Z-Quality Steel Plate Explained: Difference Between Z15, Z25 and Z35
Most steel plate specifications focus on tensile strength, yield strength, and toughness — properties measured in the rolling direction.
But in thick plate welded structures, that's not always enough.
When a steel plate is subjected to tensile stress in the thickness direction (Z-direction), internal defects such as sulfide inclusions can lead to lamellar tearing — a serious welding failure that can compromise the integrity of the entire structure.
That's why Z-quality steel plates were developed — to improve through-thickness performance and reduce the risk of lamellar tearing in heavy welded structures.
1. What Is Z-Quality Steel Plate?
Z-quality steel plate refers to steel plates with enhanced through-thickness properties. Unlike standard steel plates, Z-quality plates are tested in the thickness direction to evaluate their resistance against internal defects and lamellar tearing.
- ✔ Tested in the Z-direction (through thickness)
- ✔ Measured by reduction of area after tensile testing
- ✔ Designed to resist lamellar tearing in welded joints
- ✔ Commonly specified for thick plates and heavy fabrication
💡 In simple terms: Z-quality is about how well a steel plate performs when pulled from top to bottom — not just along its length or width.
2. Why Are Z-Quality Steel Plates Required?
Z-quality steel plates are not required for every project. But in certain applications, they are essential.
1. Reduce Risk of Lamellar Tearing
During welding — especially in thick plates and highly restrained joints — welding shrinkage, residual stress, and thickness-direction stress can combine to cause internal tearing. Z-quality plates reduce this risk significantly.
2. Improve Reliability of Heavy Welded Structures
For critical applications like pressure vessels, offshore structures, bridges, and heavy machinery, the cost of failure is far higher than the cost of upgrading to Z-quality steel.
⚠️ What we've seen: A shipyard once ordered standard steel plates for a high-restraint welded joint. The plates had no Z-quality requirement. During welding, lamellar tearing occurred. The repair cost exceeded the price difference of upgrading to Z25. That's the kind of mistake Z-quality is designed to prevent.
3. What Do Z15, Z25 and Z35 Mean?
The number in Z15, Z25, and Z35 represents the minimum percentage reduction of area measured in a through-thickness tensile test.
This is not a strength grade. It's a ductility measure in the thickness direction. Higher numbers mean better resistance to lamellar tearing.
4. Difference Between Z15, Z25 and Z35 Steel Plates
📌 Z15 — Basic Through-Thickness Performance
- Minimum 15% reduction of area in Z-direction
- Suitable for moderate requirements
- Applications: General welded structures, machinery bases
📌 Z25 — Improved Resistance to Lamellar Tearing
- Minimum 25% reduction of area in Z-direction
- Commonly selected for heavy welding joints with moderate restraint
- Applications: Pressure equipment, large welded structures, pressure vessel steel plates
📌 Z35 — Highest Through-Thickness Performance
- Minimum 35% reduction of area in Z-direction
- Used for critical applications where failure is unacceptable
- Applications: Offshore platforms, large pressure vessels, heavy industrial equipment
5. Which Steel Grades Can Be Supplied with Z-Quality Requirements?
Z-quality requirements can be applied to various carbon steel and low-alloy steel plates. The most common grades include:
- Q345R — Pressure vessel steel
- Q355 — Structural steel
- SA516 Grade 70 — ASME pressure vessel steel
- Q690 — High-strength steel
- Shipbuilding steel plates (AH36/DH36/EH36)
💡 Note: Not all steel grades are available with Z-quality. Always confirm with your supplier whether the required grade and thickness can be supplied with Z15, Z25, or Z35.
6. Z-Quality Steel Plate Standards and Testing
Z-quality requirements are governed by specific standards depending on the region and project.
