What is Slag line welds defect? how to determine and prevent it?

What is Slag line welds defect? how to determine and prevent it?

Slag line weld defect occurs when slag gets entrapped in the weld metal during the welding process. Slag is a byproduct of the flux used in welding processes such as shielded metal arc welding (SMAW) or flux-cored arc welding (FCAW). When slag becomes trapped in the weld metal, it forms a line or inclusion within the weld bead, known as a slag line defect. This defect can weaken the weld joint, leading to reduced mechanical properties and increased susceptibility to cracking or failure under load.

Example: Welding of Steel Plates

Let’s consider the welding of steel plates for the construction of a ship’s hull. Welded joints are crucial for maintaining the structural integrity and seaworthiness of the vessel.

Slag Line Weld Defect:

During the welding of steel plates, slag line defects can occur if proper welding procedures are not followed or if there are issues with flux quality or welding technique. This defect manifests as a line or inclusion within the weld bead, where slag becomes trapped during the welding process.

Determining Slag Line Defect:

  1. Visual Inspection: Slag line defects can often be visually identified by examining the surface of the weld bead. If there is a visible line or inclusion within the weld metal, it may indicate this defect.
  2. Radiographic Testing (RT): Radiographic images of the weld joint can provide detailed insight into the internal structure of the weld. Slag line defects appear as dark, linear inclusions within the weld metal in radiographic images.

Radiographic Process:

In the radiographic process for determining slag line defects:

  • X-rays or gamma rays are directed through the weld joint onto a film or digital detector.
  • The resulting radiographic image shows the internal structure of the weld, including any slag line defects.
  • Slag line defects appear as dark, linear inclusions within the weld metal in the radiographic image, indicating the presence of slag entrapped during the welding process.

Prevention Strategies:

  1. Proper Welding Technique: Ensure proper welding technique, including maintaining a consistent arc length, travel speed, and electrode angle, to minimize the risk of slag entrapment in the weld metal.
  2. Flux Quality: Use high-quality flux materials that have been properly stored and handled to minimize the presence of slag particles that can become entrapped in the weld.
  3. Slag Removal: Implement thorough slag removal procedures between welding passes to prevent slag from becoming trapped in subsequent weld layers.

Impact on Weld Quality:

Reduced Mechanical Properties:

Slag line defects weaken the weld joint, reducing its mechanical properties such as strength, ductility, and toughness, leading to increased susceptibility to failure under load.

Susceptibility to Cracking:

Welds with slag line defects are more prone to cracking, particularly at the areas of slag inclusion where stress concentrations are highest, further compromising weld quality and structural integrity.

Corrosion Potential:

Slag line defects create potential pathways for the ingress of moisture, contaminants, or corrosive substances, leading to increased susceptibility to corrosion and reduced service life of the welded component.


Slag line defects are weld defects that require careful attention to welding procedures, flux quality, and slag removal to prevent. Through thorough inspection and testing, including radiographic examination, welders and inspectors can ensure the reliability, safety, and quality of welded structures, such as ship hulls, and mitigate the risk of these defects.

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