What is Slag in Welding?

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Last Updated on October 25, 2021 by weldinghubs

Do you have any idea about slag in the welding process? If you are new in the welding field, then you may not have the concept of slag. However, slag is a crucial part of welding that provides some benefits and creates trouble in some welding techniques.

As an expert welder, I will share detailed information regarding the slag in welding, like how it develops and its benefits during the welding process. Moreover, keep in mind that flux composition is a critical factor in producing the welding slag.

However, many beginner welders are curious about “What Is Slag In Welding?” so here is my approach to slag meanings.

Welding electrodes, especially consumable electrodes, contain the flux that melts the metal and produces a slag along with shielding gas. Both slag and shielding gases protect the welding pool and the welding arc. But you can chip off the slag right after the welding process. The consumable electrode flux also provides the deoxidizers, alloying elements, and scavengers to create a robust welding pool.

Now let’s dive into a detailed discussion regarding every aspect of slag in welding.

How is slag produced during welding?

The slag is produced during the welding process through the electrode flux. It is a meltable solution that is produced during metal melting. However, flux is a particular material that presents in the electrode along with the filler material. This flux material consumes while the electric arc produces heat after having the electric arc flux melt and solidify at the place of the welding site. You can also say it as the filler metal to join two metal pieces.

So flux has multifunctional properties that produce slag along with the shield gas at the place of welding. These shielding gasses also protect the welding site from environmental gasses. Slag come over

Few welding techniques use the electrode containing flux material. These welding techniques include shielded metal arc welding and Flux-cored arc welding. Slag that is produced in this process help to protect the weld during solidification. Moreover, both these welding techniques also make a massive amount of spatter that can harm welders.

However, the process that doesn’t use the flux-containing electrodes needs to use the external shielding gasses. These shield gases include the inert or semi inert gas mixture that helps to protect the welding pool from contamination and didn’t produce a slag layer that welders need to remove after cooling the weld.

Problems caused by slag in welding

Now you know how slag produce and what is the purpose of its production ins specific welding techniques. But it is a fact that it also causes some particular problems that can create trouble for welders under specific circumstances.

Here I will share some common problems that appear due to slag in welding.

Welding slag inclusion is one of the common problems that appear during slag production. It damages the quality and reliability after cooling the weld joints. Slag inclusion seems when molten metal or slag material cannot come on the upper surface during the cooling process. There are some reasons behind the production of slag inclusion development.

The common reason behind the slag inclusion development is improper handling of flux, poor flux content material, improper use of welding techniques.  Moreover, the slag inclusion is also connected with arc welding, stick welding, and flux-cored welding. On the other hand, MIG welding is less prone to welding slag, due to which it provides clean and accurate welding pools.

However, weld slag inclusion leads the welding pool towards corrosion that damages the welding strength and effectiveness.

Slag in welding is a visible layer at the surface of the welding pool or joint where two metals join together. Sometimes slag continues to develop, but in some cases, it is discontinuous.

Development of welding slag inclusions

I already share some information regarding the welding slag inclusion, but it still needs to be more precise for new welders.

Slag is a residue during the welding process that comes from the flux of consumable electrodes. Flux contains different materials, but the deoxidation product is standard due to chemical reactions between air, oxide at the metal surface, and flux.

If two insufficient layers do have not a suitable distance, then slag may be trapped inside the weld joint. Of course, as a welder, you need to prevent this situation. But if you successfully place both layers, then there is no way of trapping the slag, and you can deal with it conveniently.

Moreover, slag can get trapped between welding joint cavities, due to which welders need to pass the weld through undercut. As a result, it can also produce an irregular surface with the other metal welding layer or piece.

These welding sag inclusion have effects on different factors like restricted welding position and join configuration. Therefore, try to avoid these situations to protect the welding slag inclusion, affecting the welding results.

Effects of flux coating on welding slag

Flux coating has specific purposes in the welding technique, but one of the primary purposes is to produce the slag that can move freely at welding and cover the welding pool. So it is essential to have the molten metal that can keep the weld joint protected from the atmospheric gasses and relevant contaminants.

Flux is available in consumable welding electrode that produces the slag, and it also has their particular characteristics. So it is essential to note the freezing rate, surface tension, and other properties that help get the quality welding joint.

Surface tension properties of welding slag are necessary to deal with challenging welding positions. Keep in mind that slag with higher velocities is better for horizontal and vertical welding positions. In addition, it provides a smooth welding layer and a layer of slag that is easy to remove after cooling.

If you are welding the metal in a vertical position, it is essential to keep the low velocity. It helps you get an adequate flow and reduces the overflow that causes trouble and irregular weld.

Moreover, the flux composition also affects the slag inclusion risk factors. Keep in mind that flux composition has a direct impact on the slag and welding joint properties. So it is essential to employ the oxidative flux that contains enough iron oxide and create weld with significant surface tension. Moreover, the welding joint composition also has excellent properties that enhance its ability to wet the welding surface and melt the metal properly.

Flux with silicate in higher content is easy to remove after cooling the weld. At the same time, the flux with lime materials is challenging to withdraw from the welding joint.

How does slag provide preventive measures?

Now you know that slag is not a problem. Although, of course, it will generate some issues but only in particular conditions. In contrast, it provides many preventive measures to the welding pool.

When you continue the welding process, you may notice that slag solidifies along with the weld pool. It is because it prevents the melted metals from atmosphere vapors and gasses that are unsuitable for welding.

Let’s review the actual process of welding and see how slag help as a protective barrier.

Welding uses the high temperature to melt the metal and join two pieces. It requires a limit of time for resolidification, and during this time, protection requires from the external atmosphere. So during this time, slag protects the welding pool and lets it cool effectively.

So it is a protective barrier between the environment and the welding site essential for every welding technique.


Summing up the above discussion, it is clear what slag is in welding and how it protects the welding pool. It is a fact that slag can cause issues for welders but only in certain circumstances. As a welder, you need to carefully weld the metal to have fewer chances of welding slag inclusion. However, slag is a protective barrier between the welding pool and external particles, vapors, and gasses.

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Does it necessary to remove the slag after welding?

Slag is not necessary for the strength of the metal after the welding process. It is a waste material that is necessary to remove. So remove the slag after cooling the welding site to geta clean and clear surface that looks like a painted side.

Can you weld over slag?

You can't weld over the slag because it is a non-conductive material that is impossible for welding. Moreover, the slag inclusion is a problematic welding defect that fails the welding strength. So you need to remove the slag first and then weld it again.

When there is slag, you drag?

There is a push and pull rule of welding that is simple and efficient. For example, it would help if you dragged the welding rod using the stick and flux core welding. In comparison, push the welding rod in the metal inert gas welding technique.

Does flux produce slag?

Flux core welding produces the slag that comes on the top after finishing the process. Therefore, it needs to remove; similarly, we do in stick welding.

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