Discontinuity
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.
TitleHot crack (solidification cracking)
ClassCrack
ViewIndicative
What it looks like
- Often visible immediately after welding, sometimes opening while the bead is still glowing
- Follows the grain boundaries, giving an irregular, branching path rather than a straight split
- Fracture faces show a rounded, oxidised, dendritic surface rather than the bright faces of a cold crack
- Commonly runs along the bead centreline where opposing grain fronts meet
Why it happens, and how it is stopped
Causes
- Sulphur, phosphorus, and other low-melting elements picked up from the base metal, forming weak liquid films
- A deep narrow bead shape that drives columnar grains to meet head-on at the centreline
- High restraint pulling on the joint during the brief mushy stage between liquid and solid
- Fully austenitic stainless deposits with no ferrite to interrupt the continuous liquid film
- Excessive travel speed producing a tear-drop shaped puddle that concentrates segregation at the centre
Prevention
- Choose filler metal that deposits a small amount of ferrite when welding austenitic stainless steel
- Keep bead width equal to or greater than depth so grains do not meet on a single weak plane
- Limit dilution from base metals high in sulphur or phosphorus by controlling penetration
- Reduce travel speed to give a more elliptical puddle and a less segregated centreline
How it is found
Which examination methods can see it, and what it is about the discontinuity that makes them able to.
- VTSolidification cracks appear immediately, so a welder or inspector watching the bead cool often sees the crack open on its own.
- PTThe branching surface crack draws penetrant readily and bleeds into developer as a distinctly irregular indication.
- MTOn carbon steel deposits the surface-breaking crack leaks the applied field and holds particles along its jagged path.
How inspectors think about it
- The branching, grain-boundary path of a hot crack tells an inspector the cause was metallurgical rather than mechanical, which changes the corrective action entirely.
- As a crack it is treated as a rejectable planar flaw regardless of length, and it rarely occurs in isolation once conditions favour it.
- Repair alone seldom solves the problem, because rewelding with the same consumable into the same chemistry reproduces the liquid films that cracked the first time.
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
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.Crater crackA 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.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.Oxide inclusionSurface oxide folded into the weld pool rather than broken up by the arc, most troublesome on aluminium where the oxide skin melts far above the metal it covers.