S355NL Within EN 10025-3
S355NL is a weldable fine grain structural steel defined in EN 10025-3, the part of the European standard that covers normalized and normalized rolled products. The designation breaks down into S for structural steel, 355 for the minimum yield strength in MPa at 16 mm, N for a normalized or normalized rolled delivery condition and L for the low-temperature impact quality. The L quality is tested at -50 °C, which is 30 °C colder than the -20 °C test applied to S355J2 of EN 10025-2 and 50 °C colder than the J0 quality. Where a project instead uses thermomechanical rolling, the corresponding grade is S355ML of EN 10025-4, which reaches similar strength with a different processing route and different weldability characteristics.
Yield Strength and Chemistry
| Thickness | Minimum yield strength |
|---|---|
| Up to 16 mm | 355 MPa |
| 16-40 mm | 345 MPa |
| 40-63 mm | 335 MPa |
| 63-80 mm | 325 MPa |
| 80-100 mm | 315 MPa |
| 100-150 mm | 295 MPa |
| 150-200 mm | 285 MPa |
| 200-250 mm | 275 MPa |
Chemistry is capped at carbon 0.20 % maximum, silicon 0.50 % maximum, manganese 0.90-1.65 %, phosphorus 0.030 % maximum, sulphur 0.025 % maximum and nitrogen 0.015 % maximum, with copper, chromium and nickel residuals reported because they enter the carbon equivalent value, which the standard limits to 0.43 % for this quality. The tensile range is 470-630 MPa for plate up to 100 mm and 450-600 MPa above it. The NL quality is impact tested at -50 °C with a minimum average absorbed energy of 27 J in the longitudinal direction.
Equivalent Grades in Other Systems
GB/T 1591 Q355E, a low-alloy structural steel with the same 355 MPa strength class and impact testing at -40 °C. Editions of GB/T 1591 before the 2018 revision designated this grade Q345E.
JIS G3106 SM490C, the Japanese welded-structure grade carrying the lowest impact test temperature of the SM490 family.
ASTM A516/A516M Grade 70 when the plate is destined for pressure equipment rather than structural use; it is supplied normalized with a 485-620 MPa tensile range and a 260 MPa minimum yield strength.
EN 10225 S355G10+M or S355G10+N for offshore structural plate, where the standard adds carbon equivalent limits and low-temperature Charpy requirements for weldable offshore structures.
Choosing Between S355JR, S355J2 and S355NL
The choice is driven by service temperature rather than by strength, because all three grades carry a 355 MPa minimum yield strength at 16 mm. S355JR is impact tested at +20 °C and is the economical option for moderate climates and indoor service. S355J2 is tested at -20 °C and is the usual choice for bridges, cranes and outdoor structures in temperate to cold regions. S355NL is tested at -50 °C and is specified for wind tower shells, offshore structures, liquefied gas projects and any application where brittle fracture at very low temperature would be unacceptable. The delivery condition also differs: S355JR and S355J2 are normally supplied as-rolled or normalized rolled under EN 10025-2, while S355NL is normalized or normalized rolled under EN 10025-3, which produces a uniform ferrite-pearlite microstructure and stable toughness through the thickness of heavy plate.
Supply Forms, Testing and Documentation
S355NL is supplied as plate, coil, round bar and pipe for construction, wind energy, bridge and offshore fabrication. Plate is available in widths up to about 4050 mm and lengths to 15000 mm, with delivery condition stated as +N for normalized or normalized rolled, and heavier sections supplied with a simulated post-weld heat treatment cycle applied to the test coupons where the fabrication sequence requires it. Ultrasonic examination to an agreed acceptance class and thickness-direction testing to Z15, Z25 or Z35 can be added. Certification includes a mill test certificate to EN 10204 3.1 with heat number, chemical analysis, carbon equivalent value, tensile and yield results and Charpy energy at -50 °C, and it can be raised to 3.2 or supported by independent third-party inspection for project work.
Frequently Asked Questions
Q: What is the S355NL equivalent material?
A: The closest structural equivalent is GB/T 1591 Q355E, formerly designated Q345E. JIS G3106 SM490C is a near neighbour in the Japanese system, and ASTM A516 Grade 70 is the usual choice when the plate is used for pressure equipment rather than for structural work.
Q: What does the NL suffix mean?
A: N means the plate is delivered normalized or normalized rolled, and L means the steel is impact tested at -50 °C with a minimum average absorbed energy of 27 J.
Q: How does S355NL differ from S355J2?
A: Both have a 355 MPa minimum yield strength, but S355J2 is impact tested at -20 °C under EN 10025-2 while S355NL is tested at -50 °C under EN 10025-3 in a normalized condition.
Q: Why does the guaranteed yield strength fall with thickness?
A: Heavier plate cools more slowly during rolling and normalizing, so the standard steps the guaranteed yield strength down from 355 MPa at 16 mm to 275 MPa at 200-250 mm.
Q: Can S355NL plate be welded without preheat?
A: The carbon equivalent value is capped at 0.43 %, which supports welding without preheat in normal thicknesses. Heavy sections and high-restraint joints are welded with preheat and hydrogen-controlled consumables under a qualified procedure.
Q: Which projects typically specify S355NL?
A: Wind tower shells, bridge girders in cold climates, offshore structures and liquefied gas facilities are the main applications, because all of them require guaranteed toughness well below freezing.
