What P275NH Is and Where It Sits in EN 10028-3
P275NH is a normalized fine grain steel grade specified in EN 10028-3, the European standard for flat products made of steels for pressure purposes with specified elevated temperature properties. It is a non-alloy steel, not a low alloy or creep resistant grade. The designation breaks down into three parts: P for pressure purposes, 275 for the minimum yield strength in MPa at the thinnest section range, and NH for normalized delivery with elevated temperature property guarantees.
The grade is used for boilers, pressure vessels and piping that carry hot fluids, where the design depends on the yield strength and allowable stress of the material at the working temperature rather than at room temperature. Its practical advantages are stable high temperature behaviour, a fine grain structure produced by normalizing, and weldability good enough that preheat is not normally required at moderate thickness. EN 10028-3 has largely replaced the older national standards it was derived from, such as the German and British pressure vessel plate specifications used before European harmonisation.
Chemical Composition to EN 10028-3
| Element, % | Requirement | Effect |
|---|---|---|
| Carbon | 0.16 max | Low carbon supports weldability and toughness |
| Silicon | 0.40 max | Deoxidation |
| Manganese | 0.80-1.50 | Primary strengthening element, range tightly controlled |
| Phosphorus | 0.025 max | Low content improves low temperature toughness |
| Sulfur | 0.015 max | Very low sulfur limits segregation and inclusion damage at high temperature |
| Aluminium, total | 0.020 min | Grain refining, ensures a fine austenitic grain size |
| Nitrogen | 0.012 max | Prevents ageing and porosity effects |
| Chromium | 0.30 max | Residual element limit |
| Copper | 0.30 max | Residual element limit |
| Molybdenum | 0.08 max | Residual element limit |
| Nickel | 0.50 max | Can improve toughness |
| Niobium | 0.05 max | Microalloying for grain refinement |
| Titanium | 0.03 max | Binds nitrogen and assists grain refinement |
| Vanadium | 0.05 max | Microalloying for precipitation strengthening |
| Niobium + titanium + vanadium | 0.05 max combined | Controls total microalloy content |
For weldability assessment the two calculated values normally quoted are the IIW carbon equivalent and the Pcm value. For P275NH the carbon equivalent falls in the region of 0.36 to 0.42 % and Pcm in the region of 0.18 to 0.22 %, calculated from mid to upper limits of the specified ranges. Both figures indicate low susceptibility to cold cracking, which is why preheating is generally not required for thicknesses below about 25 mm, provided that heat input and interpass temperature are controlled and the material is dry.
Mechanical Properties and the Thickness Effect
P275NH is thickness sensitive by design. EN 10028-3 sets lower minimum yield strength values as plate thickness increases, because thicker sections cool more slowly during normalizing and have a coarser through thickness structure.
| Thickness range | Minimum yield strength, ReH | Tensile strength, Rm | Minimum elongation |
|---|---|---|---|
| Up to 16 mm | 275 MPa | 390-510 MPa | 24 % |
| Over 16 mm up to 40 mm | 265 MPa | 390-510 MPa | 24 % |
| Over 40 mm up to 60 mm | 255 MPa | 390-510 MPa | 24 % |
| Over 60 mm up to 100 mm | 235 MPa | 370-490 MPa | 23 % |
| Over 100 mm up to 150 mm | 225 MPa | 360-480 MPa | 23 % |
| Over 150 mm up to 250 mm | 215 MPa | 350-470 MPa | 22 % |
Two consequences follow. In design, the yield strength value for the actual thickness range of the plate must be used; taking the 275 MPa figure for a 120 mm shell would overstate the capacity of the component. In procurement, the mill certificate must show the measured yield strength of the tested plate together with the thickness it applies to, and that value must not be lower than the minimum for that thickness band.
Charpy V-notch impact testing is mandatory for P275NH, which is what distinguishes it from grades supplied without a toughness guarantee. The test temperature, specimen orientation and minimum absorbed energy required are set by EN 10028-3 and by the purchase specification, and grain refined normalized plate is normally produced with a comfortable margin over the standard minimum. High temperature design data are a separate matter again: allowable stresses for service temperatures up to about 400 degrees C must be read from the pressure vessel design code, such as EN 13445 or the ASME Boiler and Pressure Vessel Code, because the room temperature yield strength cannot be used directly at elevated temperature.
