What Is Welding Slag?

Welding slag is a residue left from certain types of welding such as MIG, MMA and FCA welding, namely welding techniques that use some form of flux process (with the exception of MIG welding which does not require flux in the welding process).

Slag causes an imperfection to the work being welded as it is a deoxidation by product from the flux reaction process, when two welds are applied side by side the slag then gets trapped in the weld causing a pocket in the weld. This pocket is because the slag does not get melted out as part of the weld, the cavity formed by the slag makes the weld lose strength and durability and is known in the business as a linear defect.

Slag caught in a welding cavity can over time can cause cracks in the weld and make the weld break causing damage to the work welded. This is even more of an issue if the work is of industrial use as it tends to mean harsher environments with vibrations and heavy workloads required putting more stress on the weld.

There are a few top tips to help prevent welding slag when welding, the more experienced welders' use these techniques on a daily basis but "green" welders may not know about this simple techniques.

Ensure you use the correct welding technique to produce smooth and strong weld beads as this will reduce the chances of slag pockets forming.

Make sure the correct current is used for your work and that the welding torch travel speed is correct for your current and work.

If carrying out multi layer welds always ensure you remove slag and clean the weld area between each weld layers. You can use a wire brush and for stubborn slag forms use an angle grinder with a sand disc or in worse case scenario use a grinding disc.

Welding slag is not just an imperfection it is also something that can have a detrimental impact on the work being carried out as it can cause a weld to lose strength and durability. This in turn can mean the weld breaks later on from the stress being applied to the weld of day to day workloads.