TIG welding (Tungsten Inert Gas welding). Hydrogen is also used to boost the weld and therewith grants deeper weld penetration.
As soon as a gas mixture contains more than 1% active gas, it is no longer considered an inert gas but an active gas.
In all applications, the inert gas comes out of a welding nozzle that dispenses the gas to expel the ambient air, which is generally composed of 21% oxygen, 78% nitrogen and 1% other gases (water vapor, methane, ozone, carbon dioxide, etc.). These different gases will have a negative impact on the area heated during welding – especially oxygen, which will damage the welding result.
What are the most common applications?
A closed head – more precisely its closed chamber in which the tube is welded – will be filled with the shielding gas.
When using a prefabrication machine or an open head, the gas is sprayed at a very high flow rate like a water jet to ensure that an inert atmosphere is present at the nozzle outlet.
Other applications are linear welds – which are very common – as well as more exotic welds that require the use of a shielding gas.
Why is shielding essential for welding?
Metals generally react as soon as oxygen is present, especially when welding steel that is heated to high temperatures. This is especially the case for stainless steels. Stainless steels have a thin layer of chromium to protect them from corrosion. But as soon as the weld starts, this layer is altered and oxidation occurs almost instantly. When the welded parts corrode, this is called root porosity. Little holes build up within the weld and the burnt part can no longer be called stainless steel because the chromium layer has been destroyed. This case also applies to other noble metals such as titanium and other corrosion-resistant alloys.
Shielding and steel: An unpopular application
Some welders use shielding gases even for "conventional" steels. During welding, these steels oxidize easily on the surface and form a layer called scale, which – unlike root porosity – looks like a discoloration. If the application requires mechanical durability for moving parts that may be subject to fatigue effects, this can present problems due to defects in shape, geometry or purity. These impacts are often of microscopic size and invisible to the naked eye – despite the large dimensioning of the equipment components. Thanks to the shielding, the weld will have the same smooth and shiny appearance with the same degree of purity as a stainless steel weld and the risk of cracking over time is postponed.