6.3 Laminations, Seams, Laps, and Undercut

6.3 Laminations, Seams, Laps, and Undercut

Quick Answer: Laminations split plate mid-planes parallel to the surface—use normal-beam UT. Seams are elongated mill defects along bar/pipe length. Forging laps are folded-over metal with oxide-lined crevices. Undercut is a melted groove at the weld toe that concentrates stress and hurts fatigue life. Choose methods by plane orientation and surface access—not by habit.

These discontinuities appear repeatedly in Basic exam scenarios that link manufacturing process (Chapter 3–4 themes) to inspection method selection.

Rolling Laminations

A lamination in plate or sheet is a planar separation approximately parallel to the rolled surfaces, often near mid-thickness. Origins include:

• Pipe or porosity from the ingot/slab that was flattened during rolling
• Clustered inclusions elongated into planar weaknesses
• Insufficient discard of defective ends of billets/slabs

Because the plane of the flaw is parallel to the plate faces:

• Normal-beam (0°) UT from either face reflects strongly from the mid-plane interface—this is the textbook method.
• Angle-beam UT may miss or poorly characterize the same plane depending on setup.
• RT through the thickness often fails: the beam is perpendicular to a tight plane with little thickness gap along the ray path, so contrast is poor.
• PT and MT on the plate faces find nothing if the lamination does not break the edge or face. Edge-on PT/MT after cutting or on plate edges may show openings.

Laminations matter structurally when welding, forming, or hydrogen service drives delamination or when through-thickness tension (e.g., strained joints, T-joints with lamellar tearing risk) pulls on the weak plane. Lamellar tearing is related: terraced cracking in the base metal under a weld due to through-thickness strain and poor through-thickness ductility—often associated with planar inclusion bands. Level III should flag high through-thickness strain joints for material quality and UT screening expectations.

Seams vs Cracks

A seam is a longitudinal, elongated discontinuity on or near the surface of bar, rod, wire, or seamless tubular products, originating in billet defects, cracks, or folds that were drawn out during rolling or drawing. Seams often run parallel to the working direction (length of bar/pipe).

How seams differ from service cracks:

Both may be surface-breaking and respond to MT (ferromagnetic), PT, ET (tube/bar surface), and VT. Calling every longitudinal indication a "crack" without process context confuses root-cause and acceptance. A seam can still be rejectable under product specifications; the Level III point is correct classification language and appropriate method/specification.

Seams can open further in service and act as fatigue starters—so "mill origin" does not automatically mean "harmless."

Forging Laps

A forging lap (fold) forms when metal folds over itself during forging—excess metal, improper die fill, or wrong billet temperature/shape—and is forged into the surface. The fold typically carries oxide scale into the crevice, preventing true welding of the fold. Result: a sharp, surface-connected, oxide-lined discontinuity that behaves like a crack for stress concentration and for surface NDT.

Detection:

• VT after cleaning may show a line or fold scar
• PT and MT (if ferromagnetic) are effective when the lap is open to the surface
• Machining may partially remove or reopen laps
• UT can find deeper associated defects but surface methods dominate classic lap detection

Related forging issues include bursts (internal ruptures from excessive deformation) and flakes—different morphologies; do not label every forging defect a lap.

Weld Undercut

Undercut is a groove melted into the base metal at the toe or root of a weld and left unfilled by weld metal. Causes include excessive amperage, long arc, wrong electrode angle, fast travel, or poor technique at edges.

Geometry and risk:

• Creates a notch at a high-stress location (toe)
• Reduces section thickness locally
• Strongly elevates fatigue crack initiation risk under cyclic tension
• Depth and length are controlled by welding codes and visual acceptance criteria

NDT and control:

• VT (including weld gauges) is the primary production control
• Depth may be measured mechanically; some automated laser/profile tools exist in advanced shops
• PT/MT can show if undercut coincides with cracking, but undercut itself is a geometry condition first
• Repair is typically weld repair per procedure—not "inspect it away"

Method Selection Summary

Use this decision frame on exam scenarios:

1. Is the plane parallel to the rolled surface mid-wall? → Think lamination → normal-beam UT. Do not rely on RT through-thickness or face PT alone.
2. Is it a longitudinal surface line on bar/pipe from mill practice? → Think seam → surface methods; check product specs.
3. Is it a folded surface on a forging? → Think lap → PT/MT/VT; expect oxide-lined crevice behavior.
4. Is it a melted groove at a weld toe? → Think undercut → VT and code depth limits; flag fatigue service.
5. Is it a service crack at an undercut? → Now crack rules apply (Section 6.1): MT/PT/UT as appropriate, plus root-cause on the undercut geometry.

Interaction with Welding and Service

• Welding onto laminated plate can open laminations or enable lamellar tearing under the weld.
• Seams in pipe may be mistaken for weld defects if longitudinal seam-welded pipe is confused with seamless—know product form.
• Laps left in forged pressure parts become preferential corrosion or crack sites.
• Undercut plus cyclic pressure or vibration is a classic fatigue pairing for vessels, bridges, and rotating equipment attachments.

Level III Documentation Angle

Procedures for plate receiving inspection often specify UT lamination scanning (grid or full coverage per specification). Bar and billet may require MT or ET. Weld visual criteria always address undercut. When writing acceptance criteria references, cite the product or construction code tables for maximum undercut depth/length and for plate UT quality levels—do not invent numeric limits on the exam if not provided; know that limits exist and that undercut is geometric acceptance, not merely "indication amplitude."

Exam traps: Using only RT to clear plate for laminations; calling a mid-plane UT reflector a "porosity cluster" without orientation logic; treating undercut as irrelevant because "no crack yet"; equating seams with fatigue cracks without origin context. Orientation and process origin keep these four discontinuity types straight.