P355NH
1.0565
Weldable normalized fine-grain steel for pressure equipment with specified elevated-temperature proof strength · EN 10028-3:2017 — normalized weldable fine-grain flat products for pressure purposes · EN 10028-1:2017 — general requirements for pressure-purpose flat products · ISO 9328-3:2018 — international specification for normalized weldable fine-grain flat products
P355NH is a weldable fine-grain pressure-equipment steel whose EN 10028-3 flat-product specification combines a minimum room-temperature yield strength of 355 MPa in the thinnest range with specified 0.2% proof strength up to 400 °C. It is normally supplied normalized, offers moderate sub-zero toughness, and is principally selected for welded pressure vessels and components where higher strength and elevated-temperature design data are required. Thickness, delivery condition, test orientation and the exact product standard are essential parts of the specification.
- P / 355 / N / HGrade meaning · Pressure-purpose steel; 355 MPa minimum yield in the thinnest range; normalized or normalizing rolled; specified elevated-temperature proof strength.
- EN 10028-3:2017Flat-product standard · Apply together with EN 10028-1:2017.
- +NDelivery condition · Normalized is standard; normalizing rolling may replace furnace normalizing under the standard.
- ReH 355 to 295 MPaThickness dependence · Minimum room-temperature yield decreases from 355 MPa at t ≤ 16 mm to 295 MPa at 150 < t ≤ 250 mm.
- 30 J transverse at −20 °CImpact class · EN 10028-3 P…NH flat-product requirement; longitudinal values are different and verification is optional.
- Rp0.2 specified through 400 °CElevated-temperature data · These are material test values, not an automatic maximum service temperature or design allowable stress.
Overview
- Designation system
- EN steel name according to EN 10027; numerical designation 1.0565
- Product forms
- Plate, sheet and other flat products covered by EN 10028-3, Seamless pressure tube under EN 10216-3, subject to different requirements, Welded pressure tube under EN 10217-3, subject to different requirements, Pressure-purpose forgings, fittings and bars under their respective product standards
- Condition
- Normalized (+N), Normalizing rolled where permitted by the governing product standard, Untreated delivery only by agreement, with testing in the simulated normalized condition
- Density
- 7.85 g/cm³ (Representative density used for carbon and low-alloy steel calculations; not a grade acceptance requirement.)
What the designation defines
P355NH is a pressure-purpose fine-grain steel defined primarily by product specification, delivery condition and mechanical performance rather than by chemistry alone. In the designation, P identifies pressure-equipment steel, 355 is the minimum room-temperature yield strength in the smallest specified thickness range, N denotes normalized or normalizing-rolled delivery, and H identifies the quality with specified minimum proof strength at elevated temperature.
The designation and material number also occur in standards for tubes, forgings, fittings and bars. Those products do not inherit the EN 10028-3 plate chemistry, dimensions or mechanical-property tables. Always identify the product standard as well as P355NH/1.0565.
Delivery condition and metallurgical basis
EN 10028-3 flat products are normally supplied in the normalized condition. The manufacturer may replace furnace normalizing with normalizing rolling. This processing develops a refined ferritic-pearlitic microstructure and provides the strength-toughness balance expected from the grade. If untreated delivery is agreed, acceptance testing is performed after simulated normalizing; the processor must still ensure that the finished component achieves the required properties after its actual thermal cycle.
The fine-grain condition is supported by controlled carbon and manganese contents, aluminium deoxidation/nitrogen binding and permitted additions of niobium, titanium and vanadium. These microalloying elements are controlled both individually and by a combined limit. The specified condition can be affected by hot forming, normalizing, extensive stress relief or repeated post-weld heat treatment, so severe fabrication heat cycles should be communicated to the steelmaker and represented in qualification or simulated-heat-treatment testing where necessary.
