What Is Lamellar Tearing? Plate Welding
Lamellar tearing is a type of cracking that occurs in the base metal beneath or adjacent to welded joints, typically in rolled steel plates loaded in the through-thickness (Z) direction. The tearing follows the plane of non-metallic inclusions (primarily manganese sulfide stringers) that become elongated during the rolling process. The crack propagates in a step-like pattern parallel to the plate surface, making it a subsurface defect that is difficult to detect visually.
How Lamellar Tearing Forms
During welding, the weld metal shrinks as it cools, generating tensile stress perpendicular to the plate surface (through-thickness direction). If the plate contains planar sulfide inclusions aligned parallel to the surface, these inclusions act as internal planes of weakness. The shrinkage stress tears the steel along these inclusion planes, creating a terraced, staircase-shaped crack beneath the weld.
Susceptible Joint Configurations
| Joint Type | Risk Level | Reason |
|---|---|---|
| T-joint (full penetration) | High | Weld shrinkage pulls directly through the plate thickness |
| Corner joint | High | Through-thickness stress on the restrained plate |
| Cruciform joint | Very high | Concentrated restraint from welds on both sides |
| Nozzle-to-shell weld | Moderate to high | Radial shrinkage stresses on shell plate |
| Butt joint | Low | Shrinkage acts in-plane, not through-thickness |
Key Factors
| Factor | Effect |
|---|---|
| Sulfur content | High sulfur (>0.010%) increases inclusion volume and tearing susceptibility |
| Inclusion shape | Elongated MnS stringers are the primary culprit |
| Through-thickness ductility | Reduction of area (Z-direction) below 15-20% indicates poor resistance |
| Plate thickness | Thicker plates (>25 mm) are more susceptible |
| Restraint level | Highly restrained joints concentrate through-thickness stress |
| Weld size | Larger welds generate higher shrinkage forces |
Prevention Strategies
Key prevention methods include:
- Material selection: Use low-sulfur steel (S less than 0.005%) or Z-quality plates per EN 10164 for critical joints
- Joint design: Use balanced weld preparations (double-sided welds) to reduce one-sided shrinkage stress
- Buttering: Apply one or two layers of weld metal on the susceptible plate surface before assembling the joint; the ductile butter layers absorb shrinkage strain
- Preheat: Reduces the cooling rate and residual stress, though it alone does not eliminate the risk
- Welding sequence: Deposit smaller passes and allow stress relaxation between passes
Lamellar tearing is a base-metal defect caused by the combination of material susceptibility and joint design, not by the welder’s technique. It is detected by ultrasonic testing (UT) of the plate before welding (to identify existing laminations) and after welding (to detect tearing). For structural and pressure vessel applications involving thick carbon steel plates, through-thickness testing of the base material is a standard preventive measure.
Leave a Comment
Have a question or feedback? Send us a message.