Why Steel Plate Thickness Matters More Than You Think in Pressure Vessel Design
Two pressure vessel steel plates may have the same grade but different thicknesses. Does that mean they can be used interchangeably?
Not necessarily. Thickness affects vessel design, pressure resistance, corrosion allowance, fabrication, testing, and final acceptance.
This guide explains why pressure vessel steel plate thickness matters — and what buyers should confirm before ordering.
1. Steel Grade Is Only One Part of a Pressure Vessel Plate Specification
A buyer cannot simply specify "Q345R steel plate" or "SA516 Grade 70" and expect the correct material. The complete purchasing specification includes much more.
2. How Does Steel Plate Thickness Affect Pressure Vessel Design?
2.1 Internal Pressure
Pressure vessels require sufficient wall thickness to contain internal pressure safely. The required wall thickness depends on several factors:
- ✔ Design pressure
- ✔ Vessel diameter
- ✔ Allowable stress of the material
- ✔ Applicable design code
In general, higher design pressure or larger vessel diameter means greater required wall thickness. But the final thickness should be determined according to the applicable design code and engineering calculation — not simply by comparing steel grades.
3. Steel Plate Thickness and Corrosion Allowance
Corrosion allowance is an additional thickness added to the calculated required thickness to account for material loss during service.
Example: If the calculated required thickness is 18 mm and the design requires a 2 mm corrosion allowance, the nominal plate thickness needs to account for this additional allowance.
⚠️ Important: Corrosion allowance is a design requirement, not something a steel supplier should decide independently.
4. Does Thickness Affect the Mechanical Properties of Pressure Vessel Steel?
Yes — different standards may specify different mechanical property requirements for different thickness ranges.
- ✔ Yield strength — may decrease as thickness increases
- ✔ Tensile strength — range may vary by thickness
- ✔ Elongation — minimum requirements may differ
- ✔ Impact energy — requirements may vary
📌 Examples: Q345R has thickness-dependent yield strength requirements under GB/T 713. SA516 Grade 70 has specific mechanical property requirements that must be confirmed against the applicable ASTM/ASME version. 16MnDR is designed for low-temperature applications where impact toughness is a critical requirement.
5. Why Impact Testing Becomes Important for Certain Thickness and Applications
Impact testing is not the same as tensile testing. It evaluates the material's toughness at a specified temperature.
- ✔ Test temperature
- ✔ Impact energy (Joules)
- ✔ Specimen orientation
- ✔ Sampling location
- ✔ Applicable standard
📌 Note: 16MnDR is designed for applications where low-temperature toughness is an important requirement. It is not simply "better" than Q345R — it serves a different purpose.
6. Does Thicker Steel Plate Make Welding More Difficult?
Thicker plates can introduce additional fabrication considerations:
- ✔ Preheating
- ✔ Interpass temperature
- ✔ Welding procedure
- ✔ Hydrogen control
- ✔ Post-weld heat treatment (PWHT)
- ✔ Welding deformation and residual stress
⚠️ Important: Welding requirements depend on the material, thickness, welding process, and applicable welding procedure — not thickness alone.
7. Steel Plate Thickness and Ultrasonic Testing
Some pressure vessel projects specify ultrasonic testing (UT) to reduce the risk of internal discontinuities in plate.
- ✔ Internal discontinuities and laminations
- ✔ ASTM A578 — standard for straight-beam UT of steel plates
- ✔ Level A / Level B / Level C acceptance criteria
8. Why MTC Matters When Buying Pressure Vessel Steel Plate
When the plates arrive, the buyer should confirm:
- ✔ Chemical composition — C, Mn, Si, P, S, alloying elements
- ✔ Mechanical properties — yield, tensile, elongation
- ✔ Impact test — temperature and energy
- ✔ UT results — if required
- ✔ Traceability — heat number, plate number
💡 Key point: The MTC should be checked against the purchase specification — not simply filed away as a certificate.
9. A Simple Example: Choosing a Pressure Vessel Plate
Project Requirement:
Vessel type: Storage tank
Design pressure: X MPa
Design temperature: X °C
Material: Q345R
Calculated thickness: XX mm
Corrosion allowance: X mm
UT: Required
Impact test: According to project requirement
The buyer should not simply ask a steel supplier for "Q345R plate." Instead, provide the complete specification.
