What the Suffix Actually Controls
S355JR and S355J2 belong to the same family in EN 10025-2, the standard for hot rolled products of structural steels in the non-alloy quality group. The number 355 is the specified minimum yield strength in megapascals for the thinnest thickness range covered by the standard, and it is identical for both grades. The letters and digits after the number describe the impact test class, and that is where the two grades separate.
In the EN 10025-2 designation system, R indicates that impact energy is verified at plus 20 degrees Celsius, 0 at zero degrees Celsius, 2 at minus 20 degrees Celsius and K2 at minus 20 degrees Celsius with a higher absorbed energy requirement. S355JR is therefore tested with a Charpy requirement of 27 joules at plus 20 degrees Celsius, while S355J2 is tested with the same 27 joule requirement at minus 20 degrees Celsius. S355J0 sits between them at zero degrees Celsius, and S355K2 raises the minimum absorbed energy to 40 joules at minus 20 degrees Celsius.
Impact Energy and Service Temperature
| Grade | Impact test temperature | Minimum absorbed energy | Typical service use |
|---|---|---|---|
| S355JR | plus 20 degrees C | 27 J | Indoor structures, general fabrication, ambient service |
| S355J0 | 0 degrees C | 27 J | Outdoor structures in moderate climates |
| S355J2 | minus 20 degrees C | 27 J | Cold climate structures, dynamic and cyclic loading |
| S355K2 | minus 20 degrees C | 40 J | Severe low temperature or high impact demand |
The impact class is a design input, not a formality. A structure that will see service temperatures near freezing, or that will be loaded dynamically, is ordered with a tested impact temperature well below the lowest expected service metal temperature, so that the steel retains toughness instead of behaving in a brittle manner.
Chemistry and Carbon Equivalent
Because a lower carbon content and lower residual element levels are needed to hold toughness at lower temperature, the two grades also carry different chemical limits in EN 10025-2. S355JR is permitted a higher maximum carbon content and higher maximum phosphorus and sulphur than S355J2, while the maximum manganese level is the same for both. The carbon equivalent value, calculated from the analysis, is capped by the standard according to thickness, and the capped values are the same for the two grades at the same thickness.
The practical consequence is weldability. A J2 grade with a lower maximum carbon and lower residuals is easier to weld in thick sections and has less risk of hydrogen-assisted cracking in the heat-affected zone, which is why low-temperature structural work and heavier weldments normally specify J2 rather than JR even when the impact requirement alone could be met by JR.
Strength by Thickness
The yield strength of both grades falls as thickness rises. The value 355 MPa applies up to 16 mm. Above that the specified minimum steps down through 345 MPa for thicknesses over 16 mm up to 40 mm, 335 MPa over 40 mm up to 63 mm, 325 MPa over 63 mm up to 80 mm, and further reductions in the thicker ranges of the standard. Tensile strength is specified as a band rather than a minimum: approximately 470 to 630 MPa for thin product and 450 to 600 MPa in the normal structural thickness range. Both grades are supplied in the as-rolled, normalised or thermomechanically rolled condition according to what is agreed at order.
Because yield strength depends on thickness, the design engineer must use the value for the actual plate or section thickness rather than the headline 355 MPa figure. This is a common source of error when a quotation is compared purely on the grade name.
Product Forms and Hollow Sections
Both grades are available as plate, strip, wide flats, bars and hot rolled or cold formed structural hollow sections. For tubular and hollow section products a different standard applies: hot finished structural hollow sections are covered by EN 10210 and cold formed ones by EN 10219, and in those standards the grade carries an H suffix, so an S355J2H hollow section is the tubular equivalent of the S355J2 plate grade with the same impact class. A purchase order for pipe or hollow section should therefore quote the product standard and the H designation, not only the plate grade.
Frequently Asked Questions
Q: Do S355JR and S355J2 have the same strength?
A: Yes. The specified yield and tensile strength values are identical for both grades at the same thickness; only the impact test class and the chemical limits differ.
Q: Can S355JR be substituted for S355J2?
A: Only after engineering review. JR is verified at plus 20 degrees Celsius, so substituting it in a structure designed for minus 20 degrees Celsius service temperature removes the toughness assurance the design assumed.
Q: Which grade is better for welding?
A: S355J2 generally welds more comfortably in heavy sections because of its tighter carbon, phosphorus and sulphur limits, even though the standard applies the same carbon equivalent ceiling to both grades at a given thickness.
Q: What does the H in S355J2H indicate?
A: It identifies the grade as a structural hollow section product to EN 10210 or EN 10219 rather than a flat product to EN 10025-2, with the same 355 MPa yield class and the same impact class as the plate grade.
Q: Why is the yield strength quoted lower on thick plate?
A: Because the standard specifies yield strength per thickness range, and the minimum value decreases as section thickness increases due to the lower cooling rate and coarser microstructure in thick product.
Q: Is S355K2 ever specified instead of S355J2?
A: Yes, where a higher absorbed energy of 40 joules at minus 20 degrees Celsius is required, for example in safety-critical or high impact loading applications.