Cracks in welds: hot, cold and lamellar tearing

The one group of defects that is rejectable everywhere, without a size limit. Three different mechanisms, three different cures — and telling them apart starts with where and when the crack appeared.

Hot cracking — during solidification

Where: along the centreline of the weld, or in the crater at the end of a run. Visible immediately, sometimes while still welding.

Why: as the weld freezes, the last liquid to solidify is concentrated in low-melting-point compounds — iron sulphide above all — and it sits in a film between the growing grains. The weld is shrinking at the same time, and the film has no strength to resist it.

What makes it worse: sulphur and phosphorus in the parent metal; a deep narrow bead, where the grains grow inwards from both sides and meet in a plane down the middle; high restraint; and stopping the arc without filling the crater.

Cures: a wider, shallower bead — depth-to-width ratio around 1:1 rather than 2:1; low-sulphur consumables; fill every crater; on austenitic stainless, a filler that leaves a few per cent of ferrite in the deposit, which is exactly why 308L is used on 304 rather than a matching composition.

Cold cracking — hours after welding

Where: in the heat-affected zone, usually just under the weld toe, running parallel to the fusion line. Often invisible at the surface.

Why: three factors have to coincide. A hard martensitic structure in the heat-affected zone, from cooling too fast. Dissolved hydrogen, from moisture in the consumable or on the plate. And tensile stress, from restraint and shrinkage. Remove any one of the three and the crack does not form.

Cures, in the order they are usually applied: preheat, which attacks the first two factors at once — see the preheat calculator; low hydrogen consumables, properly baked and kept in a quiver; higher heat input, within whatever the WPS allows; and a welding sequence that lets the joint shrink instead of building restraint into it.

Whether the steel is prone to this at all comes from the composition — run it through the carbon equivalent calculator before deciding preheat is unnecessary.

Lamellar tearing — in the parent metal

Where: in the plate rather than the weld, stepped and running parallel to the surface, under a joint that loads the plate through its thickness.

Why: rolled plate contains flattened non-metallic inclusions lying in the rolling plane. Loaded across the thickness — the short transverse direction — the plate simply has less ductility there, and the inclusions link up.

Cures: this is a design problem more than a welding one. Redesign the joint so the load is not carried through the plate thickness; specify plate with guaranteed through-thickness properties (the Z quality of EN 10164); butter the surface with a low-strength layer before welding the joint; and reduce restraint.

Repairing a crack

Find both ends first — penetrant or magnetic particle testing, because a crack is always longer than the part you can see. Drill stop-holes past each end if the excavation cannot be done in one go. Gouge or grind the whole crack out, re-test the excavation to confirm it has gone, then re-weld to an approved repair procedure, usually with preheat one step above the original.

Updated: 14 August 2026