P355NL1 Steel Plate and the EN 10028-3 Standard
P355NL1 is a weldable fine-grain pressure vessel steel plate specified in the European standard EN 10028-3, which covers flat products of steels for pressure purposes in the normalised delivery condition. The designation is built from four elements: P for pressure purposes, 355 for the minimum yield strength in MPa, N for normalised delivery, and L1 for the low-temperature impact sub-grade. The L1 sub-grade is associated with impact testing at -40 °C, so P355NL1 is frequently chosen for vessels and piping that operate in cold climates or that must survive a low-temperature upset condition without brittle fracture.
Because it is used in pressure equipment, the plate is also governed by the design code under which the vessel is built, such as EN 13445 or the national codes derived from the Pressure Equipment Directive. The material certificate must therefore be read together with the code to confirm that the specified toughness and strength are acceptable for the design temperature.
Chemical Composition of P355NL1
The table below gives the typical ladle analysis limits for P355NL1. The steel is a low-alloy, fine-grain, fully killed product with aluminium added for grain refinement; carbon and manganese provide strength, while silicon, chromium, nickel, molybdenum and copper are restricted to keep the weld zone tough and the carbon equivalent low.
| Element | C max | Si max | Mn | P max | S max | Al min | N max | Cr max | Ni max | Mo max | Cu max |
|---|---|---|---|---|---|---|---|---|---|---|---|
| P355NL1, % | 0.18 | 0.50 | 0.90-1.65 | 0.025 | 0.015 | 0.020 | 0.012 | 0.30 | 0.50 | 0.08 | 0.35 |
Each element has a defined role. Carbon raises strength and hardness but reduces ductility, so it is capped at a low level. Manganese contributes solid-solution strengthening and combines with sulphur to prevent hot cracking. Silicon assists deoxidation. Phosphorus and sulphur are treated as harmful residuals: both increase brittleness, and sulphur in particular damages toughness in the through-thickness direction, so their limits are tight in this grade.
Aluminium is deliberately added to pin grain boundaries during normalising, and nitrogen is controlled in parallel because aluminium nitride formation depends on the balance between the two. This combination is what gives P355NL1 its fine ferrite-pearlite structure and its good low-temperature toughness.
Mechanical Properties
| Nominal thickness, mm | Yield strength, min, MPa | Tensile strength, MPa | Elongation, min, % |
|---|---|---|---|
| ≤ 16 | 355 | 490-630 | 22 |
| 16 < t ≤ 60 | 345 | 470-610 | 21 |
| 60 < t ≤ 100 | 335 | 450-590 | 21 |
| 100 < t ≤ 150 | 315 | 440-580 | 20 |
Strength falls as thickness increases because the mid-thickness of a heavy plate cools more slowly during normalising and receives less reduction during rolling. Charpy V-notch impact testing is carried out on the sub-grade test temperature, and transverse test pieces are normally used for plate, which gives lower and more representative values than longitudinal testing. A common engineering requirement is a minimum carbon equivalent value of 0.43 for P355NL1, which keeps the heat-affected zone hardness after welding within acceptable limits.
It is worth noting that generic figures sometimes quoted for this family of steel, such as a yield strength range of 270-400 MPa, describe the broader pressure plate population rather than P355NL1 itself. For this grade the guaranteed minimum yield strength is 355 MPa at thin sections, and this is the value that should be used in design calculations.
Delivery Condition and Processing
P355NL1 is delivered in the normalised condition, either by normalising after rolling or by normalised rolling when the rolling schedule achieves the required microstructure and is covered by the inspection documents. Plates are routinely supplied with ultrasonic testing for internal soundness when specified, and with edge preparation on request.
The steel can be cold formed, hot formed and machined using standard workshop practice. Cold forming is comfortable up to moderate strains because of the low carbon content and fine grain size; heavier deformation is usually followed by a stress-relief anneal. Hot forming should be completed within the temperature range recommended for the grade and followed by a fresh normalising treatment, since hot working destroys the grain refinement that the grade relies on.
Welding and Pressure Equipment Service
P355NL1 is intended for welded construction, and its low carbon and low carbon equivalent make it one of the easier pressure vessel steels to weld. Matching electrodes are used for the strength level required, and the low sulphur content limits the risk of solidification cracking. For thick sections a modest preheat is applied to control hydrogen, and post-weld heat treatment is specified when the design code requires it for wall thickness or when service conditions demand stress relief. Because the grade is qualified down to its low-temperature impact test temperature, impact-tested consumables should be used wherever the weld metal itself must meet the same toughness requirement.
Typical applications include pressure vessels, storage tanks, boilers and heat exchangers, gas processing equipment, low-temperature pipework and offshore structures where a combination of moderate strength, proven toughness and good weldability is more valuable than very high strength.
Frequently Asked Questions
Q: What does P355NL1 stand for?
P is for pressure purposes, 355 is the minimum yield strength in MPa, N means normalised delivery, and L1 is the low-temperature impact sub-grade tested at -40 °C.
Q: Which standard specifies P355NL1?
It is specified in EN 10028-3, the European standard for flat products of weldable fine-grain steels for pressure purposes supplied in the normalised condition.
Q: What is the minimum yield strength of the plate?
355 MPa for plate up to 16 mm thick, decreasing to 345 MPa up to 60 mm, 335 MPa up to 100 mm and 315 MPa up to 150 mm.
Q: Why is aluminium specified in the composition?
Aluminium forms fine nitride particles that restrict grain growth during normalising, which is the mechanism behind the fine-grain structure and the good low-temperature toughness of the grade.
Q: Is post-weld heat treatment always needed?
Not always; it depends on section thickness and the governing design code. When required, it relieves residual stress and tempers the heat-affected zone after welding.
Q: Which grade should be used at higher temperatures?
For elevated-temperature creep duty, a European pressure plate grade intended for high-temperature service should be selected instead, since the normalised fine-grain family is optimised for low-temperature toughness.
