Interpreting a Weld Toe Line: Undercut, Mismatch, or Crack?
During visual examination of a circumferential butt weld in 8 mm stainless steel pipe, the inspector notes a continuous dark line along the upper weld toe for about 40% of the circumference. The line is uniform in width, follows the toe contour closely, and appears on the side where the pipe sections show a slight step (about 1 mm) at the joint. The weld cap is otherwise smooth. The inspector's draft report calls it 'toe undercut, reject.' The client's engineer asks the Level III to confirm before a repair is authorized, since repair of stainless pipe is costly and carries its own risk.
1. List the candidate explanations for the dark toe line and the visual features that distinguish them.
Read the worked answer
Candidates:
- Toe undercut - a groove melted into the base metal at the toe. Visual signature: a distinct recess below the base-metal surface, often with a sharp root; shadow line follows the toe but the surface is depressed. Depth can be gauged with a pit gauge or weld gauge.
- Hi-lo / mismatch step - the ~1 mm step at the joint creates a geometric shadow at the toe on the high side. Visual signature: the line coincides with the step, is uniform, and disappears when viewed from the opposite direction or when the step is blended. It is a geometry shadow, not a depression.
- Toe crack - visual signature: irregular, often branching, may have oxidation staining, typically not uniform in width, and does not follow the toe perfectly. Requires surface method to confirm.
- Oxide/heat-tint line or spatter shadow - a color or residue line; does not represent a depression.
Key discriminator: is the surface depressed (undercut) or is the line a shadow cast by a step (mismatch)? Viewing from multiple angles and using a weld/pit gauge resolves this.
2. Describe the examination steps you would direct to discriminate among these before authorizing repair.
Read the worked answer
- Multi-angle viewing: view the toe from both sides and along the pipe axis. A mismatch shadow changes or vanishes with viewing direction; a true undercut depression persists and shows a consistent shadow.
- Quantify the step: measure the hi-lo with a bridge-type weld gauge or similar. If the step matches the line location and the line is on the high side, mismatch is the likely cause.
- Quantify depression: use a pit depth gauge or weld gauge at several points along the line. Undercut has measurable depth; a shadow does not.
- Surface method confirmation: since the material is stainless (non-magnetic), penetrant testing is the appropriate method to determine whether the line is a tight crack or a benign geometric feature. MT is not applicable.
- Compare to acceptance criteria: if undercut is confirmed, evaluate depth and length against the applicable code acceptance criteria rather than rejecting on the inspector's impression.
Only after these steps should repair be authorized.
3. The line is confirmed as a 1 mm mismatch shadow with no undercut. What should the Level III report, and what is the disposition?
Read the worked answer
Report the finding as geometric mismatch (hi-lo) of approximately 1 mm producing a toe shadow; no undercut or crack detected. Disposition:
- The dark line is an apparent indication caused by joint mismatch, not a discontinuity requiring repair.
- Evaluate the 1 mm mismatch against the applicable fabrication tolerance for pipe alignment. If it is within tolerance, accept; if it exceeds tolerance, the disposition is an engineering decision (fit-up acceptance), not a weld repair of the toe.
- Record the measured mismatch, the PT result confirming no surface-breaking crack, and the basis for acceptance.
Correct the draft report to remove the 'toe undercut, reject' call and document the discrimination steps taken.