P355NL1 and P355NL2 in the EN 10028-3 System
P355NL1 and P355NL2 are weldable, normalized fine grain steel plate grades for pressure equipment. Both are covered by EN 10028-3, the European standard for flat products of steel for pressure purposes supplied in the normalized condition, and both are widely used for boilers, pressure vessels, storage tanks, heat exchangers and pipework operating at low ambient or process temperatures.
The designation reads the same way for every grade in the family. The letter P indicates pressure purposes, the number 355 gives the minimum yield strength in MPa for plate 16 mm thick and below, and the suffix states the impact test temperature. Grades with N or NH are tested at -20 °C or above, NL1 at -40 °C or above and NL2 at -50 °C or above. That single suffix is the only technical difference between the two grades discussed here.
The Core Difference: -40 °C versus -50 °C Impact Testing
P355NL1 is impact tested at -40 °C. P355NL2 is impact tested at -50 °C, which is the stricter requirement and makes it suitable for extremely low temperature service. Everything else, including minimum yield strength, tensile strength and elongation, is identical for the two grades.
| Item | P355NL1 | P355NL2 |
|---|---|---|
| Standard | EN 10028-3 | EN 10028-3 |
| Delivery condition | Normalized (+N) | Normalized (+N) |
| Minimum yield strength, t up to 16 mm | 355 MPa | 355 MPa |
| Charpy impact test temperature | -40 °C | -50 °C |
| Maximum phosphorus | 0.025 % | 0.020 % |
| Maximum sulphur | 0.015 % | 0.010 % |
The tighter phosphorus and sulphur limits of P355NL2 are worth noting because they are not merely cosmetic. Cleaner steel means fewer non-metallic inclusions, better through thickness ductility and a lower risk of brittle fracture initiation at a weld toe or a notch in cold service.
Chemical Composition Compared
The heat analysis limits for the two grades are almost identical, and the only meaningful divergence is in the residual elements phosphorus and sulphur.
| Element | P355NL1 | P355NL2 |
|---|---|---|
| Carbon, C | 0.18 % max | 0.18 % max |
| Silicon, Si | 0.50 % max | 0.50 % max |
| Manganese, Mn | 1.10 - 1.70 % | 1.10 - 1.70 % |
| Phosphorus, P | 0.025 % max | 0.020 % max |
| Sulphur, S | 0.015 % max | 0.010 % max |
| Total aluminium, Alt | 0.020 % min | 0.020 % min |
| Nitrogen, N | 0.012 % max | 0.012 % max |
| Chromium, Cr | 0.30 % max | 0.30 % max |
| Copper, Cu | 0.30 % max | 0.30 % max |
| Molybdenum, Mo | 0.08 % max | 0.08 % max |
| Niobium, Nb | 0.05 % max | 0.05 % max |
| Nickel, Ni | 0.50 % max | 0.50 % max |
| Titanium, Ti | 0.03 % max | 0.03 % max |
| Vanadium, V | 0.10 % max | 0.10 % max |
Aluminium is deliberately added at a minimum of 0.020 % so that the steel is fully killed and the grain is refined by aluminium nitride. Carbon is held at 0.18 % maximum, which keeps the carbon equivalent low enough for routine welding of pressure equipment. Both grades are supplied in the normalized condition, marked +N.
Mechanical Properties by Thickness
The tensile strength band narrows as thickness rises, and the two grades follow exactly the same table.
| Nominal thickness | Tensile strength Rm, +N |
|---|---|
| Up to 60 mm | 490 - 630 MPa |
| Over 60 mm up to 100 mm | 470 - 610 MPa |
| Over 100 mm up to 150 mm | 460 - 600 MPa |
| Over 150 mm up to 250 mm | 450 - 590 MPa |
| Nominal thickness | Minimum yield strength ReH, +N |
|---|---|
| Up to 16 mm | 355 MPa |
| Over 16 mm up to 40 mm | 345 MPa |
| Over 40 mm up to 60 mm | 335 MPa |
| Over 60 mm up to 100 mm | 315 MPa |
| Over 100 mm up to 150 mm | 305 MPa |
| Over 150 mm up to 250 mm | 295 MPa |
Minimum elongation at fracture is 22 % for the common thickness range and 21 % for heavier plate. Hardness, like tensile strength, is a function of the normalized structure rather than of the grade suffix, so the two grades respond identically to forming, machining and welding.
Impact Energy Requirements
Charpy V-notch tests are taken transverse to the rolling direction on normalized specimens. The minimum average energy required is the same for the two grades; only the temperature at which it must be achieved changes.
P355NL1: transverse Charpy V-notch minimum energy demonstrated at -40 °C.
P355NL2: the same minimum energy demonstrated at -50 °C.
Longitudinal specimens record higher energies than transverse specimens at every temperature.
Toughness rises steeply with temperature, so both grades show a wide margin above 0 °C.
For a plant located in a cold climate, the piping specification normally fixes the minimum design metal temperature first and then selects the grade whose test temperature is at or below it with a suitable margin. That rule of thumb leads to P355NL1 for most temperate installations and to P355NL2 for arctic, cryogenic and liquefied gas applications.
Selection and Supply Notes
Where the minimum design metal temperature is -40 °C or warmer, P355NL1 gives the required assurance and is usually more readily available. Below -40 °C, P355NL2 is the correct choice, since NL1 carries no guarantee at -50 °C. Thick walled vessels and restrained nozzle welds often specify NL2 regardless of temperature, because the cleaner steel improves heat affected zone toughness and reduces the risk of lamellar tearing.
Both grades are supplied in normalized condition and can be ordered with ultrasonic testing, with restricted carbon equivalent, with through thickness tensile testing for critical joints, or with third party inspection for pressure equipment. Because the property tables are identical, plate of either grade can be used freely once the impact requirement has been settled, which simplifies substitution during procurement.
Frequently Asked Questions
Q: Is P355NL2 stronger than P355NL1?
No. Both have a minimum yield strength of 355 MPa and identical tensile strength ranges for every thickness range.
Q: What exactly distinguishes P355NL1 from P355NL2?
The impact test temperature, -40 °C for NL1 and -50 °C for NL2, plus tighter maximum phosphorus and sulphur limits in NL2.
Q: Which grade suits a vessel in an arctic climate?
P355NL2, because it is qualified at -50 °C, whereas P355NL1 offers no toughness guarantee below -40 °C.
Q: Are the two grades interchangeable?
P355NL2 can replace P355NL1 for warmer service, but the reverse substitution is not acceptable when the design temperature is below -40 °C.
Q: Why does the grade contain aluminium?
Aluminium is required at 0.020 % minimum to fully kill the steel and to refine the grain through aluminium nitride during normalizing.
Q: How does thickness affect the guaranteed properties?
Minimum yield strength falls from 355 MPa at 16 mm to 295 MPa at 250 mm, and the tensile band also narrows as thickness increases.
