S235 or S355? Selecting the Right Structural Steel

Before every steel construction project stands the same fundamental question: is an S235 sufficient, or does it have to be an S355? Both materials belong to the non-alloy structural steels to EN 10025-2 and cover the majority of all design tasks. The choice determines weight, cross-sections, welding effort and, ultimately, cost. We show you what really matters in the selection.

What the designation reveals

The short names follow a fixed scheme. The "S" stands for "structural steel". The number that follows gives the minimum yield strength in MPa (N/mm²) for the smallest product thickness: S235 thus guarantees 235 MPa, S355 correspondingly 355 MPa. Important here: these values are reference values for thin cross-sections. As the material thickness increases, the guaranteed yield strength drops in steps. Decisive are always the tabulated values of the standard in conjunction with your structural analysis.

The suffixes: JR, J0, J2, K2

The letter-number combination after the yield strength describes the guaranteed notch impact toughness, i.e. the resistance to brittle fracture as the temperature falls. The impact energy is tested in the Charpy notched-bar impact bending test:

• JR – test temperature +20 °C
• J0 – test temperature 0 °C
• J2 – test temperature -20 °C
• K2 – test temperature -20 °C at increased minimum impact energy

The lower the service temperature of your construction, the higher the toughness requirements you should choose. For unprotected steel components outdoors or at low operating temperatures, J0 or J2 is the safe choice, while JR is sufficient for many applications in a protected environment.

S235 or S355: the practical consequences

The higher yield strength of the S355 can be translated directly into the design. At the same loading, you get by with smaller cross-sections, which reduces the dead weight and allows slimmer components or greater spans. This pays off especially where weight is a cost driver, for example with beams, crane runways or wide-span hall structures.

S235 is cheaper to buy and is often entirely sufficient for lightly loaded parts. Where strength is not the limiting factor, but rather stiffness, deformation or simply the geometry, the more expensive S355 brings no advantage. The rule of thumb: strength-driven components benefit from S355, everything else you run more economically with S235.

Weldability and inspection certificate

When it comes to processing, you need not worry: both S235 and S355 are readily weldable in the common grades and can be processed without difficulty using the usual methods. With greater thicknesses and higher strengths, it is worth taking a look at preheating and process control, but in the normal case of daily practice this presents no hurdle.

For documentation and verification, we supply you on request with an inspection certificate 3.1 to EN 10204. This gives you the material properties documented and neatly filed for your quality assurance.

In short: The number in the short name is the minimum yield strength in MPa; the suffixes JR/J0/J2 stand for the notch impact toughness at +20/0/-20 °C. S355 allows lighter and slimmer constructions, S235 scores on price for lightly loaded parts. Both are readily weldable. Since all strength values are thickness-dependent, the structural analysis in conjunction with the standard remains decisive in the end, and you can obtain an inspection certificate 3.1 from us on request.