Common standards include:
- EN 10164: European standard for through-thickness properties
- ASTM A770: American standard for Z-direction testing
- Customer-specific requirements: Many EPCs and classification societies have their own Z-quality requirements
Testing method:
- Through-thickness tensile test (Z-direction tensile test)
- Ultrasonic testing (UT) — often required in addition to Z-quality to detect internal defects
7. When Should Buyers Request Z15, Z25 or Z35?
Not every project needs Z-quality steel. But when the following factors are present, it's worth considering:
- Plate Thickness: Thicker plates have higher risk of internal inclusions and lamellar tearing. Generally, plates above 40mm should be evaluated for Z-quality requirements.
- Welding Structure: High-restraint welded joints — such as T-joints, corner joints, and thick plate butt welds — benefit from Z-quality.
- Service Conditions: Pressure equipment, offshore structures, and critical load-bearing applications require higher reliability.
📌 General guideline: Z15 for basic needs, Z25 for most heavy welding applications, Z35 for critical structures where failure is not an option.
8. Z-Quality Steel Plate Buying Considerations
Before placing an order for Z-quality steel plates, confirm the following:
- ✓ Steel Grade: e.g., Q345R, SA516 Grade 70
- ✓ Thickness: Z-quality is typically required for plates ≥ 40mm
- ✓ Z-Quality Level: Z15, Z25, or Z35
- ✓ UT Requirements: Ultrasonic testing level and acceptance criteria
- ✓ Applicable Standard: EN 10164, ASTM A770, or project-specific
- ✓ Inspection Documents: MTC, third-party inspection reports
🛑 Red flag: If a supplier quotes Z-quality but cannot confirm the exact Z-grade on the MTC, proceed with caution. Z-quality testing must be documented on the certificate — it cannot be added after production.
9. Frequently Asked Questions
❓ What does Z25 mean in steel plate?
Z25 means the steel plate has a minimum of 25% reduction of area in the thickness direction (Z-direction) during tensile testing. It indicates improved resistance to lamellar tearing compared to Z15.
❓ What is the difference between Z15, Z25 and Z35?
The difference is the minimum reduction of area in the thickness direction: Z15 ≥ 15%, Z25 ≥ 25%, Z35 ≥ 35%. Higher numbers mean better through-thickness ductility and higher resistance to lamellar tearing.
❓ Why is Z-quality steel used in pressure vessels?
Pressure vessels often have thick welded joints subjected to high internal pressure and thermal stress. Z-quality steel reduces the risk of lamellar tearing in these critical joints, ensuring long-term safety and reliability.
❓ Does thicker steel plate require Z-quality?
Not automatically, but thicker plates have higher risk of internal defects. Many engineering codes require Z-quality for plates above a certain thickness (typically ≥ 40mm) in critical applications.
❓ What is lamellar tearing in steel plates?
Lamellar tearing is a type of welding defect that occurs in the base metal near the weld joint, caused by through-thickness tensile stress. It is typically associated with thick plates and high-restraint welded joints. Z-quality steel is designed to reduce this risk.
❓ Can Q345R steel plates be supplied with Z-quality requirements?
Yes — Q345R pressure vessel steel can be supplied with Z15, Z25, or Z35 requirements, depending on the project specification. Always confirm with your supplier whether the required grade and thickness are available with Z-quality.
10. Final Thoughts
Z-quality steel plates provide improved reliability for heavy welded structures by enhancing through-thickness properties.
For pressure vessels, offshore equipment, and heavy fabrication projects, selecting the correct Z-grade can help reduce production risks and improve long-term performance.
💡 Our view: The cost difference between Z15 and Z25 is often small compared to the cost of repairing a lamellar tear. If your project involves thick plates and high-restraint welding, it's worth asking the question: "Should we specify Z-quality?"
📚 Further Reading & Related Content
Need Z-Quality Steel Plates for Your Project?
CEHE STEEL supplies pressure vessel and structural steel plates with Z15, Z25, and Z35 requirements. Send us your specifications — we'll confirm availability and provide full MTC documentation.