6.3 Lamellar Tearing

6.3 Lamellar Tearing

Quick Answer: Lamellar tearing is cracking in the base metal on planes parallel to the plate surface, caused by through-thickness tensile strain acting on bands of non-metallic inclusions or segregation. Classic risks are T-joints and corner joints welded to the plate face. Prevent with Z-quality steel, better design, and buttering when specified.

WT2.5 includes lamellar tearing because inspectors who only look for “weld cracks” will miss a base-metal failure mode that is driven by joint design and plate quality as much as by welding parameters.

What Lamellar Tearing Is

Rolled steel plate is not isotropic. Non-metallic inclusions (notably elongated sulphides and other stringers) and segregation bands tend to lie in planes parallel to the rolling direction and plate surface. When a large tensile strain acts through the thickness of the plate (the short-transverse direction), those weak planes can separate. The crack grows in a characteristic stepped or terrace-like path—hence “lamellar.”

Key distinctions:

• Location is primarily parent plate, often just beneath the weld fusion zone where through-thickness stress is high
• Path follows inclusion planes, not necessarily the weld centreline solidification structure
• Initiation is tied to welding residual stress / shrinkage strain from attached welds, not to a liquid weld pool film (so it is not solidification cracking)
• Hydrogen may aggravate cracking in steels generally, but the defining feature of lamellar tearing is through-thickness ductility deficiency of the plate product

Why T-Joints and Corner Joints Are Critical

Imagine a thick plate used as a flange or node. A stem plate is fillet- or butt-welded onto the face of the flange plate. As the weld cools, shrinkage pulls on the flange plate through its thickness. If the flange plate has poor short-transverse ductility, lamellar tears open under that pull.

High-risk geometries include:

1. T-joints with full or heavy welds attaching a member normal to the plate surface
2. Corner joints that load the plate through thickness
3. Nodes and stiffener ends on thick plate with large weld volumes
4. Repair welds that add large deposits onto already restrained details
5. Configurations where the loaded plate is thick, highly restrained, and of ordinary structural quality without improved through-thickness properties

Low-risk situations include thin plate, low restraint, small welds, and joints loaded mainly in the rolling plane rather than through thickness.

Material Factors

Through-thickness ductility depends on:

• Inclusion content and morphology (sulphur level, inclusion shape control)
• Segregation from casting and rolling practice
• Plate thickness and reduction ratio
• Steelmaking route (modern clean steels perform better than older high-inclusion plates)

Z-quality steels (improved through-thickness properties; often specified with a Z reduction-of-area requirement in the short-transverse direction under product standards such as those in the EN 10164 family of concepts) are the usual material solution when design demands high through-thickness strain capacity. The inspector’s job is not to recalculate Z-values on site, but to verify that the plate grade and certificates match the specification when Z-quality is required, and that plate is not substituted with ordinary stock.

Plate orientation also matters: the face that receives the weld must be the face intended by design. Mis-oriented plate or using plate edges incorrectly can change which direction sees through-thickness strain.

Prevention Strategies

1. Material selection

• Specify improved through-thickness ductility (Z-quality or equivalent project specification) for plates that will be loaded through thickness by welding
• Prefer clean, modern steels with controlled sulphur and inclusion shape where the design standard allows

2. Joint design

• Reduce weld size and volume to the structural minimum required
• Redesign details so shrinkage does not pull heavily through the thickness of a susceptible plate (e.g. change attachment geometry, use soft toe details as designed by engineering)
• Avoid stacking large welds on both sides of a thin ligament of susceptible plate without review
• Sequence fabrication to avoid locking in maximum restraint before large welds are made, where the procedure allows

3. Buttering

Buttering deposits weld metal on the plate surface (often with a ductile consumable compatible with the procedure) before the final joint is completed. The buttering layer can:

• Place a more ductile, cleaner metal in the highest strain region
• Allow intermediate NDT of the buttered surface before the closing welds
• Sometimes be combined with stress-relief steps if the WPS/engineering specification requires them

Inspectors verify buttering is done with the qualified consumable and sequence, and that any hold points for NDT between buttering and final welding are observed.

4. Process and restraint control

Lower heat input is not a universal cure—and excessive restraint is always unhelpful. Follow the engineered WPS: some details use balanced welding, peening only if explicitly allowed, or PWHT for residual stress where the code and thickness require it. Improvised “more preheat” without design change does not convert a high-inclusion plate into Z-quality steel.

Inspection Awareness

Before welding

• Identify T, corner, and node details on drawings that impose through-thickness strain
• Confirm plate specification (including any Z-requirement) and material certificates
• Confirm buttering or special sequence notes on the WPS/ITP

During / after welding

• Watch for surface-breaking steps or cracks in the base metal near the toe, not only cracks in weld metal
• Ensure NDT methods and coverage include the base metal adjacent to the joint when the risk is lamellar tearing—not only the weld bead centreline
• Ultrasonic testing is often important for subsurface terrace cracks; surface methods alone may miss internal steps
• Treat any through-thickness crack indication in plate under a T-joint as a serious structural non-conformance requiring engineering disposition

Common confusion to avoid

Fitness and Reporting

Lamellar tears reduce section integrity and can grow under residual or service stress. The IWI-S reports location, orientation, and joint detail; recommends the NDT extent defined by the quality plan; and stops progressive welding on sister joints if a systematic plate-quality or design problem is suspected. Repair may require gouging to sound metal, redesigned joint detail, or plate replacement—decisions for welding engineering, not ad-hoc fill.

Summary

Lamellar tearing is a base-metal, through-thickness cracking mode controlled by inclusions/segregation + through-thickness strain + residual welding stress. Master the T-joint story, Z-quality steel, design relief, and buttering, and you will answer WT2.5 lamellar items and spot high-risk details during document review and VT planning.