Supply Range, Tolerances and Equivalents
| Parameter | Typical availability | Governing standard |
|---|---|---|
| Thickness | 6 mm to 300 mm | EN 10029 for tolerances |
| Width | 1500 mm to 4050 mm | EN 10029 for tolerances |
| Length | 3000 mm to 15000 mm | EN 10029 for tolerances |
| Delivery condition | Normalized, designation +N | EN 10028-3 |
| Supplementary options | Through thickness testing Z15, Z25, Z35, ultrasonic examination, HIC testing, SSC resistance testing, special inspection conditions | Order specific |
| Standard system | Comparable grade | Note |
|---|---|---|
| European, EN 10028-3 | P275NH | The base specification |
| ISO 9328-2 | P275NH | Corresponding ISO specification |
| Germany, historical | WStE 285 | Historic designation, reference only |
| United Kingdom, BS 1501 | 224 Gr. 430 | Approximate equivalent only |
| United States, ASTM and ASME | A516 Gr. 60 | Frequently referenced, but chemistry and impact requirements differ |
Comparable grades are for orientation only. P275NH is a European specification with its own chemistry limits, impact test temperature and thickness dependent strength table, so replacing it with A516 Gr. 60 or with a domestic Chinese grade requires recalculation of allowable stress, re-evaluation of toughness at the design minimum temperature and formal approval by the design authority.
Design, Heat Treatment and Welding Practice
Normalizing is the standard delivery condition and is what produces the fine grain structure: after rolling, the plate is reheated above its austenitizing temperature and cooled uniformly in air. Subsequent hot forming or welding alters that condition locally, and for heavily hot worked pressure parts a re-normalizing heat treatment may be needed to restore the specified properties. Welding is normally carried out with controlled heat input, because excessive heat input coarsens the heat affected zone and reduces toughness in a grade that depends on grain refinement. Post-weld heat treatment is applied to relieve residual stress on thick sections: under combined high temperature and pressure, residual stresses can promote stress corrosion cracking and creep damage, so the requirement is set by the design code and the component thickness rather than left to the fabricator. Supplementary testing should be defined at enquiry stage, in particular whether HIC testing or sulfide stress cracking resistance testing is needed for wet hydrogen sulfide service.
Frequently Asked Questions
Q: What is the difference between P275N, P275NH, P275NL1 and P275NL2?
The letters after the strength figure define the impact test temperature and property guarantee. P275N is the basic normalized grade for general pressure vessels, P275NH adds elevated temperature property guarantees for service up to about 400 degrees C, and P275NL1 and P275NL2 are qualified for low temperature service down to -50 degrees C and -60 degrees C respectively.
Q: Why do the yield strength values for P275NH change with thickness?
Because thicker plate cools more slowly during normalizing, which produces a slightly coarser structure and lower minimum yield strength. The standard therefore publishes a descending table of minimum values by thickness band, and design calculations must use the value for the band actually ordered.
Q: Can the room temperature yield strength be used for elevated temperature design?
No. Allowable stresses at the design temperature must be taken from the pressure vessel design code, since strength falls as temperature rises and the values account for that reduction with a safety factor applied.
Q: Is preheating required when welding P275NH?
Preheating is generally not required below about 25 mm thickness because the carbon equivalent and Pcm values indicate low cold cracking susceptibility. Thicker sections, low ambient temperature and highly restrained joints are usually handled by preheat as defined in the qualified welding procedure.
Q: Can P275NH be replaced by A516 Gr. 60 or a domestic Chinese grade?
Not directly. The grades come from different standard systems with different chemistry limits, impact test temperatures and design allowables, so any substitution has to be recalculated and approved by the design authority, and the welding procedure re-qualified for the new material.
Q: What does the H in P275NH actually guarantee?
It guarantees mechanical properties at elevated temperature in addition to the room temperature requirements, which is why the grade is selected for steam headers, boiler components and high temperature vessels rather than only for ambient temperature duty.
