Z Direction Steel Plate: The Practical Guide to Preventing Lamellar Tearing (Z15 / Z25 / Z35)

In offshore modules, bridges, wind towers, and heavy industrial structures, the most expensive weld failures are often the ones that start inside the base metal—out of sight, under a perfectly acceptable weld bead. Lamellar tearing is a classic example. It typically occurs when thick plate is used in highly restrained joints, and weld shrinkage loads the steel through its thickness (the “Z direction”).

That’s where Z direction steel plate (also called through-thickness or Z-quality plate) becomes a procurement requirement rather than an upgrade. Specifying the right Z class can prevent rework, missed delivery windows, and inspection disputes—without overpaying for a Z level your joint detail doesn’t actually need.

If you’re building an RFQ package for plate supply, start with LYH Steel’s plate categories such as Carbon Steel Plate (https://lyhsteel.com/carbon-steel-plate/) and Shipbuilding Steel Plate (https://lyhsteel.com/shipbuilding-steel-plate/). For inspection scope and third-party coordination, see Quality Inspection (https://lyhsteel.com/quality-inspection/). When you’re ready, send project specs through https://lyhsteel.com/contact-us/.

1) What Is Z Direction (Through-Thickness) Steel Plate?

Rolled steel plate is not perfectly uniform in every direction. Most mechanical properties are typically verified along the plate surface—in the rolling direction (X) and transverse direction (Y). However, certain welded joints impose strain perpendicular to the plate surface (Z direction). In the Z direction, steel can be more vulnerable to cracking because of the way inclusions are elongated during rolling.

Z direction steel plate is not a different steel grade.
It is a performance requirement applied to a base grade to ensure improved deformation capacity through thickness—primarily to reduce lamellar tearing risk in heavy weldments.

The best-known framework for Z-quality in structural plate sourcing is EN 10164, which specifies through-thickness deformation properties and acceptance criteria based on reduction of area (RA) in through-thickness tensile testing. A publicly accessible standard preview is available here: https://preview.sist.si/sist-preview/41580/b098216690104743bf1790272409a600/SIST-EN-10164-2018.pdf.

2) Lamellar Tearing Explained: Why It Happens in Thick Welded Joints

Lamellar tearing is a form of cracking that develops in the base metal, typically parallel to the plate surface, beneath or adjacent to welds. It is most commonly associated with three conditions occurring together:

  1. Thick plate (risk increases as thickness and restraint increase; many projects begin paying attention around the 40–50 mm range, but joint design matters more than a single number)

  2. Highly restrained weld details, such as T-joints, corner joints, cruciform joints, or heavy fillet welds where the parts cannot relieve shrinkage strain

  3. Weld shrinkage strain acting through thickness, often amplified by multi-pass welding and large weld volumes

The material susceptibility component is often linked to non-metallic inclusions elongated by rolling—classically manganese sulfide “stringers”—which reduce ductility in the short-transverse direction. A concise, engineering-focused overview of the phenomenon and mitigation hierarchy is discussed in the Steel Construction Institute guidance note on through-thickness properties: https://www.steelconstruction.info/images/b/b7/GN_3-02.pdf.

Procurement takeaway: Lamellar tearing is not “a weld problem” alone. It is an interaction between joint detail + restraint + plate through-thickness ductility.

3) How Z-Quality Is Measured: Reduction of Area (RA) in Through-Thickness Tensile Testing

EN 10164 defines Z-quality by reduction of area (RA) from tensile tests conducted on specimens oriented through thickness (Z direction). EN 10164 references tensile testing methodology aligned with EN ISO 6892-1; the ISO standard entry is here: https://www.iso.org/standard/78322.html.

In American specification language, buyers may also see ASTM A770 / A770M, which explicitly describes through-thickness tension testing and acceptance criteria as a way to measure resistance to lamellar tearing: https://www.astm.org/a0770_a0770m-03r18.html.

Why RA matters: RA is a practical indicator of how much deformation the steel can accommodate through thickness before fracture—exactly the direction that matters when weld shrinkage pulls the plate “apart” internally.

4) Z15 vs Z25 vs Z35: What the Numbers Mean (and How to Choose)

Under EN 10164, Z classes are defined by minimum RA requirements (average of three tests plus an individual minimum). In simplified procurement language:

  • Z15: RA ≥ 15% (average), with a minimum individual requirement

  • Z25: RA ≥ 25% (average), with a minimum individual requirement

  • Z35: RA ≥ 35% (average), with a minimum individual requirement

For the exact acceptance rules, including individual minimum values and sampling, reference the EN 10164 standard preview: https://preview.sist.si/sist-preview/41580/b098216690104743bf1790272409a600/SIST-EN-10164-2018.pdf.

