The Grade Ladder for Hot Rolled H Beams
EN 10025-2 sets out a ladder of structural grades in which the two numbers state the minimum yield strength and the letters describe impact quality. For hot rolled H beams the five grades ordered most often are S235JR, S275JR, S355JR, S355J0 and S355J2. The strength step from S235 to S355 allows a reduction of roughly a third in section mass for a strength governed design, while the J suffix step decides the lowest service temperature at which notch toughness is certified.
Because H beams concentrate material in parallel flanges, they resist bending in the plane of the web and, to a lesser extent, about the weak axis, and the straight inner faces simplify end plate and stiffener welding. The grade chosen must therefore be matched to both the load level and the temperature of the service environment, since the two are certified separately.
Mechanical Properties and Impact Classes
| Grade | ReH min up to 16 mm (MPa) | Tensile strength (MPa) | Charpy requirement |
|---|---|---|---|
| S235JR | 235 | 360 - 510 | 27 J at +20 C |
| S275JR | 275 | 410 - 560 | 27 J at +20 C |
| S355JR | 355 | 470 - 630 | 27 J at +20 C |
| S355J0 | 355 | 470 - 630 | 27 J at 0 C |
| S355J2 | 355 | 470 - 630 | 27 J at -20 C |
Yield strength falls as thickness increases, so the value for the actual flange thickness should be taken from the standard rather than from the headline number. S235JR and S275JR are general building grades, S355JR is used where strength or section depth control the design, and S355J0 and S355J2 are selected for cold climate structures and for members subject to dynamic loading, where the notch toughness test temperature matters more than the strength value.
Hot Rolling and Microstructure
H beams are rolled at 1,100 - 1,250 C, which allows the steel to be shaped in universal stands without excessive roll force and produces a dense, fine grained structure as the section cools. Straightening after the last pass removes bow and sweep, and controlled cooling keeps flange and web properties uniform along the length.
The resulting microstructure is fine ferrite and pearlite, which gives good weldability and stable mechanical properties in both the flange and the web. Low sulfur and phosphorus contents, each limited to 0.035 percent in the JR grades, limit the risk of hot cracking and lamellar tearing during fabrication.
Dimensions, Tolerances and Delivery
Dimensions and mass are taken from EN 10365 for HEB, HEA and IPE series, and shape tolerances are stated in the same standard family for out-of-straightness, flange squareness and web off-centre. Sections are supplied as rolled, or shot blasted and primed where a painted system is specified, and cut to length in the mill or at a processing centre.
Where a structure is subject to tension in the through thickness direction, for example at a heavy welded tee joint, plate and section properties in the thickness direction can be specified to EN 10164 with Z15, Z25 or Z35 quality, and the requirement should be stated at the enquiry stage because it affects steelmaking route and lead time.
Welding, Bolting and Protection
Fabrication follows AWS D1.1/D1.1M or EN 1090-2 at the execution class stated in the project specification, with welding procedures qualified to ISO 15614-1. The J quality grades weld with the same consumables as the corresponding JR grades, but the lower test temperature of S355J2 implies use in cold service where preheat and hydrogen control are usually specified for thicker sections.
Bolted connections use high strength structural bolts to ASTM F3125 or EN 15048. Outdoor structures are commonly hot dip galvanized to EN ISO 1461, with the coating thickness related to steel thickness, and site repairs to the coating are made with a zinc-rich product of compatible composition.
Frequently Asked Questions
Q: What is the difference between S355JR, S355J0 and S355J2?
A: The three grades have identical minimum yield and tensile strength, and differ only in impact toughness: S355JR is tested at plus 20 C, S355J0 at 0 C and S355J2 at minus 20 C, each requiring 27 J minimum absorbed energy. Selection depends on the minimum service metal temperature.
Q: Can S275JR be used instead of S235JR?
A: Yes. S275JR gives a higher yield of 275 MPa against 235 MPa, so a slightly smaller section can be used, and both grades share the same room temperature impact class. The structural check must be repeated for deflection, which does not improve with grade.
Q: Why does EN 10025-2 reduce yield strength with thickness?
A: Thicker sections cool more slowly after rolling, producing a coarser microstructure with lower yield strength. The standard therefore defines yield strength in thickness classes so that designers use a value that is actually achievable in the delivered section.
Q: Are these grades suitable for galvanizing?
A: Yes, all five are routinely hot dip galvanized to EN ISO 1461. The silicon content of the heat influences coating appearance and thickness, so for critical appearance the steelmaking route is discussed with the galvanizer before the order is placed.
Q: Which grade should be specified for a cold store or a winter outdoor structure?
A: S355J0 covers service down to 0 C and S355J2 down to minus 20 C with 27 J minimum Charpy energy. For lower temperatures, a normalized or thermo-mechanically rolled grade with a lower impact test temperature is selected instead of a standard EN 10025-2 grade.
Q: What tolerance applies to flange squareness?
A: Flange squareness and out-of-straightness are covered by the shape tolerance standard for the section family, and the permitted deviation is expressed as a fraction of the nominal dimension. The actual values should be taken from the standard corresponding to the section series being supplied.