When specifying pressure vessel steel plates under the EN 10028-3 standard, engineers and procurement professionals frequently encounter two closely related grades: P355NH (1.0565) steel and P355NL1 (1.0566) steel.
At first glance, these two normalized fine-grain steels appear nearly identical-they share similar minimum yield strengths, overlapping chemical compositions, and common applications in welded pressure equipment.
However, their critical distinction lies in low-temperature toughness qualification, which can significantly impact material selection for projects operating in cold climates, cryogenic-adjacent services, or under sudden cooling scenarios.
Choosing incorrectly between P355NH (1.0565) vs P355NL1 (1.0566) can lead to unnecessary cost overruns or, worse, material unsuitability for the intended service conditions.
Understanding P355NH (1.0565) Steel and P355NL1 (1.0566) Steel – Standard and Designation Breakdown
Before diving into the technical differences, it is essential to decode what the designations P355NH (1.0565) and P355NL1 (1.0566) actually signify under the EN 10028-3 standard for weldable fine-grain pressure vessel steels. Both grades belong to the same material family, but the subtle differences in their designation suffixes hold the key to understanding their respective service envelopes.
What Does P355NH (1.0565) Steel Mean?
Breaking down the P355NH (1.0565) designation:
- P: Pressure purposes-indicating the grade is specifically developed and certified for pressure-containing equipment
- 355: Minimum yield strength of 355 MPa for thicknesses up to 16 mm
- N: Normalized delivery condition-the plate undergoes controlled heating above the upper critical temperature followed by air cooling, producing a refined, uniform grain structure
- H: High-temperature properties-the material is qualified for elevated-temperature service, with verified mechanical properties at temperatures up to approximately 400°C
- 1.0565: The unique steel number assigned within the European steel numbering system
What Does P355NL1 (1.0566) Steel Mean?
Breaking down the P355NL1 (1.0566) designation:
- P: Pressure purposes (same as P355NH)
- 355: Minimum yield strength of 355 MPa for thicknesses up to 16 mm (same as P355NH)
- N: Normalized delivery condition (same as P355NH)
- L1: Low-temperature toughness qualified at -40°C-this is the critical differentiator
- 1.0566: The unique steel number in the European numbering system
The core takeaway is this: P355NH (1.0565) is optimized for elevated-temperature service with impact testing at -20°C, while P355NL1 (1.0566) is optimized for low-temperature service with impact testing at -40°C. This fundamental design intent drives all the downstream differences in specification requirements and application suitability.
Chemical Composition Comparison of P355NH (1.0565) Steel vs P355NL1 (1.0566) Steel
| Element | P355NH (1.0565) – Max % | P355NL1 (1.0566) – Max % | Notes |
|---|---|---|---|
| Carbon (C) | 0.18 | 0.18 | Identical limit |
| Silicon (Si) | 0.50 | 0.50 | Identical limit |
| Manganese (Mn) | 1.10 – 1.70 | 1.10 – 1.70 | Identical range |
| Phosphorus (P) | 0.025 | 0.025 | Identical limit |
| Sulfur (S) | 0.015 | 0.015 | Identical limit |
| Chromium (Cr) | 0.30 | 0.30 | Identical limit |
| Nickel (Ni) | 0.50 | 0.50 | Identical limit |
| Molybdenum (Mo) | 0.08 | 0.08 | Identical limit |
| Niobium (Nb) | 0.05 | 0.05 | Identical limit |
| Titanium (Ti) | 0.03 | 0.03 | Identical limit |
| Vanadium (V) | 0.10 | 0.10 | Identical limit |
| Nitrogen (N) | 0.012 | 0.012 | Identical limit |
| Aluminum (Al) | 0.020 min | 0.020 min | Identical requirement |
| Copper (Cu) | 0.30 | 0.30 | Identical limit |
| Nb + Ti + V | ≤ 0.12 | ≤ 0.12 | Identical micro-alloy cap |
Room Temperature Mechanical Properties – P355NH (1.0565) vs P355NL1 (1.0566)
| Nominal Thickness (mm) | Min. Yield Strength ReH (MPa) – Both Grades | Tensile Strength Rm (MPa) – Both Grades | Min. Elongation A (%) – Both Grades |
|---|---|---|---|
| ≤ 16 | 355 | 490 – 630 | 22 |
| 16 > to ≤ 40 | 345 | 490 – 630 | 22 |
| 40 > to ≤ 60 | 335 | 490 – 630 | 22 |
| 60 > to ≤ 100 | 315 | 470 – 610 | 21 |
| 100 > to ≤ 150 | 305 | 460 – 600 | 21 |
| 150 > to ≤ 250 | 295 | 450 – 590 | 21 |
Impact Energy Requirements – P355NH (1.0565) vs P355NL1 (1.0566)
| Parameter | P355NH (1.0565) | P355NL1 (1.0566) |
|---|---|---|
| Impact Test Temperature | -20 °C | -40 °C |
| Typical Min. Impact Energy (KV, J) – Transverse | 27 J | 27 J |
| Temperature Qualification Focus | Elevated temp. (up to ~400 °C) | Low-temperature service |
| Design Intent | Boilers, steam drums, hot process equipment | Cold-region vessels, cryogenic-adjacent storage, low-ambient installations |
When Low-Temperature Toughness Matters – Application Scenarios
The enhanced low-temperature toughness of P355NL1 (1.0566) becomes critical in several specific scenarios:
- Arctic and sub-Arctic installations: Pressure vessels and storage tanks operating in ambient temperatures that can drop below -20°C, where P355NH (1.0565) lacks guaranteed toughness properties
- Cryogenic-adjacent LPG storage: While not suitable for fully cryogenic service (which requires nickel-alloy steels or stainless steels), P355NL1 (1.0566) can handle the cold metal temperatures encountered in LPG spheres and bullets during depressurization events
