What Is Slag Inclusion In Welding?

Welding is a vital part of many manufacturing and upkeep processes, but it can also come with its own set of challenges. One of these is called “slag inclusion”, which is an undesired welding defect that can have an impact on the quality and integrity of a welded joint.

This article will look at what slag inclusion is and how it’s caused, its effects, and how it can be prevented, read on to find out more! Globally, welding processes account for overhalf of a trillion dollars in annual manufacturing costs, and 235 processes are seen to account for 91% of all welding done. Slag inclusion is a defect that affects around 15-30% of welds, costing businesses in terms of having to discard unusable welds and starting over, resulting in time and money losses.

What is Slag Inclusion in Welding?

Slag inclusions are one of the common defects that occur during welding. They result from solidified molten flux, which appears as a sediment or foreign material left behind in the weld zone.

Slag inclusions are formed during the welding process, when the molten weld metal initially enters the joint and creates undissolved gas or slag particles. In arc welding, welding flux is used to protect the weld from oxidation.

The welding flux also helps control the temperature of the weld, prevent porosity and slag inclusion in the weld. During welding, the flux melts and leaves a coating around the weld pool.

The molten flux then solidifies and forms slag cover on surface of the weld.

Formation of Slag Inclusion

The formation of slag inclusions occurs when the molten weld metal does not completely dissolve the flux material. These inclusions remain as particles of solid embedded in the weld bead, resulting in slag inclusion.

Slag inclusions are formed due to several reasons.

  • Improperly cleaned joint edge surface.

  • Low welding current.
  • Improperly positioned welding electrode.
  • Flux with high melting point.

  • Low welding speed.
  • Lack of education about welding technique.
  • High flux-to-metal ratio.

Effects of Slag Inclusions

Slag inclusions can have a severe negative impact on weld’s strength and its other mechanical properties. Some of the effects of slag inclusions include:
  • The presence of slag inclusions can reduce tensile strength, decrease fatigue and corrosion resistance, and affect the weld’s toughness.
  • Slag inclusions can cause porosity, affecting the welding procedure.
  • Slag inclusions can decrease the precision of the weld.
  • Slag inclusions can also cause cracks and warping at the weld area.
  • Can reduce the mechanical properties of welded joints.

Preventive Measures

Slag inclusions can be prevented by partnering routine maintenance with the right equipment and high-quality weld materials and accessories.
  • Clean the joint edges properly.
  • Use welding current according to metal thickness.
  • Use low-hydrogen electrodes.
  • Perform multiple passes.
  • Use preheating and post-heating.
  • Increase welding speed.
  • Start the welding process with low current.
  • Regularly check and replace damaged or worn-out electrodes.
  • Know the correct welding techniques.
  • Maintaining the right welding angle and distance between the electrodes.
By following these preventive measures, the chances of slag inclusions in welding can be greatly reduced and the weld quality can be improved.


Slag inclusions are common welding defects that are caused by the solidified molten flux, which appears as foreign material left behind in the weld zone. Slag inclusions can reduce the tensile strength of welds, affect the precision of the weld, and can cause cracks and warping at the weld area. Preventive measures such as using the right equipment and high-quality weld materials and accessories, properly cleaning the joint edges, increasing welding speed, and following the correct welding techniques should be implemented to reduce the chances of slag inclusions in welding. Citations Urlhttps://www. engineersedge. com/welding/slag_inclusions. htmhttps://www. fronius. com/en-za/applications/arc-welding-migmag/weld-defects/slag-inclusions/ https://www. bueker-welding-consulting. com/index. php/en/blog/slag-inclusion-arc-weldinghttps://www. boneham. com/Downloads/Slag_Inclusion_in_Welding. pdf

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