What the P275NH and P355NH Designations Mean
P275NH and P355NH belong to EN 10028-3, the European standard covering weldable, normalized fine-grain steel flat products for pressure purposes. The designation is built from four signals: the letter P marks pressure service, the following number is the minimum yield strength in MPa at room temperature, N states that the plate is delivered in the normalized condition, and the trailing H confirms that the grade carries verified elevated-temperature properties for hot-wall equipment.
Both grades are produced from fully killed, fine-grain-treated steel with a ferritic-pearlitic microstructure refined by aluminum and microalloying additions. EN 10028-3 covers plate thicknesses from roughly 6 mm up to 250 mm, which makes these grades suitable for shells, dished ends, tube plates and flat covers in the same equipment.
Chemical Composition Comparison
P355NH carries slightly wider manganese freedom and higher carbon and silicon ceilings than P275NH, which is how the mill reaches the extra 80 MPa of yield strength while staying within a normalized structure.
| Element, max % | P275NH | P355NH |
|---|---|---|
| C | 0.16 | 0.20 |
| Si | 0.40 | 0.50 |
| Mn | 0.50-1.40 | 0.90-1.70 |
| P | 0.030 | 0.030 |
| S | 0.020 | 0.025 |
| Al, min | 0.020 | 0.020 |
| Cr / Cu | 0.30 / 0.30 | 0.30 / 0.30 |
| Mo / Ni | 0.08 / 0.50 | 0.08 / 0.50 |
| Nb / V / Ti | 0.05 / 0.05 / 0.03 | 0.05 / 0.10 / 0.03 |
Aluminum is held at a minimum of 0.020% so that dissolved nitrogen is bound as aluminum nitride and grain growth during normalizing is restrained. Residual chromium, copper, molybdenum and nickel are capped to protect weldability and toughness, and the carbon equivalent is assessed as CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, reported on the inspection certificate for each heat.
Mechanical Properties Across Thickness
The strength advantage of P355NH is constant, but both grades step down as the plate gets thicker, because grain coarsening and lower rolling reduction cannot be fully recovered by normalizing.
| Thickness, mm | P275NH ReH min, MPa | P355NH ReH min, MPa | Tensile range P275NH, MPa | Tensile range P355NH, MPa | Elongation min, % |
|---|---|---|---|---|---|
| ≤16 | 275 | 355 | 390-510 | 490-630 | 24 / 22 |
| >16 to 35 | 275 | 355 | 390-510 | 490-630 | 24 / 22 |
| >35 to 50 | 265 | 345 | 390-510 | 490-630 | 24 / 22 |
| >50 to 70 | 255 | 325 | 390-510 | 490-630 | 24 / 22 |
| >70 to 100 | 235 | 315 | 370-490 | 470-610 | 23 / 21 |
| >100 to 150 | 235 | 295 | 350-470 | 450-590 | 23 / 21 |
Impact testing is mandatory for the NH condition, with Charpy V-notch specimens taken in the longitudinal direction and tested at -20 °C; the standard requires a minimum absorbed energy of 27 J. Because the impact requirement is linked to the delivery condition rather than to a single thickness band, P275NH and P355NH remain usable for cold-climate installations where a non-tested structural plate would be unacceptable.
Where Each Grade Is Used
P275NH is the economical choice for steam drums, air receivers, storage tanks, condensers and column shells where the calculated wall thickness stays moderate and the design stress is low. It is widely used for equipment working in the 200-400 °C range where elevated-temperature yield data must be available.
P355NH is selected when the same duty has to be met with a thinner wall: high-pressure separators, larger diameter vessel shells, penstock liners, digesters and heavy tube plates. Upgrading from P275NH to P355NH reduces wall thickness by roughly 20-25% at equal design pressure, which lowers weld volume, lifting weight and support steel. Where the service temperature extends below the NH impact test level or the wall exceeds 150 mm, a quenched and tempered grade from the EN 10028-6 family becomes the practical alternative.
Fabrication, Welding and Quality Control
Both grades are supplied normalized, with the option of normalizing rolling in place of a separate heat treatment when the rolling schedule is qualified. Plates can be delivered as-rolled for flat product, or with supplementary ultrasonic examination to EN 10160 at class levels agreed for pressure-retaining service. Thickness-direction properties to EN 10164 (Z15, Z25, Z35) are available for nozzle and support attachment zones.
Welding follows conventional practice for normalized fine-grain pressure steels: low-hydrogen consumables, controlled heat input, interpass temperature limits and, for heavy sections, preheat matched to the carbon equivalent reported on the certificate. Post-weld heat treatment is normally carried out between 560 °C and 600 °C when the wall thickness or the applicable pressure vessel code demands it, and the cooling rate is controlled to preserve the -20 °C impact properties.
Each delivery is documented to EN 10204 with inspection certificates covering heat analysis, product analysis, tensile testing, impact testing, dimensional checks and, on request, hardness survey and non-destructive examination records. Cutting is performed by plasma, laser or waterjet, while flame cutting of thick sections requires preheating to avoid a hardened cut edge.
Frequently Asked Questions
Q: What is the main difference between P275NH and P355NH?
A: The minimum yield strength. P275NH is specified at 275 MPa and P355NH at 355 MPa at room temperature, while both remain normalized fine-grain steels under EN 10028-3.
Q: Are P275NH and P355NH impact tested?
A: Yes. Both grades in the NH condition are Charpy V-notch tested at -20 °C with a minimum absorbed energy of 27 J.
Q: What delivery condition do these plates have?
A: They are delivered normalized, either by a separate normalizing treatment after rolling or by a qualified normalizing rolling route.
Q: Can the two grades be welded to each other?
A: Yes. They are metallurgically similar, and a matching low-hydrogen consumable together with a preheat and interpass schedule based on the higher carbon equivalent gives sound joints.
Q: What thickness range is normally available?
A: EN 10028-3 covers plate from about 6 mm to 250 mm; most pressure equipment work is supplied between 8 mm and 150 mm.
Q: Which grade should be chosen for a low-temperature application?
A: Neither grade is intended for service below its -20 °C impact level; below that temperature a quenched and tempered EN 10028-6 grade with a lower test temperature should be used.
