Z Direction Steel Plate (Z15 / Z25 / Z35): Preventing Lamellar Tearing
Z direction steel plate, also called through-thickness or Z-quality steel plate, is specified where welded joints impose significant strain perpendicular to the plate surface. Selecting the correct Z class can reduce lamellar tearing risk in offshore structures, bridges, wind towers and other highly restrained heavy welded structures.
Why Z Direction Steel Plate Matters
In offshore modules, bridges, wind towers and heavy industrial structures, some of the most expensive weld failures can begin inside the base metal, beneath an otherwise acceptable weld bead. Lamellar tearing is one of the classic examples.
It typically develops when thick plate is used in highly restrained welded joints and weld shrinkage loads the material through its thickness—the Z direction.
That is where Z-quality plate becomes a procurement requirement rather than simply an upgrade. Specifying the appropriate Z class can help prevent rework, inspection disputes and schedule disruption without automatically paying for a higher Z level than the joint actually needs.
1. What Is Z Direction (Through-Thickness) Steel Plate?
Rolled steel plate does not behave identically in every direction. Most conventional mechanical properties are verified along the plate surface—in the longitudinal rolling direction and transverse direction.
Some welded joints, however, impose strain perpendicular to the plate surface. This is the Z direction, also known as the short-transverse or through-thickness direction.
Through thickness, steel can be more susceptible to cracking because non-metallic inclusions may become elongated during rolling and reduce ductility in this direction.
A widely used framework is EN 10164, which specifies through-thickness deformation properties and acceptance criteria based on reduction of area in through-thickness tensile testing.
Reference: EN 10164 standard preview .
2. Lamellar Tearing Explained: Why It Happens
Lamellar tearing is cracking that develops in the base metal, generally parallel to the plate surface beneath or adjacent to a weld.
It is most commonly associated with several conditions acting together:
Thick Plate
Risk tends to increase as plate thickness and structural restraint increase. Joint geometry matters more than a single thickness threshold.
High Restraint
T-joints, corner joints, cruciform joints and heavy fillet welds can prevent components from relieving weld shrinkage strain.
Through-Thickness Strain
Multi-pass welding and large weld volumes can create significant shrinkage strain perpendicular to the plate surface.
Material susceptibility is also associated with elongated non-metallic inclusions—classically manganese sulfide stringers—which reduce short-transverse ductility.
An engineering overview of lamellar tearing and through-thickness properties is available in the Steel Construction Institute guidance note .
3. How Z-Quality Is Measured
EN 10164 defines Z-quality using reduction of area (RA) from tensile specimens oriented through the plate thickness.
Reduction of area indicates how much deformation the steel can accommodate before fracture in the direction that becomes critical when weld shrinkage pulls on the plate through thickness.
EN 10164 references tensile-testing methodology aligned with EN ISO 6892-1. The ISO standard entry is available here: ISO 6892-1 .
Buyers working with American specifications may also encounter ASTM A770 / A770M, which covers through-thickness tension testing and acceptance criteria associated with lamellar tearing resistance.
Reference: ASTM A770 / A770M .
4. Z15 vs Z25 vs Z35: What the Classes Mean
Under EN 10164, the Z classes are differentiated by minimum through-thickness reduction-of-area requirements.
| Z Class | Average RA | Typical Procurement Context | Selection Consideration |
|---|---|---|---|
| Z15 | ≥ 15% | Lighter structural applications and lower-restraint details | Use only where engineering assessment supports the lower requirement |
| Z25 | ≥ 25% | General structural welding with moderate restraint | Common middle-ground specification for many welded structures |
| Z35 | ≥ 35% | Critical, highly restrained or thick welded joints | Often selected for offshore, bridge and other demanding nodes |
The values above describe simplified procurement-level class distinctions. Exact individual minimum values, testing frequency and sampling requirements should be confirmed against the applicable EN 10164 edition and the project specification.
5. Clean Steelmaking and Inclusion Control
Higher through-thickness ductility generally requires controlled steelmaking practices, including low sulfur levels, improved chemistry control and better inclusion morphology.
In mill production routes, buyers may encounter secondary metallurgy processes such as:
- LF — Ladle Furnace for chemistry and temperature control.
- VD — Vacuum Degassing for gas removal and refining.
- Inclusion modification and cleanliness control.
- Sulfur control appropriate to the required grade and Z-quality level.
6. Inspection and Documentation Requirements
A. Through-Thickness Test Results
Your Mill Test Certificate should clearly identify:
- The specified Z class, such as EN 10164 Z25 or Z35.
- The measured through-thickness reduction-of-area results.
- Applicable sampling information where required by the project.
- Heat-number traceability.
B. Ultrasonic Testing
Ultrasonic testing is used to identify laminations and other internal discontinuities in plate.
Common plate-testing references include:
- EN 10160 — ultrasonic testing of flat steel products.
- ASTM A578 / A578M — straight-beam ultrasonic examination of rolled steel plates.
To define inspection scope before placing an order, review LYH Steel's Steel Quality Inspection service .
7. Engineering Best Practices for Preventing Lamellar Tearing
Z-quality plate is an important material-control measure, but it does not replace good joint design and welding engineering.
