S355NL and S355ML HEA200 wide flange beams belong to the same 355 MPa strength class but to two different European delivery routes. S355NL is defined in EN 10025-3 for normalised or normalised rolled steel, while S355ML is defined in EN 10025-4 for thermomechanically rolled steel. Both grades are impact tested at -50 °C, and both are supplied as HEA200, a compact wide flange beam that is widely used for columns, trusses and crane masts. This guide compares the two materials, sets out the HEA200 dimensions and section properties, and explains how the choice between them affects welding, hot forming and service temperature.
Reading the S355NL and S355ML Designations
In both standards the letter S identifies structural steel and the number 355 is the minimum yield strength in megapascals at the lowest thickness range. The capital letter in the middle describes the delivery condition: N means normalised or normalised rolled, that is, the final properties are obtained by an austenitising heat treatment after rolling, while M means thermomechanically rolled, where the properties come from a controlled combination of rolling temperature and deformation, usually followed by accelerated cooling. The trailing L is the low temperature impact class, and for these two standards it requires a minimum Charpy V-notch energy of 27 J on longitudinal specimens tested at -50 °C.
The practical consequence is that both grades suit cold climate and refrigerated service, but they reach that toughness by different routes. The normalised route is more tolerant of subsequent heating, while the thermomechanical route achieves its properties with lower carbon and alloy content, which is an advantage for welding.
HEA200 Dimensions and Section Properties
HEA is the lightest of the three European parallel flange series and its proportions differ from HEB and HEM sections: an HEA profile is wider than it is deep, so the HEA200 beam has a depth of 190 mm and a flange width of 200 mm. This wide, shallow shape is efficient for columns and for members where the buckling length about the weak axis has to be kept short.
| Parameter | Value |
|---|---|
| Profile | HEA200 |
| Depth h | 190 mm |
| Flange width b | 200 mm |
| Web thickness tw | 6.5 mm |
| Flange thickness tf | 10 mm |
| Root radius r | 18 mm |
| Cross sectional area | 53.8 cm2 |
| Nominal mass | 42.3 kg/m |
| Second moment of area Iy / Iz | 3692 cm4 / 1336 cm4 |
| Elastic section modulus Wely / Welz | 389 cm3 / 133.6 cm3 |
| Radius of gyration iy / iz | 8.28 cm / 4.98 cm |
The 6.5 mm web of HEA200 is thin compared with the flange, so the section is sensitive to local buckling and to damage during handling. Web stiffeners, cope holes and lifting points should be detailed with that thin web in mind, and weld sizes should not be increased beyond what the design requires.
Chemical Composition and Mechanical Requirements
The two standards set similar limits because the strength level is the same. The thermomechanical grade normally has a leaner analysis, which lowers its carbon equivalent and improves weldability, while the normalised grade relies on a fine grained microstructure produced by the heat treatment.
| Element | C | Si | Mn | P | S | N | Al (min) |
|---|---|---|---|---|---|---|---|
| S355NL, EN 10025-3, max % | 0.20 | 0.55 | 1.60 | 0.025 | 0.020 | 0.025 | 0.020 |
| S355ML, EN 10025-4, max % | 0.16 | 0.55 | 1.60 | 0.025 | 0.020 | 0.025 | 0.020 |
Yield strength: minimum 355 MPa for thickness up to 16 mm, stepping down with increasing thickness in both standards
Tensile strength: 470 to 630 MPa for the common plate and beam thicknesses of these grades
Elongation: minimum 22 % on a 50 mm gauge length at the lowest thickness range
Impact energy: minimum 27 J at -50 °C, Charpy V-notch, longitudinal specimens
Fine grain practice: aluminium or an equivalent grain refining addition is required
Carbon equivalent and weldability: the thermomechanical grade is generally supplied with the lower carbon equivalent
The mill certificate should always be read against the actual product thickness, because yield strength, tensile range, elongation and impact values all change with section size, and because both standards allow the producer a choice of product analysis within the specified limits.
Normalised versus Thermomechanical Delivery
A normalised S355NL beam is austenitised and air cooled after rolling, which refines the grain structure and evens out the properties of thick sections. The benefit is stability: the material can be hot formed, stress relieved or welded with preheat and post weld heat treatment without losing the properties that were certified, provided the treatment temperatures are selected sensibly. That is why the normalised grade remains the first choice for heavy welded fabrications, for equipment that must operate above ambient temperature, and for members that will be thermally cut and formed before assembly.
A thermomechanically rolled S355ML beam obtains its fine grain structure from controlled rolling with low finishing temperature and accelerated cooling, so it needs less carbon and fewer alloying elements for the same strength. The result is lower carbon equivalent, easier welding and often better toughness at the same strength. The trade off is thermal history: heating the steel above roughly 580 to 600 °C after delivery can coarsen the microstructure and reduce the properties that were certified, so hot forming, galvanising at high temperature and post weld heat treatment have to be agreed with the supplier before they are applied.
Welding, Fabrication and Applications
Both grades are welded with conventional arc processes using low hydrogen consumables and a preheat chosen from the carbon equivalent and the section thickness. Two points deserve attention in HEA200 work. First, the thin web cools quickly, so heat input should be controlled to avoid hard, crack sensitive heat affected zones; a modest preheat on thick flange and web junctions helps. Second, the impact requirement applies to the finished structure as much as to the steel, so weld procedure qualification for low temperature service should include impact tests on the heat affected zone.
Typical applications include building columns and trusses in cold regions, bridge and viaduct components, ship and offshore structures, energy engineering frames, low temperature storage racks, and heavy machinery and crane masts where a compact wide flange section is needed. In each case the L class is specified because the design service temperature is below the range covered by ordinary structural grades.
Frequently Asked Questions
Q: What is the difference between S355NL and S355ML?
S355NL is normalised or normalised rolled to EN 10025-3 and S355ML is thermomechanically rolled to EN 10025-4; both have a 355 MPa minimum yield strength and are impact tested at -50 °C.
Q: What are the HEA200 dimensions?
HEA200 has a depth of 190 mm, a flange width of 200 mm, a 6.5 mm web, a 10 mm flange, a cross sectional area of 53.8 cm2 and a nominal mass of 42.3 kg/m.
Q: Why is the L suffix important?
The L class requires 27 J minimum Charpy V-notch energy at -50 °C, which qualifies the steel for cold climate and refrigerated service where ordinary grades would be at risk of brittle fracture.
Q: Can S355ML be welded more easily than S355NL?
Usually yes, because the thermomechanical route uses lower carbon and a lower carbon equivalent, which reduces hardenability and the preheat needed for a sound joint.
Q: Can S355ML be hot formed after delivery?
Heating above roughly 580 to 600 °C can reduce the certified properties, so hot forming or post weld heat treatment should be agreed with the supplier and verified by testing afterwards.
Q: Are these grades suitable for high temperature service?
Both are structural grades intended for ambient and low temperature duty; the normalised grade is the more stable of the two when moderate temperature exposure or thermal treatment is unavoidable.