AWSCWI.COM team inspecting for weld slag inclusion to support welding quality.

What is Slag Inclusion? | 4 Amazing Ways to Prevent it

In Slag Free Weld Slag Inclusion by Matthew BehlenLeave a Comment

What is Weld Slag Inclusion?

Slag inclusions are a common problem in welding, and can lead to a number of defects in the weld, including porosity, cracks, and poor mechanical properties. Slag inclusions are caused by the entrapment of slag in the weld pool during welding. This can happen for a number of reasons, including:

  • Improper cleaning of the weld joint
  • Inadequate shielding gas coverage
  • Incorrect welding parameters
  • Welding in a dirty or dusty environment

Slag inclusions can be prevented by taking steps to ensure that the weld joint is clean and free of contaminants, that the welding parameters are correct, and that the weld is protected from contamination by dust and dirt. If slag inclusions do occur, they can be removed by grinding or chipping them out of the weld.

Slag inclusions can have a significant impact on the quality of the weld, and can lead to a number of defects. It is important that welders and inspectors take their time understanding weld discontinuities. These steps help to prevent slag inclusions from occurring in order to ensure that the weld is strong and durable.

What is a Slag Free Weld?

A slag free weld is a weld that does not contain any slag inclusions. Slag inclusions are small pieces of slag that become trapped in the weld metal during the welding process. They can cause a number of problems, including weakening the weld, reducing its corrosion resistance, and making it more difficult to machine.

How to prevent slag inclusions:

  • Using clean welding rods or wires
  • Maintaining a clean welding area
  • Using the correct welding technique
  • Adjusting the welding parameters

If slag inclusions do occur, they can be removed by grinding or chipping them out of the weld. However, it is always better to prevent slag inclusions from occurring in the first place.

For more information on slag free welding, please consult a qualified welding inspector.

How to Repair a Slag Inclusion in a Weld

First Identify and Assess the Slag Inclusion

  • Visual Inspection: With the knowledge of an AWS CWI or other certified welding inspector, check the weld surface for visible signs of slag inclusions.
  • Non-Destructive Testing (NDT): Use methods such as ultrasonic testing, radiographic testing, or magnetic particle inspection to locate and assess the extent of inclusions beneath the surface.

Preparation for Slag Inclusion Repair

  • Remove Defective Area: Grind or gouge out the area containing slag inclusions to remove all contaminated weld metal. Ensure removal is complete to sound metal.
  • Clean the Area: After removal, clean the area thoroughly to eliminate any remaining slag or contaminants. Use wire brushes, grinding, or a suitable cleaning method.
AWSCWI.COM team inspecting for weld slag inclusion to support welding quality.

AWSCWI.COM inspectors identifying slag inclusion in a weld.

Slag Inclusion Repair Welding

  • Preheat (if necessary): Preheat the area according to the material specifications to reduce the risk of further welding defects and improve weld quality.
  • Select Appropriate Filler Material: Choose a filler material compatible with the base metal and the initial welding process.  Always check the WPS to confirm.
  • Adjust Welding Parameters: Modify welding parameters (current, voltage, speed) to ensure optimal penetration and fusion while preventing further slag formation.
  • Welding Technique: Use appropriate welding techniques to fill the prepared area. Techniques may vary based on the welding process and the type of material being welded.

Post-Repair Slag Inclusion Inspection

  • Visual Inspection: Perform a thorough visual inspection of the repaired area to ensure the absence of surface slag inclusions.
  • NDT Verification: Apply non-destructive testing methods again to confirm that the repair has effectively eliminated the slag inclusions and no new defects have been introduced.
  • Mechanical Testing (if required): Conduct mechanical tests such as tensile testing, bend testing, or impact testing to verify the repair meets the required mechanical properties.


There are a number of ways to prevent slag inclusions from occurring. One is to use a welding technique that minimizes the amount of slag that is produced. Another is to use a slag remover to remove any slag that does form.

If slag inclusions do occur, they can be removed by grinding or machining the weld. However, this can be a time-consuming and difficult process.

In order to produce a slag free weld, it is important to follow the tips give in this publication.

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