Slag Inclusion in Welding: Definition, Causes & How to Avoid

Marcus Colson Last updated on September 4, 2023
Reading Time: 4 Minute

In the world of welding, the term slag inclusion is very familiar to the ear.

To minimize this welding flaw, in this article, we will discuss what is inclusion in welding, as well as information about what slag inclusion is, why it can occur, and how to deal with it.

Stay here. Let’s start this discussion.

What is Slag Inclusion in Welding?

There are typically two sorts of welding slag inclusions.

The first group includes metal slag inclusions, also known as tungsten inclusions and copper inclusions, which are leftover metal particles like copper and tungsten in the weld.

The second type is inclusions of non-metallic slag, which refers to metal fragments still present in the weld after welding. Slag includes electrode coatings or fluxes, sulfides, oxides, and nitrides, among other things.

Based on how they formed, it can be separated into point slag inclusions, linear slag inclusions, and solid slag inclusions.

Slag inclusions will decrease the weld’s flexibility and toughness.

Those with sharp corners will concentrate tension at the ends, eventually becoming a source of cracks, especially in welds with a strong tendency for air cooling. 

Slag inclusions in the weld will first experience cracks under casting pressure, which will spread and weaken the weld and cause it to shatter.

Read Also : What is Undercut in Welding, Causes and How to Avoid Them

Causes of Slag Inclusion and How to Overcome

Many factors cause weld slag inclusions.

If there are many inclusions in the weld material (including base metal, gas, and welding material, etc.), it will inevitably cause inclusions in the weld. 

The following are factors that lead to slag inclusions and strategies for avoiding or resolving them.

1. Flux Composition 

Flux composition may increase the risk of slag inclusions.

This immediately impacts the welded junction’s profile and slag removal ease. A reduced oxygen level in the weld pool will likely result in increased surface tension.

Hence, effective hydration of the parent material is not possible.

To prevent the welding defects as well, using a flux containing iron oxide will help create a weld pool with lower surface tension and a more concave weld joint profile, allowing the weld pool to wet the base metal properly. 

A flux with a higher silicate content creates a glassy slag that tends to loosen on its own more quickly.

Flux coatings with lime content more frequently produce slag, which is difficult to remove.

2. Incorrect Welding Technique 

Slag inclusions will result from the improper procedure.

Because of this, it’s crucial to use the proper welding process to avoid slag inclusions.

A good place to start is by handling the electrode appropriately, which produces weld beads with the proper form and enough overlap.

By employing the optimum technique, you can prevent pockets from forming that could catch slag.

According to the connection type, selecting the suitable electrode size is crucial.

Also, remember that a good welding angle guarantees sufficient weld penetration and a smooth weld profile.

Planning their welding for numerous passes is one way expert welders avoid difficulty.

The flux won’t become entrapped in the weld bead during successive passes if the beads are suitably aligned.

Furthermore, welding at the higher amperage setting range will guarantee that you can keep an accurate arc and produce clean welds.

Less powerful welders won’t have the ability to maintain a focused arc, push slag, or combine metals.

It is necessary to propel the slag into the weld pool with adequate force, but it must also rise at the right time.

By maintaining a constant travel speed when welding, you can avoid burning the metal and give the slag enough time to peak.

If you proceed too quickly, slag will remain in your final weld.

3. Slag is Not Cleaned 

Slag inclusions are likely to form when uncleaned slag is present between weld edges, weld joints, and the weld bead, particularly when using alkaline electrodes;

To prevent this, clean the welding area of any dangerous pollutants, including rust, oil stains, and other contaminants, before welding.

It’s important to wipe out the slag from the groove, its sides, and the spaces between layers, and the groove angle shouldn’t be too tiny.

4. Incorrect Welding Process Parameters

Too low amperage, too far arc, wrong welding angle, and too small seam angle can also result in slag inclusions.

In addition, a welding current that is too low results in inadequate heat being produced during the melting of the metal and slag, which reduces the fluidity of the mixture.

The molten metal, therefore, hardens rapidly, depriving the slag of the chance to climb to the surface.

To prevent and overcome; In order for the slag to float smoothly to the liquid pool’s surface, it is important to choose the appropriate welding process parameters through evaluation of the welding process. 

This includes choosing the electrode diameter according to the thickness of the plate and adjusting the electrode angle, and conveying method as needed based on the direction in which the slag flows.

5. Improper Chemical Composition of Base Metal and Welding Consumables 

The products (oxides, nitrides, sulfides, etc.) will stay in the weld when the molten metal rapidly hardens if the molten pool contains a lot of oxygen, nitrogen, sulfur, and other components.

Choose the right welding materials to avoid this and improve deoxidation and desulfurization.

Avoid using damaged or expired welding consumables; only use those with good process performance.

Read Also : Lack of Fusion in Welding: Definition and How to Avoid

How Can Slag Inclusions Be Fixed?

It is best to cut off the problematic portion of the bead and then reweld it in order to correct slag inclusions.

Many welders, especially those employing cellulosic rods like the E6011, attempt to eliminate slag inclusions by burning them with additional passes.

These rods feature an extremely aggressive arc that penetrates the metal deeply and has the ability to burn remaining impurities.

The finished bead could look beautiful even if there are no obvious inclusions. 


The stick and flux-cored procedures frequently have slag inclusions as a flaw.

The joint becomes weak and prone to failure if there are too many. They might also begin rusting.

Low amperage, an improper electrode angle, and slag left over from a prior welding operation are frequently the causes of slag inclusions.

Inclusion in welding work is a very challenging job and tough. To deepen our skills, of course, we have to practice a lot and deepen our knowledge.

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Marcus Colson
Marcus Colson

Welding is more than a hobby for me - it's a passion. The art of fusing metal together to create something new and functional never gets old. From intricate sculptures to sturdy structures, I love the endless possibilities that welding offers.

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