Optimize Joint Design
- Reduce restraint where practical.
- Avoid unnecessary concentration of weld shrinkage strain through thickness.
- Review heavy T-joints, cruciform joints and node details carefully.
- Consider transition pieces or forged nodes where appropriate.
Control Weld Volume and Sequence
- Reduce unnecessary weld volume where design permits.
- Use balanced welding sequences to manage shrinkage.
- Coordinate WPS and fabrication planning with structural restraint.
Manage Weldability of Thick Plate
Thick carbon and low-alloy plate may also require careful preheat, hydrogen control and WPS management.
Carbon equivalent is often used as an early weldability screening parameter. LYH Steel provides a Carbon Equivalent Calculator for initial CE / IIW calculations.
8. Sourcing Red Flags Buyers Should Catch Before Ordering
For critical Z-quality plate procurement, verify these items before issuing the final purchase order:
- Z class clearly stated — EN 10164 Z15, Z25 or Z35, or project-approved equivalent criteria.
- RA results included — do not accept ordinary X/Y tensile results as a substitute for through-thickness testing.
- UT requirement clearly defined — including the applicable standard and acceptance class.
- Chemistry transparency — including sulfur and heat-number traceability.
- Inspection plan agreed before production — including third-party witnessing where required.
LYH Steel's Quality Inspection workflow can be used to align inspection scope, documentation and pre-shipment requirements before ordering.
9. RFQ Checklist for Z-Direction Steel Plate
Include the following information in your RFQ to reduce clarification cycles and obtain a more accurate technical quotation.
-
Base steel grade and standard
State EN, ASTM, JIS or other required standard and approved equivalents. -
Dimensions
Plate thickness, width, length and required tolerances. -
Z-quality requirement
Specify EN 10164 Z15, Z25 or Z35, or applicable ASTM A770 acceptance criteria. -
Ultrasonic testing requirement
State EN 10160 class or ASTM A578 acceptance requirement. -
Application and joint detail
T-joint, cruciform joint, structural node, heavy fillet weld or other configuration. -
Required documentation
EN 10204 3.1 MTC, Z-test results, UT reports and third-party inspection requirements. -
Processing
State cut-to-size, profiling and preparation requirements. See Laser Cutting and Shearing . -
Export packaging
Include marking, corrosion protection and packaging requirements. -
Delivery terms
Destination port, Incoterms, required delivery window and shipment split requirements.
10. Why Buyers Use LYH Steel for Z-Quality Plate Projects
Z-direction requirements can appear simple on a purchase specification, but problems arise when the Z class, UT requirement, documentation or fabrication requirements are not aligned before production.
Plate Sourcing
Start with Carbon Steel Plate or Shipbuilding Steel Plate .
Quality Verification
Coordinate documentation, testing and third-party inspection through Quality Inspection .
Processing Support
Reduce downstream fabrication work with Laser Cutting and Shearing .
For project-specific plate dimensions, Z-class requirements, UT acceptance criteria and delivery requirements, contact LYH Steel .
Frequently Asked Questions
1. What is Z direction steel plate?
Z direction steel plate is steel plate specified with improved deformation properties perpendicular to the plate surface. It is commonly ordered using EN 10164 Z-quality classes to reduce lamellar tearing risk in heavily restrained welded joints.
2. What causes lamellar tearing?
Lamellar tearing generally results from the combination of through-thickness strain generated by weld shrinkage, structural restraint and limited short-transverse ductility of the plate.
3. What is the difference between Z15, Z25 and Z35?
Under EN 10164, the classes correspond to progressively higher through-thickness reduction-of-area requirements. Z15 has a 15% average RA requirement, Z25 has 25% and Z35 has 35%, subject to the detailed acceptance and individual-minimum criteria in the standard.
4. Does ultrasonic testing replace Z testing?
No. Ultrasonic testing evaluates internal discontinuities and laminations, while Z testing measures through-thickness deformation capacity. Critical projects may require both.
5. What should appear on a Z-quality plate Mill Test Certificate?
The MTC should identify the required Z class, the corresponding through-thickness reduction-of-area results, applicable chemistry and heat-number traceability, together with UT information where required.
6. Is Z-quality plate always necessary for thick plate?
No. Plate thickness is one risk factor, but joint geometry, restraint, weld volume and resulting through-thickness strain are also critical. The appropriate requirement should be confirmed by the project engineer or fabricator.
7. How can I obtain a quotation for Z15, Z25 or Z35 plate?
Send the base grade, plate dimensions, Z class, UT requirement, documentation requirements, destination and Incoterms through the LYH Steel contact page .
Technical References
- EN 10164 standard preview: Steel products with improved deformation properties perpendicular to the surface
- Steel Construction Institute: Through-thickness properties and lamellar tearing guidance
- ISO: ISO 6892-1 tensile testing
- ASTM: ASTM A770 / A770M
- EN 10160: Ultrasonic testing of flat steel products
- ASTM: ASTM A578 / A578M
Need Z15, Z25 or Z35 Steel Plate for Your Project?
Send your steel grade, plate dimensions, required Z class, UT standard, inspection requirements and destination. LYH Steel can review the sourcing and documentation requirements before quotation.