What is Interpass slag inclusions weld defect? how to determine

What is Interpass slag inclusions weld defect? how to determine

Interpass slag inclusions in welding occur when slag from a previous welding pass becomes trapped between subsequent weld passes, leading to discontinuities within the weld joint. Slag inclusions can weaken the weld and compromise its integrity. They can also serve as initiation points for other defects such as cracks or porosity.

Example: Welding of a Steel Structure

Let’s consider the welding of a steel structure, such as a bridge or a building. Structural welds are critical for ensuring the stability and safety of such constructions.

Interpass Slag Inclusions Weld Defect:

During the welding of a steel structure, interpass slag inclusions can occur if proper welding procedures are not followed or if there are issues with slag removal, weld cleaning, or welding technique. This defect manifests as discontinuities or pockets of slag trapped between subsequent weld passes, visible upon visual inspection or through non-destructive testing techniques.

Determining Interpass Slag Inclusions Weld Defect:

  1. Visual Inspection: Interpass slag inclusions can often be visually identified by examining the surface of the weld bead and the transition area between weld passes. Discontinuities or pockets of slag between weld passes indicate the presence of this defect.
  2. Non-Destructive Testing (NDT): Non-destructive testing techniques, such as radiographic testing (RT) or ultrasonic testing (UT), can be employed to detect and evaluate the size, location, and extent of slag inclusions within the weld joint.

Radiographic Process:

In radiographic testing (RT) for interpass slag inclusions detection:

  • X-rays or gamma rays are directed through the weld joint onto a film or digital detector.
  • Interpass slag inclusions appear as irregular-shaped, dark indications within the weld metal in radiographic images, indicating areas of discontinuity or trapped slag.

Prevention Strategies:

  1. Proper Slag Removal: Ensure thorough slag removal between welding passes to prevent slag from becoming trapped between subsequent weld passes. Use proper slag removal techniques, such as chipping, brushing, or grinding, to remove slag effectively.
  2. Weld Cleaning: Clean the weld area and surrounding surfaces thoroughly before starting each weld pass to remove any contaminants, such as oil, grease, or rust, that can contribute to slag inclusion formation.
  3. Welding Technique: Employ proper welding techniques, including maintaining the correct arc length, travel speed, and electrode manipulation, to minimize the risk of slag inclusion formation and promote proper fusion between weld passes.

Impact on Weld Quality:

Weakened Weld Strength:

Interpass slag inclusions weaken the weld joint, reducing its load-bearing capacity and increasing the risk of sudden failure under mechanical loads or stress.

Susceptibility to Cracking:

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

Increased Risk of Corrosion:

Interpass slag inclusions 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.


Interpass slag inclusions in welding are critical defects that require careful attention to welding procedures, slag removal, weld cleaning, and welding technique to prevent. Through thorough inspection and testing, including visual examination and non-destructive testing techniques, welders and inspectors can ensure the reliability, safety, and quality of welded components, such as steel structures, and mitigate the risk of these defects.

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