- Refrigeration system components: Ammonia receivers and other pressure equipment in industrial refrigeration where operating temperatures routinely dip below -20°C
- Offshore and marine pressure equipment: North Sea and other cold-water offshore installations where design codes mandate -40°C impact qualification
- Sudden depressurization scenarios: Even in normally temperate locations, rapid depressurization of gas-filled vessels can cause localized cooling-NL1 provides a wider safety margin
When Elevated-Temperature Stability Matters – The P355NH Advantage
Conversely, P355NH (1.0565) is the correct choice when the primary service condition involves sustained high temperatures rather than cold exposure:
- Steam boiler drums: Operating at 250–350°C under continuous thermal load
- Industrial heat exchangers: Handling hot process fluids at 200–400°C
- Power plant pressure components: Superheater headers and steam piping
- Chemical reactor vessels: Elevated-temperature synthesis processes
The "H" designation in P355NH indicates that the steel has been specifically validated for mechanical property retention at elevated temperatures-a characteristic that P355NL1 (1.0566) does not carry as a primary design intent, even though it may perform adequately in many elevated-temperature scenarios.
Price and Availability of P355NH (1.0565) Steel vs P355NL1 (1.0566) Steel
From a procurement perspective, P355NH (1.0565) and P355NL1 (1.0566) steel availability and pricing can differ:
P355NH (1.0565): More widely produced and stocked globally due to its broad applicability in power generation and general pressure vessel fabrication. Typically more readily available with shorter lead times.
P355NL1 (1.0566): Slightly less common than NH due to its specialized cold-service qualification. May carry a modest price premium in some markets, reflecting the additional process control and more demanding impact testing requirements.
GNEE Steel maintains inventory of both grades in standard thicknesses (6 mm to 150 mm) and can source or produce thicker plates upon request. Our dual-grade stocking strategy ensures that clients receive competitive pricing and fast delivery regardless of which grade their project requires.
Quick Selection Guide – P355NH (1.0565) vs P355NL1 (1.0566)
| Selection Factor | Choose P355NH (1.0565) | Choose P355NL1 (1.0566) |
|---|---|---|
| Minimum design temperature | Down to -20°C | Down to -40°C |
| Maximum operating temperature | Up to ~400°C | Elevated temp. possible but not primary design intent |
| Cold-climate installation | Not recommended (below -20°C) | Strongly recommended |
| LPG / ammonia storage | Not typically suitable | Suitable option |
| Steam boiler service | Ideal choice | Acceptable but may be over-specified |
| Cost sensitivity | Generally more economical | Modest premium for cold toughness |
| Code minimum impact requirement | -20°C is sufficient | -40°C mandated |
Don't leave your pressure vessel steel selection to chance. Contact GNEE Steel today. Send us your specifications-grade, dimensions, quantity, destination, and any special requirements-and receive a competitive quotation within hours. Our team is ready to provide technical advice, material availability confirmation, and complete certification details.
Request your free quote now and make the right choice between P355NH and P355NL1 steel for your next project.
What is P355NH material?
P355NH is a pressure vessel steel to EN10028-3 that is used for high temperatures. It is normalised and fine grained with excellent weldability. Like other pressure vessel steel grades we carry large quantities in stock for immediate delivery.
What is P355?
P355NL steel is a fine-grain, low-alloy steel composed of high carbon and minimal sulphur. Used in applications requiring high tensile strength, most commonly for pressurised systems and boilers.
What is the yield strength of P355NH?
Material Description. 1) Remark: Steel P355NH is a weldable fine grained steel with a yield strength of max. 355 N/mm².
What is the material specification 1.0565?
1.0565 is the EN numerical definition for this material. P355NH is the EN chemical designation. It has a very high embodied energy among processed carbon or unalloyed steels. In addition, it has a fairly high basic cost and moderately high electrical conductivity.
What is material specification 1.0566?
P355NL1/TStE355 (1.0566) is a fine-grain structural steel characterized by its high notch toughness and excellent weldability, making it suitable for low-temperature applications. It is designed to perform well under pressure, making it ideal for pressure vessels and boiler construction.
What is the impact test for P355NL1?
The designation P355NL1 steel pipe shows that this is steel for pressure purposes (P) with minimum yield strength of 355 MPa (355) which is impact tested in the transverse direction at -40 deg C (NL1). P355NL1 steel tube this material is mainly used to fabricate boiler and pressure vessel.
What is the ASME material equivalent to P355NL1?
What is the asme material equivalent to P355NL1? ASME material equivalent to EN 10028-3 P355NL1 is ASME SA516 Grade 70.