A practical selection matrix (engineering judgement still required)

Use the matrix below as a first-pass screening—then confirm with the fabricator’s joint detail and restraint level:

  • Z15 is often sufficient for lighter welded structures, thinner sections, and low-restraint details

  • Z25 is commonly specified for general structural welding with moderate thickness and restraint

  • Z35 is typically chosen for critical joints, high-restraint nodes, offshore/bridge nodes, seismic-sensitive structures, and thicker plate with large weld volumes

A useful caution from SCI guidance: specifying Z-quality “by default” can add cost without improving outcomes if the joint detail doesn’t generate significant through-thickness strain. That guidance is discussed here: https://www.steelconstruction.info/images/b/b7/GN_3-02.pdf.

5) Clean Steelmaking and What It Really Means (LF/VD and Inclusion Control)

High Z-quality generally requires “cleaner” steel: lower sulfur, controlled inclusion morphology, and more consistent through-thickness ductility. In practical mill routes, buyers will often hear terms such as LF (ladle furnace) and VD (vacuum degassing), plus other secondary metallurgy steps. These processes support:

  • lower dissolved gases

  • improved chemistry control

  • inclusion modification and reduced harmful inclusion stringers

Important procurement note: Do not rely on marketing phrases like “near-zero sulfur.” Sulfur targets vary by mill route, base grade, and project specification. What matters is that the MTC clearly reports sulfur and Z-test results, and that the Z-quality class required is stated explicitly (EN 10164 Z15/Z25/Z35 or equivalent acceptance criteria).

6) Inspection and Documentation: What You Must Specify to Avoid Disputes

A) Through-thickness (Z) test results

Your Mill Test Certificate should clearly show:

  • the Z class (e.g., EN 10164 Z25 or Z35)

  • the reduction of area (RA) results for the through-thickness tensile tests

  • sampling details if your project requires specific locations (e.g., flange region for sections, thickness limits, etc.)

B) Ultrasonic testing (UT): internal soundness is related—but not identical

UT is used to detect laminations and internal discontinuities. For plate sourcing, common frameworks include:

UT helps confirm internal soundness, but it does not replace Z-ductility verification. In many projects, the most robust approach is to specify both:

  • a Z-quality requirement (EN 10164 Z class or ASTM A770 acceptance), and

  • a UT acceptance class (EN 10160 or ASTM A578 level/class) aligned with project NDT requirements

If you want to align inspection scope before ordering, LYH Steel’s Quality Inspection overview is designed exactly for this planning step: https://lyhsteel.com/quality-inspection/.

7) Engineering Best Practices: Prevent Lamellar Tearing Beyond Material Selection

Z-quality is powerful, but it is not a substitute for good fabrication engineering. The highest-ROI mitigation actions usually come from design and welding procedure control:

1) Optimize joint design (reduce through-thickness strain)

  • redesign node details to reduce restraint where feasible

  • limit large single-pass fillets in thick plate nodes

  • consider transition pieces or forged nodes for complex connections (common in offshore nodes)

2) Control weld volume and sequence (shrinkage management)

  • reduce weld volume where design allows

  • balance weld sequences to reduce peak restraint stress

  • avoid “overmatching” practices that increase through-thickness stress concentration in susceptible details (SCI guidance discusses this effect in the context of lamellar tearing risk): https://www.steelconstruction.info/images/b/b7/GN_3-02.pdf.

3) Manage weldability risk on thicker sections

When thick carbon/low-alloy plate is involved, weldability management (preheat strategy, hydrogen control, WPS discipline) becomes part of risk control. A quick procurement check many teams use early is carbon equivalent (CE). LYH Steel provides a CE (IIW) calculator for initial discussion and WPS alignment: https://lyhsteel.com/carbon-equivalent-calculator/.

8) Sourcing “Red Flags” Buyers Should Catch Before Ordering

If you’re procuring Z-quality plate for critical infrastructure, treat these as non-negotiable checkpoints:

  1. Z class clearly stated (EN 10164 Z25/Z35, or ASTM A770 acceptance criteria)

  2. Z-test RA results reported on the MTC (not just standard tensile results in X/Y directions)

  3. UT acceptance clearly defined (EN 10160 class or ASTM A578 acceptance) with documentation

  4. Chemistry transparency (especially sulfur reporting) and traceability to heat number

  5. Inspection plan agreed (pre-shipment inspection, third-party witness, container loading supervision where required)

This is exactly where many projects lose time—so align it early. LYH Steel’s inspection workflow is designed to reduce the “late clarification” loop: https://lyhsteel.com/quality-inspection/.

