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S235 or S355, and what the letters after them mean
The number is the yield strength. The letters are the toughness, and they are the half that gets left off a drawing and then costs a week.
S235 and S355 are non-alloy structural steels to EN 10025-2. Both sit in group 1 of ISO/TR 15608, which is the group most welding certificates cover, and both are welded in this hall every week. The number is the minimum yield strength in newtons per square millimetre for the thinnest product range: 235 for the first, 355 for the second.
That difference is about fifty percent more yield strength, and it is the reason to reach for S355. For a load-limited part it lets a thinner section carry the same load, which takes weight out of the structure, takes volume out of the welds that join it, and often takes cost out of the job as a whole even though the steel costs more per tonne.
Where S355 stops paying
Not every part is load-limited. If the section is set by stiffness, by a bolt pattern, by a bearing bore or by something that has to fit, then a stronger steel buys nothing: the deflection of a beam depends on the modulus and the geometry, and the elastic modulus of S355 is the same as that of S235. Paying more per tonne for strength you cannot use is a common and entirely avoidable line on a bill of materials.
S355 also asks a little more of the shop. It springs back further in the press brake and wants a larger bend radius, so a part designed around a tight radius in S235 may not fold in S355 without cracking. At the thicker end its higher carbon equivalent can bring preheat into the picture, which is a real cost and a real schedule item.
The letters are the important half
A grade written as "S355" is incomplete, and it is the most common gap on the drawings we quote. The suffix states the impact toughness: the temperature at which the steel must still absorb a specified energy in a Charpy V-notch test.
- JR: tested at room temperature, 20 degrees Celsius.
- J0: tested at 0 degrees Celsius.
- J2: tested at minus 20 degrees Celsius.
- K2: tested at minus 20 degrees Celsius to a higher absorbed energy than J2.
Which one you need follows from the lowest service temperature, the thickness and how the joint is stressed; EN 1993-1-10 is the standard that turns those into a subgrade. The practical rule for outdoor plant in central Europe is that JR is often not enough and J2 usually is, but that is a rule of thumb and not a substitute for the calculation.
What this means for a drawing
Write the full designation, S355J2 rather than S355, and write it once in a place that governs the whole part rather than three times in three places that can disagree. If different members genuinely need different grades, say which. And if you are not sure of the subgrade, say that too: it is a five-minute conversation at enquiry and a re-order if it surfaces after the plate is cut.
Our certified welding scope is ISO/TR 15608 group 1 up to 40 mm and group 8, austenitic stainless, from 3.0 to 16.0 mm. S235 and S355 both fall in group 1. Aluminium is not in the scope and is not offered.