EN 10025-2 is the foundational European standard for hot-rolled structural steel products — flat, long, and hollow sections in grades S235 through S460. Every structural fabrication shop, structural engineer, and offshore designer references EN 10025-2 daily. The grade designation encodes yield strength, Charpy impact sub-grade, and delivery condition. Understanding the designation system, the mechanical property tables (especially the thickness-dependent yield strength reduction), and the carbon equivalent (CEV) limits is essential for correct material specification and weldability assessment.
1. Grade Designation System
The EN 10025-2 designation encodes four pieces of information:
Example: S355J2+N
S = structural steel
355 = minimum yield strength [MPa] (for t ≤ 16 mm)
J2 = Charpy sub-grade (27 J at −20°C)
+N = normalised rolling delivery condition
1.1 Yield Strength Grades
| Grade | ReH min (t ≤ 16 mm) | Typical Offshore Use |
|---|---|---|
| S235 | 235 MPa | Grating, handrail, non-structural secondary |
| S275 | 275 MPa | General secondary structural, platforms |
| S355 | 355 MPa | Primary offshore structural steel — most common for jacket, topside, module |
| S420 | 420 MPa | Weight-critical primary structures |
| S460 | 460 MPa | High-strength connections, crane booms, demanding lifting appliances |
2. Yield Strength vs Thickness — The Critical Table
The most important and most often overlooked aspect of EN 10025-2 is that yield strength decreases with plate thickness. The headline grade number (e.g. 355 in S355) applies only for thickness ≤ 16 mm. For offshore structures using thick plates, the actual design yield strength may be significantly lower:
| Thickness Range | S235 ReH | S275 ReH | S355 ReH | S460 ReH |
|---|---|---|---|---|
| t ≤ 16 mm | 235 MPa | 275 MPa | 355 MPa | 460 MPa |
| 16 < t ≤ 40 mm | 225 MPa | 265 MPa | 345 MPa | 440 MPa |
| 40 < t ≤ 63 mm | 215 MPa | 255 MPa | 335 MPa | 420 MPa |
| 63 < t ≤ 80 mm | 215 MPa | 245 MPa | 325 MPa | 400 MPa |
| 80 < t ≤ 100 mm | 215 MPa | 235 MPa | 315 MPa | 390 MPa |
| 100 < t ≤ 150 mm | 195 MPa | 225 MPa | 295 MPa | 370 MPa |
3. Tensile Strength
EN 10025-2 specifies minimum tensile strength Rm as a range (not just a minimum). The tensile strength is less thickness-dependent than yield strength:
| Grade | Rm (t ≤ 100 mm) | Rm (100 < t ≤ 150 mm) |
|---|---|---|
| S235 | 360–510 MPa | 350–500 MPa |
| S275 | 410–560 MPa | 400–540 MPa |
| S355 | 470–630 MPa | 450–600 MPa |
| S460 | 550–720 MPa | 530–710 MPa |
Note: the tensile strength is specified as a range, not just a minimum. This is important for fracture mechanics calculations — the upper bound Rm is used in some defect assessment methods (BS 7910 Failure Assessment Diagram).
4. Charpy Impact Sub-Grades: JR, J0, J2, K2
The Charpy sub-grade specifies the minimum absorbed energy and the test temperature:
| Sub-Grade | Min. Energy | Test Temperature | Typical Use |
|---|---|---|---|
| JR | 27 J | +20°C | Indoor/sheltered, warm-climate applications |
| J0 | 27 J | 0°C | Moderate climate, non-critical |
| J2 | 27 J | −20°C | North Sea standard — most common for offshore |
| K2 | 40 J | −20°C | Higher energy at same temperature — splash zone |
The minimum design temperature (MDT) governs sub-grade selection. NORSOK M-001 and DNV-OS-C101 require that the Charpy test temperature is at or below the MDT. For North Sea applications:
- Atmospheric zone (above water): MDT typically −20°C → S355J2 minimum
- Splash zone: MDT −20°C with higher energy → S355K2 or S355J2+N with supplementary requirement
- Submerged zone: MDT typically 0°C → S355J0 is acceptable in some cases, but S355J2 is specified for primary structural members
- Arctic locations: MDT can be −40°C → requires supplementary Charpy at −40°C (ordered as S355J2+N with EN 10025-2 Annex requirement or NORSOK M-001 special grade)
5. Delivery Conditions
EN 10025-2 defines delivery conditions that affect material strength, ductility, and weldability. The condition is designated with a suffix:
| Suffix | Condition | Key Property |
|---|---|---|
| +AR | As-rolled | No heat treatment after rolling; lowest cost |
| +N | Normalised / normalising rolled | Refined grain; better toughness and weldability; most common offshore choice |
| +M | Thermomechanically rolled (TMCP) | Higher strength at lower CEV; excellent weldability without pre-heat for thick plates; good for S355ML/S420ML |
| +Q | Quenched and tempered | Highest strength grades (S420/S460); very fine grain; high toughness |
| +QT | Quenched and tempered + tempered | Same as +Q for thick sections |
The +M suffix (TMCP) is particularly important: thermomechanically rolled plates can achieve S355 or higher strengths with a lower carbon equivalent than +N grades, making them easier to weld without pre-heat in thick sections. This is why S355ML (M = TMCP, L = lower Charpy test temperature −50°C) appears in arctic and subsea applications.
