S235 and S355 are the two most common non-alloy structural steel plate grades used in European design, and the choice between them is made on almost every steel structure package. Both are specified in EN 10025-2, both are carbon-manganese steels with guaranteed mechanical properties, and both weld readily with conventional processes. The differences that decide the specification are minimum yield strength, toughness class, available thickness and the behaviour of the finished member.
What the Grade Designation Tells You
In EN 10025-2 the letter S stands for structural steel, and the number is the minimum yield strength ReH in MPa for the smallest thickness range, up to 16 mm. So S235 guarantees 235 MPa and S355 guarantees 355 MPa at that thickness. The suffix describes the impact (Charpy V-notch) test condition, and the delivery condition is added after the suffix.
| Impact suffix | Test temperature | Minimum absorbed energy, longitudinal |
|---|---|---|
| JR | +20 °C | 27 J |
| J0 | 0 °C | 27 J |
| J2 | -20 °C | 27 J |
| K2 | -20 °C | 40 J |
Delivery conditions such as +AR (as rolled), +N (normalised) and +M (thermomechanically rolled) follow the grade, for example S355J2+N. The suffix is not decorative: it defines the lowest service temperature at which the plate has verified notch toughness, which is why cold-store, crane and outdoor structures in cold climates are usually ordered as J2 or K2 rather than JR.
Chemical Composition and Mechanical Properties
The chemistry of S235 and S355 is similar; the strength difference comes mainly from micro-alloying, controlled rolling and cooling practice rather than from a fundamentally different analysis. The table below gives EN 10025-2 ladle limits for the most widely ordered sub-grades.
| Grade | C max (%) | Mn max (%) | P max (%) | S max (%) | N max (%) | Cu max (%) |
|---|---|---|---|---|---|---|
| S235JR | 0.17 | 1.40 | 0.035 | 0.035 | 0.012 | 0.55 |
| S235J0 | 0.17 | 1.40 | 0.030 | 0.030 | 0.012 | 0.55 |
| S235J2 | 0.17 | 1.40 | 0.025 | 0.025 | - | 0.55 |
| S275JR | 0.21 | 1.50 | 0.035 | 0.035 | 0.012 | 0.55 |
| S355JR | 0.24 | 1.60 | 0.035 | 0.035 | 0.012 | 0.55 |
| S355J0 | 0.20 | 1.60 | 0.030 | 0.030 | 0.012 | 0.55 |
| S355J2 | 0.20 | 1.60 | 0.025 | 0.025 | - | 0.55 |
Tighter toughness classes carry tighter phosphorus and sulphur limits. Tables that quote 0.05 % P and S for all three strength levels belong to withdrawn national grades such as the old BS 4360 series, not to EN 10025-2, and must not be used for specification or inspection.
Yield strength falls as plate thickness rises, because thicker sections receive less reduction during rolling. EN 10025-2 sets a descending staircase of minimum values.
| Nominal thickness (mm) | S235 ReH min (MPa) | S275 ReH min (MPa) | S355 ReH min (MPa) |
|---|---|---|---|
| up to 16 | 235 | 275 | 355 |
| over 16 to 40 | 225 | 265 | 345 |
| over 40 to 63 | 215 | 255 | 335 |
| over 63 to 80 | 215 | 245 | 325 |
| over 80 to 100 | 215 | 235 | 315 |
| over 100 to 150 | 195 | 225 | 295 |
| over 150 to 200 | 185 | 215 | 285 |
Tensile strength ranges are 360-510 MPa for S235, 410-560 MPa for S275 and 470-630 MPa for S355 across the usual plate thicknesses. At 100 mm the gap narrows to 315 MPa against 215 MPa, yet S355 still offers roughly 45 % more usable yield stress than S235.
Strength Is Not the Only Difference
The elastic modulus of all these grades is essentially identical, about 210 GPa, so stiffness does not improve when a higher grade is ordered. What improves is the stress at which the section yields. Elongation and notch toughness, by contrast, are highest in the lower grades; as strength rises, ductility margins and welding tolerance become tighter.
Deflection-governed members gain little from S355, because deflection depends on modulus and geometry.
Strength-governed members can use thinner plate or lighter sections and cut steel mass.
A higher carbon equivalent calls for closer control of heat input, consumables and preheat.
Heavy sections, thick flanges and long spans are where S355 pays back most clearly.
Equivalent Grades in Other Standards
| EN 10025-2 grade | ASTM | JIS | China GB |
|---|---|---|---|
| S235JR | A36, A283 Gr C | SS400 | Q235B (GB/T 700) |
| S275JR | A572 Gr 42 | SM400A | Q275B (GB/T 700) |
| S355JR | A572 Gr 50 | SM490A | Q355B (GB/T 1591) |
Equivalents are a starting point, not a substitution licence. ASTM A36 has a minimum yield strength of 250 MPa and no mandatory impact test, while S235JR requires 27 J at +20 °C and holds phosphorus and sulphur to 0.035 % each. Q235B and Q355B carry their own limits under GB/T 700 and GB/T 1591. Grade chemistry, impact class and thickness de-rating should all be compared before a cross-standard substitution is accepted.
How to Choose Between S235 and S355
Start from the governing limit state. If the member is controlled by deflection or by the stability of a slender section, S235 is usually adequate and simpler to weld. If it is controlled by bending or axial stress, S355 allows a lighter structure because the cost per kilogram of the two grades is similar.
Then check the service temperature. Below 0 °C, or where impact and dynamic loading occur, JR grades should be replaced by J0, J2 or K2 regardless of strength level. Finally confirm availability in the required thickness and delivery condition, since J2 and K2 plate in heavy gauges is produced in narrower thickness windows than JR.
Frequently Asked Questions
Q: Is S355 stronger than S235?
Yes. S355 has a minimum yield strength of 355 MPa up to 16 mm against 235 MPa for S235, an increase of about 51 %.
Q: Can S355 replace S235 without changing the section?
Yes, and the resistance increases, but stiffness and deflection stay the same because the elastic modulus is unchanged.
Q: What does the JR suffix mean?
JR means the plate is impact tested at +20 °C with a minimum absorbed energy of 27 J in the longitudinal direction.
Q: Are S235 and S355 chemically very different?
No. S355 normally has slightly higher manganese and is produced with controlled rolling or micro-alloying to reach the higher yield strength.
Q: Which grade suits low-temperature service?
Use J0 for 0 °C, J2 for -20 °C and K2 where 40 J at -20 °C is required; JR grades are qualified at +20 °C only.
Q: Does S355 weld as easily as S235?
It welds with the same processes, but the higher carbon equivalent calls for controlled heat input, suitable consumables and preheat in thick sections.
