The Basic Difference in Strength
S235 and S355 are both defined in EN 10025-2. The numbers indicate the minimum yield strength in MPa for plate thickness up to 16 mm, so S235 starts at 235 MPa and S355 starts at 355 MPa. The higher strength of S355 means that a lighter section can carry the same load, which reduces dead weight in large structures. The difference narrows slightly in heavier thickness ranges, where both grades lose strength progressively.
Tensile Properties and Ductility
S235 has a tensile strength of about 360 to 510 MPa, while S355 reaches about 470 to 630 MPa. Both require a minimum elongation of 22 percent in the common sub-grades, so the higher strength of S355 does not come at the cost of ductility in the as-rolled condition. Both grades are fully killed and fine grained, which keeps toughness consistent and supports welding without special measures.
Sub-Grades and Toughness Classes
Each grade offers the same family of impact sub-grades. The JR variant is tested at plus 20 degrees C with 20 J minimum, J0 requires 27 J at 0 degrees C, J2 requires 27 J at minus 20 degrees C, and K2 requires 40 J at minus 20 degrees C. This parallel structure makes it straightforward to select the correct toughness class for the service temperature while keeping the same strength level.
Chemical Composition and Weldability
S235 permits a maximum carbon content of about 0.17 percent, and S355 allows about 0.20 to 0.24 percent depending on the sub-grade. Manganese is limited to roughly 1.40 percent in S235 and 1.60 percent in S355, with restricted phosphorus and sulfur in both. Because the carbon equivalent of both grades is modest, they can be welded with conventional procedures, and the lower carbon content of S235 makes it slightly more forgiving in poor fit-up or high restraint conditions.
Cost and Selection Guidance
S235 is usually less expensive per tonne and is available in a wide range of sections, but S355 can be more economical overall because it reduces the weight of steel required. Choose S235 for lightly loaded structures such as low rise frames, platforms, railings, supports and general fabrication. Choose S355 for bridges, high rise buildings, crane structures, heavy machine bases and long span members where weight saving and higher capacity justify the higher grade.
Frequently Asked Questions
Q: What is the main difference between S235 and S355?
The main difference is strength. S235 has a minimum yield strength of 235 MPa and S355 has 355 MPa for thickness up to 16 mm, while their sub-grade structure is the same.
Q: Is S355 stronger than S235?
Yes. S355 offers roughly 50 percent higher minimum yield strength and a higher tensile range, which allows lighter sections for the same load.
Q: Can S235 and S355 be welded the same way?
Both grades are readily weldable with conventional procedures, although S355 has a slightly higher carbon content and carbon equivalent, so procedure control matters more in thick sections.
Q: Which grade is cheaper?
S235 generally costs less per tonne, but S355 can be more economical in a finished structure because it reduces total steel weight.
Q: When should S355 be selected instead of S235?
Select S355 for bridges, high rise buildings, cranes and heavily loaded structures where higher strength and weight reduction are required.
