Slag inclusion: what it is and how to spot it

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://filletgauge.com/charts/weld-discontinuities/slag-inclusion.svg" alt="Two views of slag inclusion. A: a multi-pass groove weld cross-section with an angular dark pocket on a pass boundary and a smaller one against the sidewall. B: a radiograph strip with a long ragged dark streak along the lighter weld band" width="100%"><p><a href="https://filletgauge.com/guides/slag-inclusion?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=slag-inclusion.svg">A cut section above, the same flaw on film below. Original drawing, not to scale. by Fillet Gauge</a></p>

How to recognize a slag inclusion

1. Check the shape. Slag is irregular: angular corners, ragged edges, sometimes a long stringer. It is never the neat round void that gas leaves.
2. Check where it sits. Slag collects where cleaning is hard. The usual spots are pass boundaries, the toe of an earlier bead where it meets the sidewall, and deep valleys between two beads laid side by side.
3. Check the process. No flux, little chance of slag. A GTAW root is unlikely to hold slag; an SMAW fill pass may.
4. Read the film. On a radiograph, slag is less dense than steel, so it shows darker than the weld, with ragged outlines. Long lines of it parallel to the weld are often called wagon tracks when they run along both toes of a root bead.

How it is found and sized

Most slag is buried, so visual inspection only catches it when it breaks the surface, for example at the face of a bead that wasn't cleaned before a cap pass. Radiography is the usual way to find it in a finished groove weld, and ultrasonic testing picks up larger inclusions. On a cut section or macro etch, slag looks like a dark, crumbly pocket. Where the governing document limits slag, it usually works on length, width or the spacing of inclusions, so record those carefully and look the limit up in that document.

Common mix-ups

• Slag or porosity. Round and smooth is gas. Angular and ragged is slag. Location helps too: porosity can be anywhere, slag likes pass boundaries.
• Slag or incomplete fusion. Both can sit on a sidewall. Incomplete fusion is a thin, even line that follows the groove face. Slag is a chunkier pocket of solid material.
• Slag or a tungsten inclusion. On film, slag reads dark and tungsten reads bright.

Why it forms

Poor interpass cleaning is the big one. Others are a convex bead that leaves sharp notches at its toes, an electrode angle that pushes slag ahead of the pool, too low a current to float slag to the surface, and a groove too narrow to reach. Undercut on the sidewall of an earlier pass makes a ready trap for the next layer's slag.

Slag on the CWI exam

On Part B, slag belongs to Welding Inspection and Discontinuities, at least 34% of the practical, and turns up again under NDE Application when you read a radiograph or a report. On Part A, it fits Visual Examination, NDE Fundamentals and Welding Application and Control, where cleaning practice lives. On Part C you look up how your code treats it in the NDT acceptance criteria.

Practice it

Test your read of radiograph images on the NDE methods quiz. Then compare slag with its round cousin, weld porosity, and with the flat flaw it hides beside, incomplete fusion. The weld discontinuities guide puts every type side by side.

Check the rest of Part B

Take the free 10-question check. It's weighted to the Part B outline, scored by domain and needs no signup, so you see where slag, porosity and the rest stand against your other domains.

Sources

Show the list

• Defects / Imperfections in Welds: Slag Inclusions (Job Knowledge 43), TWI. Retrieved 25 Oct 2026.
• CWI Part B Practical Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
• Discontinuity names follow AWS A3.0, Standard Welding Terms and Definitions. Every description here is our own wording.