9) RFQ Checklist (Copy/Paste) for Z-Direction Plate

If you want accurate pricing and fewer back-and-forth emails, include these items in your RFQ:

  1. Base grade + standard (EN/ASTM/JIS) and any approved equivalents

  2. Plate thickness / width / length + tolerances

  3. Z-quality requirement: EN 10164 Z15 / Z25 / Z35 (or ASTM A770 acceptance criteria)

  4. UT requirement: EN 10160 class or ASTM A578 level/class (state project requirement)

  5. Application & joint detail: T-joint / cruciform / node / heavy fillet, plus restraint notes if known

  6. Documentation: EN 10204 3.1 MTC, RA results, UT report, plus third-party inspection if required

  7. Processing: cut-to-size / profiling / bevel needs (LYH processing includes Laser Cutting https://lyhsteel.com/laser-cutting/ and Shearing https://lyhsteel.com/shearing/)

  8. Packaging: export packing, corrosion protection, marking requirements

  9. Delivery: destination port, Incoterms, shipment split preferences, required lead-time window

To speed up quotation and technical confirmation, submit your RFQ via https://lyhsteel.com/contact-us/ and reference the plate form you need from the Products hub: https://lyhsteel.com/products/.

Why Buyers Use LYH Steel for Z-Quality Plate Projects

Z-direction requirements are easy to write and easy to misinterpret. The cost isn’t only the plate premium—it’s the downstream cost of weld repair, UT rejection, schedule disruption, and re-fabrication if the Z class or inspection scope is ambiguous.

LYH Steel supports Z-quality projects by aligning:

FAQ

1) What is Z direction steel plate?
It is plate specified with improved deformation properties perpendicular to the surface (through thickness), commonly ordered under EN 10164 Z-quality classes to reduce lamellar tearing risk in heavily welded joints. For the EN 10164 framework, see the standard preview: https://preview.sist.si/sist-preview/41580/b098216690104743bf1790272409a600/SIST-EN-10164-2018.pdf.

2) What causes lamellar tearing?
Lamellar tearing typically results from a combination of high through-thickness strain from weld shrinkage, a restrained joint detail, and plate susceptibility linked to limited short-transverse ductility. A concise technical discussion is available in the SCI guidance note: https://www.steelconstruction.info/images/b/b7/GN_3-02.pdf.

3) How do I choose Z15 vs Z25 vs Z35?
As a first pass: Z15 for light/low-restraint welding, Z25 for common structural welding with moderate restraint, and Z35 for critical, highly restrained joints and thicker sections. Final selection should be confirmed against joint design and restraint level. EN 10164 defines the RA acceptance criteria for each class: https://preview.sist.si/sist-preview/41580/b098216690104743bf1790272409a600/SIST-EN-10164-2018.pdf.

4) Does UT replace Z testing?
No. UT (such as EN 10160 https://www.evs.ee/en/evs-en-10160-2000 or ASTM A578 https://www.astm.org/a0578_a0578m-17.html) checks internal discontinuities and laminations, but Z testing verifies through-thickness deformation capacity (RA) that relates directly to lamellar tearing resistance.

5) What should I see on the Mill Test Certificate for Z-quality plate?
You should see the Z class (e.g., EN 10164 Z25/Z35), the through-thickness RA test results, and any UT acceptance information required by your RFQ. If your project uses ASTM language, ASTM A770 describes through-thickness tension testing and acceptance for lamellar tearing resistance: https://www.astm.org/a0770_a0770m-03r18.html.

6) Is Z-quality plate “always necessary” for thick plate?
Not always. Thickness increases risk, but joint design and restraint drive whether through-thickness strain becomes critical. Over-specifying Z-quality can add cost without improving outcomes. The SCI guidance note provides a practical decision context: https://www.steelconstruction.info/images/b/b7/GN_3-02.pdf.

7) How do I get a fast, accurate quote from LYH Steel?
Send base grade + thickness + plate dimensions + required Z class + UT requirement + destination/Incoterms via https://lyhsteel.com/contact-us/. If you include the RFQ checklist items above, you’ll typically reduce clarification cycles and get a more approval-ready quotation.

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