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Lamination in steel plate: what it is and how to spot it

Fillet Gauge editorial teamSources checked 4 min read

A lamination is a flat separation inside rolled metal, running parallel to the surface, usually near the middle of the plate's thickness. It is a base metal discontinuity that comes from the steel mill, not from welding: impurities or voids in the original ingot or slab get flattened into thin planes as the steel is rolled.

Two views of a lamination. A: the cut edge of a plate with a thin dark line through the middle of its thickness. B: two plates beveled for a V-groove, with the same kind of line showing along one bevel face before welding
A mill flaw, best found before anyone welds near it. Original drawing, not to scale.
Use this diagram

How to recognize a lamination

  1. Look at cut edges. A lamination that reaches a sheared, sawn or thermally cut edge shows as a thin line along the edge, parallel to the plate surfaces, often near mid-thickness.
  2. Look at bevels before welding. Preparing a groove exposes a fresh edge. A line along the bevel face is a warning sign, because welding over it can open it up.
  3. Don't expect to see it from the face. A lamination fully inside the plate gives no sign on the rolled surface. Plate that sounds or rings oddly, or a bulge after heating, are rare clues.
  4. Think plate, not weld. If the flaw lies in the plate, runs with the rolling direction and isn't tied to the weld bead, it is likely a lamination.

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Welding Inspection and Discontinuities

A convex fillet weld replica is checked with a leaf-type fillet gauge. The drawing calls for a single fillet size. The vertical leg fits the 3/8 in blade and the horizontal leg the 5/16 in blade. What size is recorded?

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How it is found and recorded

Straight-beam ultrasonic testing is the standard method. A lamination lies parallel to the surface, so sound sent straight into the plate bounces off it early, before it reaches the back wall. Radiography is poor at finding laminations, because on plate the beam crosses the thin plane at right angles, so the gap adds almost nothing to the thickness it travels through and the film shows little contrast. At edges, visual inspection and penetrant or magnetic particle testing work. Record location, length and depth. Many documents set rules for laminations found at cut edges or bevel faces, so read the one that governs the job.

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Common mix-ups

  • Lamination or lamellar tearing. A lamination is already in the plate when it arrives. Lamellar tearing is a crack that forms during or after welding, in steps, because weld shrinkage pulled on a plate with weak through-thickness ductility.
  • Lamination or incomplete fusion. Incomplete fusion is an unbonded line at the boundary between weld and base metal. A lamination is entirely inside the base metal, often well away from the weld.
  • Lamination or a scab or seam. Seams and scabs are rolling flaws on the surface. A lamination is internal.

Why it matters for welding

A lamination is a plane of weakness. Welding near or across one can open it, let it carry a crack into the joint, or trap gas that comes out as porosity at the bevel. It also blocks ultrasonic testing of the weld beyond it. That is why some jobs call for ultrasonic scanning of plate near tee and corner joints before fabrication starts.

Laminations on the CWI exam

On Part A, laminations fit Base Materials, Heat Treatments and Metallurgy and NDE Fundamentals, where you match a flaw's orientation to the method that sees it. On Part B they fit Welding Inspection and Discontinuities and NDE Application. On Part C, look up how your code treats discontinuities at cut edges in your own copy.

Practice it

Drill which NDE method sees a flat, plate-parallel flaw in the NDE methods quiz. Then read its welding-made cousin, lamellar tearing, and the other crack that hides beside a weld, underbead cracks. The weld discontinuities guide lists every type.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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