10. Common Mistakes When Purchasing Pressure Vessel Steel Plate
❌ Mistake 1: Only specifying the steel grade
The grade alone does not include thickness, delivery condition, or testing requirements.
❌ Mistake 2: Ignoring thickness tolerance
Nominal thickness is not the same as actual measured thickness.
❌ Mistake 3: Not confirming impact test requirements
Impact testing is often required for pressure vessel applications.
❌ Mistake 4: Forgetting UT requirements
If UT is required, specify it in the inquiry — don't leave it to the supplier to guess.
❌ Mistake 5: Not checking delivery condition
Normalized, as-rolled, and TMCP plates have different properties.
❌ Mistake 6: Comparing quotations without comparing specifications
Two quotes for "Q345R" may not include the same testing or delivery requirements.
❌ Mistake 7: Not checking the MTC carefully
The MTC should be checked against the purchase specification.
❌ Mistake 8: Assuming a thicker plate can automatically replace a thinner plate
Thicker is not always better — it affects weight, fabrication, welding, and design.
11. How to Specify Pressure Vessel Steel Plate Correctly
Standard:
Grade:
Thickness:
Width × Length:
Delivery Condition:
Impact Test:
UT:
Heat Treatment:
MTC:
Third-Party Inspection:
Quantity:
12. Pressure Vessel Steel Plate: What Should Buyers Check Before Ordering?
- ☐ Correct standard
- ☐ Correct grade
- ☐ Correct thickness
- ☐ Correct dimensions
- ☐ Delivery condition confirmed
- ☐ Impact test confirmed
- ☐ UT requirement confirmed
- ☐ MTC requirement confirmed
- ☐ Heat treatment requirement confirmed
- ☐ Inspection requirement confirmed
13. Frequently Asked Questions
❓ What determines pressure vessel steel plate thickness?
Design pressure, vessel diameter, allowable stress, corrosion allowance, and the applicable design code all determine the required thickness.
❓ Can a thicker steel plate replace a thinner plate?
Not automatically. Thicker plates affect weight, fabrication, welding, and design. Always check with the design engineer before substituting.
❓ Does steel plate thickness affect mechanical properties?
Yes — different standards may specify different mechanical property requirements for different thickness ranges. Always check the applicable standard.
❓ What is corrosion allowance in pressure vessel design?
Corrosion allowance is an additional thickness added to the calculated required thickness to account for material loss during service.
❓ Does thicker pressure vessel steel require special welding procedures?
Thicker plates may require preheating, controlled heat input, and PWHT. The specific requirements depend on the material, thickness, and welding procedure.
❓ What is UT testing for pressure vessel steel plate?
UT (Ultrasonic Testing) is used to detect internal discontinuities such as laminations. Some projects specify UT according to ASTM A578.
❓ What information should be included when ordering pressure vessel steel plate?
Standard, grade, thickness, dimensions, delivery condition, impact test, UT, heat treatment, MTC, inspection, and quantity.
❓ What is the difference between Q345R, 16MnDR and SA516 Grade 70?
Q345R is a Chinese pressure vessel steel for general applications. 16MnDR is a low-temperature pressure vessel steel. SA516 Grade 70 is an ASME pressure vessel steel commonly used in international projects.
14. Final Thoughts
For pressure vessel steel, buying the right grade is only the beginning. The thickness, testing, delivery condition, and inspection requirements all need to match the actual project.
When requesting a quotation, providing a complete material specification helps suppliers offer a more accurate price and reduces the risk of changes later.
💡 Our view: Looking for Q345R, 16MnDR, SA516 Grade 70, or other pressure vessel steel plates? Send us your specification, thickness, and quantity — we'll check the available options and confirm the delivery details.
📚 Further Reading & Related Content
- 🔗 Pressure Vessel Steel Plate Thickness Tolerance: Nominal vs Actual
- 🔗 Q345R vs SA516 Grade 70 – Comparison Guide
- 🔗 Ultrasonic Testing (UT) for Steel Plates – Buyer Guide
- 🔗 How to Read a Steel Plate Mill Test Certificate (MTC)
- 📄 GB/T 713.2-2023, ASME SA-516, ASTM A578, EN 10028
Need Pressure Vessel Steel Plates?
CEHE STEEL supplies Q345R, 16MnDR, SA516 Grade 70, and other pressure vessel steel plates. Send us your specification, thickness, and quantity — we'll provide a quotation with full MTC documentation.