Welding Structural Steel: S235, S355 and the Rules of EN 1090
Knowledge · Welding in contract manufacturing
What do designations like S355J2 actually say?
S stands for structural steel, the number for the minimum yield strength in N/mm² at small thicknesses: S235 carries from 235 N/mm², S355 from 355 N/mm². The letter group describes notch toughness, the safety against brittle fracture in cold conditions: JR is tested at room temperature, J0 at 0 °C, J2 at −20 °C. For welded, dynamically loaded or outdoor structures, S355J2 is the standard in machine building; +N (normalised) ensures a uniform microstructure.
Why are structural steels so weldable?
Because of their low carbon content. The decisive figure is the carbon equivalent CEV, which also weights manganese, chromium and other elements. As a rule of thumb, steels up to about CEV 0.45 are weldable without special measures. S235 and S355 sit clearly below that in common thicknesses and join quickly and reliably with MAG without preheating. This combination of strength, weldability and price makes S355 the workhorse of steel construction.
When is preheating required?
When the seam would cool too fast and harden: at large plate thicknesses (orientation: from about 30 to 40 mm depending on joint detail and heat input), at low workshop temperatures, and for high-strength fine-grain steels as a matter of course. Preheating to typically 100 to 150 °C slows cooling, reduces hardening in the heat-affected zone and drives out hydrogen that could cause cold cracking. The welding coordinator specifies it in the welding procedure specification.
What does EN 1090 require for load-bearing parts?
That only certified companies place load-bearing steel components on the market. EN 1090-1 governs factory production control and CE marking, EN 1090-2 the execution: classes EXC 1 to 4, welder qualifications to EN ISO 9606-1, a qualified welding coordinator, approved procedures and seam qualities to EN ISO 5817. In machine and plant building, EXC 2 is the usual standard. For buyers: ask for the certificate, fix the execution class in the order, agree the documentation.
Common structural steels at a glance
| Material | Yield strength (t ≤ 16 mm) | Impact test | Typical use |
|---|---|---|---|
| S235JR | 235 N/mm² | 27 J at +20 °C | light frames, substructures |
| S355J2+N | 355 N/mm² | 27 J at −20 °C | machine frames, load-bearing steelwork |
| S460 / fine grain | from 460 N/mm² | per grade | weight-optimised, highly loaded structures |
A practical tip: choosing S355J2 over S235 costs little extra on material but allows slimmer sections and smaller welds. Less weld volume means less welding time and less distortion; calculated across the whole assembly, the stronger steel is often the cheaper solution.
Frequently asked questions
What is the difference between S235 and S355?
Minimum yield strength: 235 versus 355 N/mm². S355 carries about 50 percent more at the same cross-section and is the machine-building standard for frames; both weld very well.
What does J2 mean in S355J2?
Notch toughness verified at minus 20 °C (27 joules). For welded, outdoor or dynamically loaded structures, J2 is the usual minimum grade against brittle fracture.
Does structural steel need preheating?
At common thicknesses up to about 30 mm usually not. For thick sections, cold conditions or high-strength grades the welding coordinator specifies preheating, typically 100 to 150 °C.
Who may weld load-bearing steel components?
Only companies certified to EN 1090-1 with welders qualified to EN ISO 9606-1 and a qualified welding coordinator. The execution class, usually EXC 2, belongs in the purchase order.