What the R and 0 Suffixes Really Mean
S355JR and S355J0 belong to the same strength family in EN 10025-2. The letter J marks a Charpy V-notch impact requirement, and the character that follows records the test temperature: R means the test is carried out at plus 20 C, while 0 means it is carried out at 0 C. In both cases the minimum absorbed energy is 27 J on longitudinal specimens. A widespread misconception is that JR stands for junior and therefore describes a weaker impact class. The suffix has nothing to do with grade status; it simply records a warmer test temperature than J0, J2 or K2.
Chemical Composition Compared
| Element, ladle analysis (max) | S355JR | S355J0 |
|---|---|---|
| Carbon | 0.24 % | 0.22 % |
| Silicon | 0.55 % | 0.55 % |
| Manganese | 1.60 % | 1.60 % |
| Phosphorus | 0.035 % | 0.030 % |
| Sulfur | 0.035 % | 0.030 % |
| Nitrogen | 0.012 % | 0.012 % |
The two grades look almost identical on the strength side, but the composition limits differ in a way that matters. S355J0 is held to a lower maximum carbon content of 0.22 % against 0.24 % for S355JR, and its phosphorus and sulfur caps are tightened from 0.035 % to 0.030 %. Lower carbon improves weldability and reduces hardness in the heat-affected zone, while lower phosphorus and sulfur raise notch toughness and lower the risk of cracking in thick welded joints. These tighter limits are consistent with the colder impact test demanded by the J0 designation.
Impact Toughness: The Deciding Difference
The essential difference between the two grades is the temperature at which the Charpy V-notch test is performed and must be passed. S355JR certifies 27 J minimum at plus 20 C. S355J0 certifies the same 27 J minimum at 0 C. The absorbed energy is identical; only the test temperature changes. That means S355J0 has to retain its toughness at a lower temperature, which is why it commands a more tightly controlled composition and is the correct choice when the design metal temperature is close to freezing.
Design practice usually follows the minimum metal temperature expected in service. If the coldest condition the component will experience stays comfortably above plus 20 C, S355JR provides the required evidence. If service temperature drops towards 0 C, S355J0 supplies the appropriate certification, and for colder duties such as minus 20 C the J2 or K2 designations are normally specified instead. This is why the impact suffix, not the strength figure, often decides which grade is ordered.
Mechanical Properties and Thickness
| Property (minimum values) | S355JR | S355J0 |
|---|---|---|
| Yield strength, 16 mm and under | 355 MPa | 355 MPa |
| Yield strength, over 16 to 40 mm | 345 MPa | 345 MPa |
| Yield strength, over 40 to 63 mm | 335 MPa | 335 MPa |
| Tensile strength, 16 mm and under | 470-630 MPa | 470-630 MPa |
| Impact energy, longitudinal | 27 J at plus 20 C | 27 J at 0 C |
The strength tables for S355JR and S355J0 are the same in EN 10025-2. Both reach a minimum yield of 355 MPa for thin sections, both fall away gradually as thickness grows, and both are certified in the same 470-630 MPa tensile window for light and medium plate. Any suggestion that one grade is stronger than the other is a misunderstanding: the grades differ in toughness certification and in composition limits, not in strength level. Thickness still governs the actual values, so the certificate must be read against the ordered thickness.
Weldability, Processing and Applications
Both grades weld readily with conventional consumables matched to the parent strength, and both are commonly sheared, flame cut, drilled and cold formed. The extra carbon in S355JR slightly raises hardenability, so thick sections and highly restrained joints need the carbon equivalent value to be checked carefully when welding procedures are qualified. S355J0, with its lower carbon and lower residual phosphorus and sulfur, is the more forgiving grade in heavy welded fabrication and in structures exposed to moderately low temperatures.
General building frames and indoor steelwork: S355JR is usually sufficient.
Outdoor structures in temperate climates: either grade, decided by the design metal temperature.
Bridges, crane structures and cold-store steelwork: S355J0 or S355J2, depending on the lowest service temperature.
Heavy welded joints and thick plate: S355J0, for its tighter compositional limits.
Grade selection should always be recorded on the purchase order together with the required impact test temperature, because the designation itself is the certification. Requesting the wrong suffix is one of the most common causes of rejected mill certificates in supply contracts.
Frequently Asked Questions
Q: Is S355J0 the same as S355JR?
No. They share the same strength class, but S355J0 must pass 27 J at 0 C and is held to tighter carbon, phosphorus and sulfur limits.
Q: Does JR mean a junior grade in S355JR?
No. The letter R records that the Charpy impact test is carried out at plus 20 C. It is not a statement about grade status.
Q: Which grade is tougher, S355JR or S355J0?
S355J0, because its impact energy of 27 J is demonstrated at the lower temperature of 0 C rather than at plus 20 C.
Q: What is the minimum absorbed energy for both grades?
27 J on longitudinal Charpy V-notch specimens for both grades; only the test temperature differs.
Q: Are the yield and tensile strengths different between them?
No. The strength requirements in EN 10025-2 are the same for both grades, with yield values decreasing as plate thickness increases.
Q: Can S355JR be used below freezing?
Only with project approval. Where service temperature falls to 0 C or lower, S355J0, S355J2 or S355K2 should be specified instead.
