S355 and S275 are the two most widely specified structural steel grades in European construction, both covered by EN 10025-2. Understanding their differences is essential for structural engineers, fabricators and procurement professionals.
Mechanical Properties Comparison
The grade designation directly indicates minimum yield strength in MPa at room temperature for material thickness ≤ 16 mm:
**S275**: Minimum yield strength 275 MPa, tensile strength 410–560 MPa, minimum elongation 23% (for long specimens). S275 offers good ductility and is easily formed and welded.
**S355**: Minimum yield strength 355 MPa, tensile strength 470–630 MPa, minimum elongation 22%. S355 provides approximately 29% higher yield strength compared to S275, allowing significant weight savings in structural members.
Impact Toughness and Sub-Grades
Both grades are available in sub-grades that specify impact toughness requirements per Charpy V-notch testing: JR (20°C), J0 (0°C), J2 (−20°C), and K2 (−20°C with higher energy requirement). Selection depends on service temperature and consequence class per EN 1993-1-10.
For structures in Nordic climates or exposed to sub-zero temperatures, J2 or K2 sub-grades are typically required. The EN 1090-2 execution class and the National Annex will often specify minimum toughness requirements.
Weldability
Both grades are readily weldable. However, S355 has a higher carbon equivalent value (CEV), typically 0.45 maximum versus 0.40 for S275. This means S355 may require preheating for thicker sections (generally above 25–30 mm) to prevent hydrogen-induced cracking.
Welding procedures for both grades should be qualified per EN ISO 15614-1. For S355 in thickness above 30 mm, a preheat of 75–150°C is common depending on CEV, hydrogen content of the consumable, and combined thickness at the joint.
Cost and Weight Optimization
S355 is typically 5–10% more expensive per tonne than S275. However, the 29% increase in yield strength often allows significantly lighter structural members. In beam and column design, the weight saving frequently outweighs the material cost premium.
For example, a simply supported floor beam designed to Eurocode 3 (EN 1993-1-1) might require an IPE 400 in S275 but only an IPE 360 in S355 for the same loading condition — a saving of approximately 12% in steel weight.
When to Use Which Grade
**Choose S275** when: ductility is critical (seismic zones), forming and bending requirements are severe, cost per tonne is the primary driver, or for lightly loaded secondary members.
**Choose S355** when: structural efficiency and weight savings are important, for primary load-bearing members (beams, columns, trusses), in high-rise or long-span structures, or where foundation loads need to be minimized.
Both grades are fully covered by the Eurocode 3 design framework and can be used in all execution classes per EN 1090-2.