S355 is the workhorse structural steel of the European system, standardised in EN 10025-2. It is a low carbon steel with a nominal minimum yield strength of 355 MPa, and it is bought far more often by suffix than by name, because the suffix determines impact toughness and delivery condition. Getting that suffix wrong is the most common material selection error in fabricated steelwork.
What the Suffixes Mean
The letters after the strength figure describe the guaranteed Charpy impact energy and the test temperature, while the plus sign describes the rolling or heat treatment condition: AR means as-rolled, N means normalized or normalized rolled. So S355J2+N is a normalized plate with 27 J guaranteed at minus 20 degrees C, and the same grade in the as-rolled condition is written S355J2+AR.
| Designation | Impact energy, min | Test temperature (C) | Delivery condition |
|---|---|---|---|
| S355JR | 27 J | +20 | AR or N |
| S355J0 | 27 J | 0 | AR or N |
| S355J2 | 27 J | -20 | AR or N |
| S355K2 | 40 J | -20 | N |
Strength and Thickness
Yield strength is a function of thickness, not a single number. The 355 MPa figure applies only to the thinner end of the range, and heavier plate is tested to a reduced minimum. Designers working to a European code will already apply these reductions, but fabricators buying plate against a nominal grade should confirm that the delivered thickness class matches the calculation.
| Thickness (mm) | Yield strength, min (MPa) | Tensile strength (MPa) | Elongation, min (%) |
|---|---|---|---|
| t <= 16 | 355 | 470-630 | 22 |
| 16 < t <= 40 | 345 | 470-630 | 22 |
| 40 < t <= 63 | 335 | 470-630 | 21 |
| 63 < t <= 80 | 325 | 470-630 | 20 |
| 80 < t <= 100 | 315 | 470-630 | 20 |
Chemistry and Weldability
Carbon and manganese are held low enough to keep the heat affected zone sound, and the carbon equivalent value is capped for the same reason. The J2 quality carries tighter phosphorus and sulphur limits than JR, which is why it is the usual choice for dynamically loaded and low temperature structures.
| Element (mass %) | S355JR | S355J0 / J2 / K2 |
|---|---|---|
| C, max | 0.24 | 0.22 |
| Si, max | 0.55 | 0.55 |
| Mn, max | 1.60 | 1.60 |
| P, max | 0.045 | 0.035 |
| S, max | 0.045 | 0.035 |
| N, max | 0.012 | 0.012 |
For the JR, J0 and J2 qualities the carbon equivalent value is normally limited to 0.45 for thicknesses up to 30 mm and slightly higher above that, which keeps the material comfortably weldable by manual metal arc, submerged arc and gas shielded processes with the usual low hydrogen consumables.
Equivalent Grades in Other Systems
| System | Designation | Practical note |
|---|---|---|
| ASTM | A572 Grade 50 | Comparable strength, different chemistry and impact rules |
| EN / DIN | St52-3 | Historic German designation still seen on old drawings |
| JIS | SM490 | Japanese welded structural grade, similar properties |
| GB | Q355 | Chinese structural grade, suffix system covers impact temperature |
Equivalence tables are a starting point for conversation, not a basis for substitution. Design codes pair each steel with its own partial factors and thickness rules, and impact requirements do not travel across systems unchanged.
Where S355 Is Used
Buildings and industrial frames, bridge girders and cross bracing, crane runways and jibs, heavy machinery bases, storage tank supports, offshore secondary structures and transport equipment chassis. It is chosen because the strength to weight ratio allows lighter sections than an ordinary 235 MPa grade while welding procedures stay conventional and plate remains easy to cut, drill and form.
Frequently Asked Questions
Q: What is the difference between S355JR and S355J2?
A: The impact test condition. JR guarantees 27 J at plus 20 degrees C and J2 guarantees 27 J at minus 20 degrees C, so J2 is used where cold service or dynamic loading applies.
Q: What does the +N mean in S355J2+N?
A: It means the plate was normalized or normalized rolled, a condition that refines grain size and gives more consistent toughness than the as-rolled condition.
Q: Is S355 the same as A572 Grade 50?
A: They are close in strength and are frequently interchanged in practice, but the chemical ranges, impact testing and tolerances come from different standards and must be checked against the design code in force.
Q: Can S355 be welded without preheat?
A: For common thicknesses and low carbon equivalents, low hydrogen consumables and normal workshop practice are sufficient. Thick sections, high restraint joints and cold ambient conditions call for preheat and a qualified welding procedure.
Q: What is the carbon equivalent limit for S355JR?
A: It is capped for weldability rather than left open, typically at 0.45 for thicknesses up to 30 mm, with a slightly higher value permitted above that thickness.
Q: What thickness does S355 plate come in?
A: EN 10025-2 covers plate from a few millimetres up to 400 mm, but yield strength drops across the thickness classes, so heavy plate must be specified with the thickness class in mind.
