What Is Slag Inclusion? Weld Defect

Slag inclusion is a weld defect in which non-metallic solid material (slag) becomes trapped within the weld metal or at the interface between weld passes. Slag is the solidified residue of flux used in processes such as SMAW (stick welding), FCAW (flux-cored), and SAW (submerged arc). When slag is not properly removed between passes, the next layer of weld metal covers it, creating an internal discontinuity that weakens the joint.

When Slag Inclusions Occur

Slag inclusions are found almost exclusively in flux-based welding processes. They form when the welder fails to remove the solidified slag layer between passes, when the groove geometry traps slag in tight corners, or when improper welding technique allows slag to flow ahead of the weld pool.

Causes and Remedies

Cause	Remedy
Incomplete slag removal between passes	Chip and wire-brush each pass thoroughly before depositing the next
Narrow groove angle	Increase the bevel angle for better electrode access and slag escape
Incorrect electrode angle	Angle the electrode to push slag behind the weld pool
Too low heat input	Increase amperage so the weld pool is fluid enough to float slag to the surface
Excessive weave width	Reduce weave to control slag flow, especially in vertical-up welding
Undercut from previous pass	Grind undercut smooth before the next pass to prevent slag pockets
Using damaged or damp electrodes	Store and handle electrodes per manufacturer recommendations

Types of Slag Inclusions

Type	Description	Severity
Isolated	Small, individual inclusions scattered in the weld	Low (if within code limits)
Linear	Aligned along the weld axis, typically between passes	High (acts like a planar defect)
Interpass	Trapped between two weld layers	Moderate to high
Wagon tracks	Two parallel lines of slag at the bevel edges of the root pass	High (indicates poor root cleaning)

Common Mistake

Do not rely on visual inspection of the slag coating to confirm it has been fully removed. A thin, transparent slag film can remain on the weld surface after chipping and brushing. Always grind suspect areas between passes, especially in tight groove welds and root passes on carbon steel pipe.

Detection and Acceptance

Slag inclusions are detected by radiographic testing (RT), where they appear as dark, irregular lines or spots on the film. Ultrasonic testing (UT) also detects larger inclusions. Acceptance criteria vary by code:

• ASME B31.3 / ASME Section V: Evaluated per the RT acceptance criteria of the applicable code paragraph; linear slag is typically rejectable.
• AWS D1.1: Isolated inclusions limited to 2/3 of wall thickness; aggregate length limited per weld length.
• API 1104: Individual slag inclusion max 50 mm in 300 mm weld length; width must not exceed 1.6 mm.

When slag inclusions exceed the acceptance criteria, the defective area must be excavated by grinding or arc gouging, re-welded, and re-inspected per the project inspection plan.

Read the full guide to fillet welds