Chemical composition for EN 10028-3 flat products
| Element | Requirement | Technical significance or qualification |
|---|---|---|
| C | ≤ 0.18 | Controls strength, toughness and weldability. |
| Si | ≤ 0.50 | Deoxidizing and strengthening addition. |
| Mn | 1.10–1.70 | For nominal thickness below 6 mm, a lower minimum may be permitted by the standard. |
| P | ≤ 0.025 | Residual element controlled for toughness and weldability. |
| S | ≤ 0.010 | Restricted for toughness and through-thickness integrity. |
| Al total | ≥ 0.020 | May be below this value when Nb, Ti or V is additionally used for nitrogen binding. |
| N | ≤ 0.012 | If aluminium alone binds nitrogen, Al/N ≥ 2 applies. |
| Cr | ≤ 0.30 | Cr + Cu + Mo must also satisfy the combined limit. |
| Cu | ≤ 0.30 | A lower Cu limit or Cu + 6Sn ≤ 0.33% may be agreed where hot formability matters. |
| Mo | ≤ 0.08 | Cr + Cu + Mo ≤ 0.45%. |
| Nb | ≤ 0.05 | Fine-grain/microalloying addition. |
| Ni | ≤ 0.50 | Controlled residual or intentional addition. |
| Ti | ≤ 0.03 | Grain refinement and nitrogen binding. |
| V | ≤ 0.10 | Microalloy strengthening. |
| Nb + Ti + V | ≤ 0.12 | Combined microalloy limit. |
| Cr + Cu + Mo | ≤ 0.45 | Additional combined limit. |
These are EN 10028-3 cast-analysis requirements for flat products. Product-analysis tolerances are specified separately and must not be applied as heat-analysis limits.
Room-temperature tensile requirements
| Nominal thickness t (mm) | Minimum yield strength ReH (MPa) | Tensile strength Rm (MPa) | Minimum elongation A (%) |
|---|---|---|---|
| t ≤ 16 | 355 | 490–630 | 22 |
| 16 < t ≤ 40 | 345 | 490–630 | 22 |
| 40 < t ≤ 60 | 335 | 490–630 | 22 |
| 60 < t ≤ 100 | 315 | 470–610 | 21 |
| 100 < t ≤ 150 | 305 | 460–600 | 21 |
| 150 < t ≤ 250 | 295 | 450–590 | 21 |
Values apply to the normalized condition and are thickness dependent. They must not be transferred to P355NH tube, forging or fitting products without checking the applicable product standard.
The nominal 355 MPa designation therefore does not mean that every thickness has a 355 MPa minimum yield strength. Pressure design, material selection and substitution checks must use the thickness-specific value. The specified tensile range also shifts downward above 60 mm, reflecting the reduced through-section strengthening achievable in heavier products.
Specified proof strength at elevated temperature
| Nominal thickness t (mm) | 50 °C | 100 °C | 150 °C | 200 °C | 250 °C | 300 °C | 350 °C | 400 °C |
|---|---|---|---|---|---|---|---|---|
| t ≤ 16 | 343 | 323 | 299 | 275 | 252 | 232 | 214 | 202 |
| 16 < t ≤ 40 | 334 | 314 | 291 | 267 | 245 | 225 | 208 | 196 |
| 40 < t ≤ 60 | 324 | 305 | 282 | 259 | 238 | 219 | 202 | 190 |
| 60 < t ≤ 100 | 305 | 287 | 265 | 244 | 224 | 206 | 190 | 179 |
| 100 < t ≤ 150 | 295 | 277 | 257 | 236 | 216 | 199 | 184 | 173 |
| 150 < t ≤ 250 | 285 | 268 | 249 | 228 | 209 | 192 | 178 | 167 |
The standard describes these values as minimum values based on furnace-normalized test pieces. They are material properties for the specified test condition; allowable stress must come from the governing pressure-equipment design code.
The existence of a 400 °C proof-strength value does not by itself establish a 400 °C service limit. Design temperature, time-dependent damage, oxidation, cyclic duty, joint efficiency, weld properties and the selected construction code must all be considered.
Impact toughness and test orientation
| Test-piece orientation | −20 °C | 0 °C | +20 °C | Applicability |
|---|---|---|---|---|
| Transverse | 30 J | 40 J | 50 J | Specified for P…N/P…NH flat products over the applicable 5–250 mm thickness range. |
| Longitudinal | 45 J | 65 J | 75 J | Verification is an ordering option; tabulated longitudinal values apply up to 40 mm. |
The basic P355NH toughness class is not the same as P355NL1 or P355NL2. A 40 J minimum may be specified by agreement at temperatures where the standard otherwise gives a lower minimum.
P355NH is appropriate where moderate sub-zero toughness is required, but it is not the low-temperature member of the P355 family. For colder minimum-metal-temperature requirements, P355NL1 or P355NL2 may be more appropriate, subject to the design code and the required impact-test temperature, energy, orientation and thickness. Impact results from tube specifications must not be substituted for the EN 10028-3 plate values.
