Discontinuity
Crater crack
A small star-shaped or linear crack in the depression left where the arc was broken, formed because the last shallow pool of metal cooled fast and shrank into a cavity it could not fill.
What it looks like
- A star, cross, or wishbone pattern radiating from the centre of the crater at a stop
- Sometimes a single short crack running back along the bead axis from the crater
- The crater itself is a dished depression sitting below the surrounding bead face
- Small enough to hide under slag or spatter, so it is easily missed on a dirty stop
Causes
- Breaking the arc abruptly instead of filling the crater before extinguishing it
- The high surface-to-volume ratio of the shallow crater pool causing very rapid cooling
- Concave crater geometry that concentrates shrinkage stress at the centre of the depression
- Welding materials with a wide solidification range, where the last metal to freeze is weakest
- Excessive current at the end of a pass leaving a deeper crater than the technique can fill
Prevention
- Fill the crater deliberately by pausing, back-stepping, or reversing over the bead before breaking the arc
- Use the machine's crater-fill or downslope function where the equipment provides one
- End passes on run-off tabs so the crater lands outside the finished joint and is removed afterwards
- Grind out any crater that is not going to be welded over, rather than trusting the next pass to consume it
How it is found
- VT
- Cleaning the crater and examining every stop with good light finds most crater cracks, since they are surface-breaking by definition.
- PT
- Penetrant collects in the fine star cracks and bleeds out clearly, revealing arms too tight to see with the unaided eye.
- MT
- On steel, a crater crack disrupts the magnetic field and holds particles in the distinctive radiating pattern.
How inspectors think about it
- Crater cracks are still cracks, so their small size does not soften how they are judged: a sharp tip in a stress-concentrating depression is exactly where a fatigue failure begins.
- Inspectors pay particular attention to craters left at the end of a final pass, because there is no subsequent pass to melt the defect out.
- Repeated crater cracking across a job points at welder technique or machine setup, so the practical response is retraining or a downslope setting rather than repair after repair.
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
Longitudinal crackA crack running parallel to the weld axis, either down the centreline of the bead or along the fusion line, driven by shrinkage stress acting across the joint as the deposit cools and contracts.Hot crack (solidification cracking)A crack that forms while the weld is still partly liquid, when low-melting films between growing grains are pulled apart by shrinkage before the metal has any strength to resist it.UnderfillA groove weld whose face or root sits below the surface of the adjoining base metal, so the joint has less cross-section than the surrounding material it is supposed to match.Arc strikeA small localised melted spot where the arc touched the base metal outside the joint, quenched instantly by the surrounding cold plate into a hard, brittle, crack-prone patch.
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.