Free reference
Weld discontinuities
A discontinuity is an interruption in the uniformity of a weld. It becomes a defect only when it is bad enough that the governing code rejects it — and knowing that distinction is most of the job.
On acceptance
These pages explain how inspectors reason about each discontinuity — why a planar flaw is treated differently from a rounded one, why orientation decides whether ultrasonics will see it, why location in the load path matters. They deliberately do not reproduce any acceptance table. The numbers that decide accept or reject live in the code or specification governing your job, and nowhere else.
Cracks
9 pagesPlanar, sharp-tipped, and judged more harshly than anything else.
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.Transverse crackA crack running across the weld axis, at right angles to the direction of travel, caused by longitudinal shrinkage stress in a weld that is stronger or harder than the metal around it.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.Toe crackA 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.Root crackA crack in or immediately beside the root pass, the thinnest and most highly restrained part of a groove weld, where shrinkage stress from every later pass concentrates on the least metal.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.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.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.Lamellar tearingA step-like separation in the base metal below a weld, where rolled-in inclusion layers pull apart because the joint loads the plate through its thickness rather than along its rolling direction.
Cavities and porosity
4 pagesGas that did not get out before the metal froze.
Uniformly scattered porosityRounded gas pockets frozen into the weld metal and spread fairly evenly through the deposit, rather than bunched together or lined up, showing that shielding or cleanliness was marginal for the whole pass.Cluster porosityA tight group of gas pores concentrated in one localised area of the weld with sound metal on either side, marking a moment when shielding or cleanliness broke down instead of a whole-pass problem.Linear porosityGas pores lined up in a row, almost always along the weld root or between passes, because the gas source is itself a line such as an unclean root face or a contaminated interpass surface.Piping porosity (wormhole)An elongated tubular gas cavity that grew as the solidification front chased a rising gas bubble, leaving a hole shaped like a worm burrow rather than a sphere inside the weld metal.
Fusion and penetration
2 pagesMetal that was deposited but never joined.
Incomplete fusionWeld metal that lay against the joint face or a previous bead without melting into it, leaving a tight unbonded plane that carries no load even though the joint looks completely filled.Incomplete joint penetrationA groove weld that did not reach through the joint as the design required, leaving unfused root faces and an effective throat smaller than the drawing calls for.
Shape and profile
5 pagesThe weld is sound but the wrong shape, which is its own problem.
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.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.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.Excessive reinforcementA weld face built up far higher than the joint needs, producing an abrupt change of section at the toes that concentrates stress despite the extra metal looking like extra strength.Misalignment (high-low)Two members offset from one another across the joint so their surfaces do not line up, forcing load to change path through the weld and often leaving part of one root face unfused.
Inclusions
3 pagesSomething that is not weld metal, trapped inside the weld metal.
Slag inclusionNon-metallic slag trapped inside the weld instead of floating to the surface, most often left in a corner or undercut of a previous pass and then buried by the next one.Tungsten inclusionA fragment of tungsten electrode broken off into the weld pool during gas tungsten arc welding, leaving a hard, brittle, very dense particle embedded in the deposit.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.
Surface conditions
2 pagesDamage to the parent metal beside the weld.
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.SpatterDroplets of molten metal thrown out of the arc and stuck to the plate around the weld, mostly a cleanliness and coating problem but occasionally a source of local hard spots.