Welding, forming and fabrication
P355NH is intended to be weldable using established processes for fine-grain ferritic steels. Welding procedure qualification should nevertheless account for the actual heat analysis or carbon equivalent, combined thickness, restraint, consumable hydrogen level, heat input, preheat/interpass temperature and required weld-metal and heat-affected-zone toughness. The grade designation alone does not establish a universal preheat temperature.
Low-hydrogen practice is prudent, particularly for heavy or highly restrained joints. Consumables should be selected to satisfy the required joint strength and impact properties after any specified post-weld heat treatment. Welding recommendations for ferritic steels are provided by EN 1011-1 and EN 1011-2, while the construction code and approved welding procedure remain controlling.
Cold forming is feasible, but strain, bend radius, edge condition and thickness affect cracking risk and the retained properties. Hot forming or subsequent heat treatment can replace or disturb the original normalized microstructure. For significant hot forming, heavy cold deformation, repeated PWHT or long stress-relief cycles, agree the processing route with the steelmaker and verify the final condition where required. The optional Cu/Sn restriction in EN 10028-3 is particularly relevant when hot formability is important.
Pressure-service considerations
P355NH supplies elevated-temperature proof-strength data but is not a creep-strength alloy comparable with chromium-molybdenum steels. Selection for sustained high-temperature duty must follow the applicable pressure-vessel or piping code and its permitted-material, design-stress and time-dependent-property rules. The grade also has no inherent corrosion resistance; corrosion allowance, coating, lining, inhibitor use or a corrosion-resistant alloy may be required for the process environment.
Standard P355NH is not automatically qualified for wet H2S or other sour environments. EN 10028-3 allows hydrogen-induced-cracking testing to EN 10229 to be ordered, with the test solution and acceptance criteria defined as applicable. Sour-service procurement normally requires explicit chemistry restrictions, HIC acceptance criteria, hardness controls and fabrication requirements rather than reliance on the grade name alone.
Specifying and purchasing P355NH plate
| Item | What to state or verify |
|---|---|
| Product specification | EN 10028-3:2017 together with EN 10028-1:2017, not merely P355NH or 1.0565. |
| Dimensions | Nominal thickness, width, length and applicable dimensional-tolerance standard. |
| Delivery condition | P355NH+N; identify any untreated-delivery or simulated-heat-treatment agreement. |
| Inspection document | Required EN 10204 document, commonly type 3.1 where project requirements call for specific inspection. |
| Impact testing | Temperature, transverse or longitudinal orientation, energy requirement and any mid-thickness sampling. |
| Elevated-temperature testing | Whether elevated-temperature tensile verification is required by the project or design code. |
| Weldability control | Whether the optional maximum CEV is required and the applicable thickness band. |
| Fabrication heat cycle | Any hot forming, normalizing or severe/multiple PWHT cycle that must be simulated during testing. |
| Internal soundness | Required ultrasonic quality class and acceptance standard. |
| Sour service | HIC test method, solution, specimen location and acceptance criteria; any additional chemistry or hardness limits. |
| Hot formability | Any agreed lower Cu limit or Cu + 6Sn restriction. |
| Regulatory compliance | Applicable pressure-equipment legislation, harmonized material route, design code and project material specification. |
A certificate showing only “P355NH” is insufficient for technical acceptance unless the product standard, edition, dimensions, condition, heat analysis, test results and required options can also be confirmed.
Substitution and identification cautions
P355NH should not be substituted solely on the basis of nominal yield strength. A technically valid comparison must include product form, governing specification, thickness-specific yield and tensile values, elevated-temperature proof strength, impact-test temperature and orientation, delivery condition, chemistry and carbon equivalent, heat-treatment history, inspection requirements and acceptance under the governing design code.
Likewise, P355N, P355NH, P355NL1 and P355NL2 share much of their room-temperature tensile-property framework but are not interchangeable quality suffixes. P355NH adds the elevated-temperature proof-strength table, while NL1 and NL2 provide progressively more demanding low-temperature toughness. P355GH belongs to a different EN 10028 part and has a different metallurgical and property basis.
Sources
- BS EN 10028-3:2017 — Flat products made of steels for pressure purposes: Weldable fine grain steels, normalizedBSI
- EN 10028-3:2017 standard previewSIST / iTeh Standards
- ISO 9328-3:2018 — Steel flat products for pressure purposes, Part 3International Organization for Standardization
- Industry catalogue — P355NH EN 10028-3ArcelorMittal
- Weldable fine-grain structural steels for pressure vesselsSalzgitter Flachstahl
- P355NH under EN 10217-3Mannesmann
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