6. Carbon Equivalent — Weldability
The carbon equivalent (CEIIW) quantifies a steel's susceptibility to hydrogen-induced cracking (HIC) during welding. EN 10025-2 §7.2.3 provides maximum CEV values per grade and thickness:
Calculated from ladle analysis (heat certificate values)
Maximum CEV limits per EN 10025-2 (for plates, selected grades):
| Grade | t ≤ 30 mm | 30 < t ≤ 40 mm | t > 40 mm |
|---|---|---|---|
| S235J2 | 0.35 | 0.35 | 0.38 |
| S275J2 | 0.40 | 0.40 | 0.42 |
| S355J2 | 0.43 | 0.45 | 0.47 |
| S355K2+N | 0.43 | 0.45 | 0.47 |
| S355M/ML | 0.39 | 0.39 | 0.39 |
| S460J2 | 0.47 | 0.49 | 0.49 |
| S460M/ML | 0.43 | 0.43 | 0.43 |
The benefit of +M (TMCP) grades is clear: S355M has a maximum CEV of 0.39 vs 0.47 for S355J2+N at the same thickness. For thick plates (t > 40 mm), this is the difference between requiring significant pre-heat and welding without pre-heat — saving substantial fabrication cost and schedule.
7. Chemical Composition Limits
EN 10025-2 Table 2 specifies maximum ladle analysis chemical composition. Key elements for S355:
| Element | S355 max (t ≤ 40 mm) | Notes |
|---|---|---|
| Carbon (C) | 0.20% (J2/K2); 0.18% (N/M) | Main driver of hardness and cracking risk |
| Manganese (Mn) | 1.60% (J2/K2); 1.65% (N/M) | Primary strengthening element |
| Phosphorus (P) | 0.030% | Embrittlement at grain boundaries |
| Sulfur (S) | 0.025% | Reduces ductility, promotes lamellar tearing |
| Silicon (Si) | 0.55% | Deoxidation |
| Copper (Cu) | 0.55% | Weather resistance (COR-TEN grades) |
| Nitrogen (N) | 0.014% | Strain ageing; controlled in fine-grain grades |
8. Through-Thickness Quality — Z Grades
Standard EN 10025-2 plates have no requirement for through-thickness ductility. When plates are loaded perpendicular to the rolling direction (through-thickness tension, typical in T-joint and cruciform welds), lamellar tearing can occur. For these applications, order steel with Z-quality per EN 10164:
| Z Class | Min. Through-Thickness Reduction in Area Z (%) | Application |
|---|---|---|
| Z15 | 15% | Low-severity T-joints |
| Z25 | 25% | Medium-severity cruciform joints |
| Z35 | 35% | High-severity — required by DNV-OS-C101 for most offshore T-joints with thick flanges |
The Z class is ordered as a supplementary requirement to EN 10025-2: e.g. S355J2+N+Z35. The EN 10204 material certificate must confirm the Z-test result for each heat.
9. Cross-Reference Map
| Standard | Relationship to EN 10025-2 | KB Status |
|---|---|---|
| EN 10204 | Material test certificates — EN 10025-2 §9 requires test certificates per EN 10204; structural steel for primary offshore use typically requires Type 3.1 or 3.2 (inspection certificate with third-party witness) | ✅ Ingested |
| EN 1090-2 | Execution of steel structures — references EN 10025-2 as the base plate material standard for EXC1–EXC4 fabrications; the execution class determines the required material certificate type | ✅ Ingested · blogged |
| NORSOK M-001 | Material selection — specifies minimum Charpy sub-grade and supplementary requirements (Z-quality, CEV limits) for North Sea offshore applications built from EN 10025-2 grades | ✅ Ingested |
| DNV-OS-C101 | Structural design — defines when Z35 through-thickness quality is required, minimum Charpy requirements per structural category, and references EN 10025-2 as the plate material specification | ✅ Ingested · blogged |
| DNV-ST-0377 | Structural systems — maps structural category (Special/Primary/Secondary) to minimum material grade and sub-grade from EN 10025-2 series | ✅ Ingested · blogged |
10. Common Errors and Pitfalls
- Using the nominal grade yield strength (355 MPa) for thick plates without applying the thickness reduction table — a 60 mm S355J2 plate has ReH,min = 335 MPa; using 355 MPa is a 6% non-conservative error
- Specifying S355JR for offshore primary steel — JR is tested at +20°C and is not suitable for North Sea atmospheric zone where minimum design temperature is −20°C; J2 is the minimum for primary offshore structural steel
- Confusing yield strength ReH with tensile strength Rm — structural calculations use ReH as the characteristic material resistance; Rm is used only in fracture mechanics and some connection design checks
- Not ordering Z35 quality for thick flange plates in T-joints — EN 10025-2 standard plates have no through-thickness ductility guarantee; lamellar tearing in the heat-affected zone of T-joint welds on 50+ mm flanges is a documented failure mode that Z35 prevents
- Ordering +AR (as-rolled) when +N or +M is required — the delivery condition affects CEV, toughness, and grain size; some procurement specs allow the mill to supply any condition unless explicitly specified; always specify +N or +M for primary offshore steel
- Using the ladle (heat) CEV from the mill cert for weldability assessment without checking the product (check) analysis — EN 10025-2 allows product analysis to differ from ladle analysis within specified tolerances; the product analysis is what's actually in the plate; use the higher of the two
- Applying S355J2 without specifying fine grain practice (+N or +M) for plates >40 mm on subsea/arctic applications — fine grain practice (Al ≥ 0.02% or Nb/V/Ti microalloying) is required by NORSOK M-001 for primary members >25 mm; the sub-grade alone (J2) does not guarantee fine grain structure
Ask Leide Navigator about EN 10025-2
EN 10025-2:2019 is ingested in the Navigator knowledge base (65 chunks), together with EN 10204, EN 1090-2, and NORSOK M-001. Ask about yield strength for a specific grade and thickness combination, sub-grade selection for a design temperature, CEV limits for weldability, or Z-quality requirements for a T-joint configuration.