Discontinuity
Toe crack
A crack starting at the junction of the weld face and the base metal, where the abrupt change in section concentrates stress and the heat-affected zone is at its hardest and least forgiving.
What it looks like
- A fine line following the weld toe, usually just outside the visible bead edge
- Generally starts at the surface and grows into the heat-affected zone at an angle to the plate face
- Often associated with a sharp, steep toe angle or a small undercut notch
- May appear hours or days after welding rather than immediately, particularly on higher-strength steel
Causes
- Stress concentration at a sharp toe angle where a convex bead meets the plate abruptly
- Hard, brittle heat-affected zone microstructure produced by fast cooling in hardenable steel
- Hydrogen from damp consumables or contaminated surfaces diffusing into the heat-affected zone
- Residual shrinkage stress in a restrained joint pulling directly at the toe notch
- Undercut or overlap at the toe adding a mechanical notch to an already stressed location
Prevention
- Preheat to slow cooling through the transformation range and keep the heat-affected zone soft enough to yield
- Blend the toe into the base metal so the transition is gradual instead of a sharp step
- Use low-hydrogen consumables and keep them dry, since hydrogen is the usual accomplice
- Avoid excessively convex beads, which steepen the toe angle and raise the local stress concentration
How it is found
- MT
- The crack is surface-breaking in ferromagnetic steel, and a field crossing the toe line leaks sharply where the crack interrupts it.
- PT
- Reveals tight toe cracks on stainless and aluminium where magnetic particle inspection cannot be used.
- VT
- Careful cleaning and low-angle lighting can show the line, though a tight toe crack is easy to lose against the toe shadow.
- UT
- Angle-beam scanning from the plate surface picks up the crack as it grows down into the heat-affected zone, beyond the reach of surface methods.
How inspectors think about it
- Toe cracks sit at the exact location where fatigue cracking naturally initiates, so their presence is treated as far more serious than the small dimensions suggest.
- Inspectors consider whether the crack is confined to weld metal or has entered the heat-affected zone, because base metal cracking widens the scope of any repair.
- Delayed appearance is a known behaviour, so a joint that passed inspection immediately after welding may need re-examination once it has cooled fully.
General reasoning only. The accept/reject limits for your work come from the code or specification governing the job, which this site does not reproduce.
Related discontinuities
UndercutA groove melted into the base metal at the weld toe and left unfilled by weld metal, reducing the section of the parent material and leaving a sharp notch exactly where stress concentrates.Underbead crackA crack lying in the heat-affected zone beneath a sound-looking weld bead, produced by hydrogen trapped in hard martensite that cracks under residual stress after the joint has cooled.Cold crack (hydrogen-induced cracking)A crack that appears after the weld has cooled, sometimes many hours later, when dissolved hydrogen collects at defects in a hard microstructure and residual stress finishes the job.OverlapWeld metal that has rolled over the toe and sits on unmelted base metal, forming a lip with a sharp crevice underneath that carries no load and hides an unfused interface.
Can you name it on sight?
The gauge trainer's identification mode shows you a weld and four names. Reading about porosity and picking it out of a line-up are not the same skill.