EN 10025-6 S690QL: High-Strength Structural Steel for Heavy Engineering & Offshore
EN 10025-6:2019 · Published: 2026-07-15 · Updated: July 2026
S690QL is a thermomechanically rolled and quenched + tempered (Q+T) high-strength structural steel under EN 10025-6, providing 690 MPa minimum yield strength — 2.8× the strength of S355 and 2.0× A572 Grade 50. This extraordinary strength enables...
S690QL is a thermomechanically rolled and quenched + tempered (Q+T) high-strength structural steel under EN 10025-6, providing 690 MPa minimum yield strength — 2.8× the strength of S355 and 2.0× A572 Grade 50. This extraordinary strength enables dramatic weight reduction: a beam replacing S355 with S690QL can reduce section weight by 40-50% while maintaining equivalent load capacity. S690QL is specified for mobile crane booms (Liebherr, Tadano), offshore jacket structures (oil & gas platforms), wind turbine towers and foundations, heavy-duty transport trailers, and bridge segments where weight savings translate directly to operational advantage. The 'QL' designation means the steel is Quenched and tempered with Low-temperature impact guarantee (≥30J at -20°C, and ≥27J at -50°C for S690QL1). Despite the high strength, S690QL retains good weldability — the thermomechanical rolling process produces a fine-grained microstructure (microalloyed with Nb, V, Ti) that provides both strength and toughness without the high carbon content that would compromise weldability. Maximum carbon equivalent (CEV) is limited to 0.65 for thicknesses ≤50mm.
Quick Facts
| Category | Carbon Steel |
| Standard | EN 10025-6:2019 |
| Density | 7.85 g/cm³ |
| Yield Strength | 690 MPa (100 ksi) minimum |
| Tensile Strength | 770-930 MPa (112-135 ksi) |
Detailed Mechanical Properties
| Elongation | 14% minimum (longitudinal) |
| Hardness | 210-280 HB |
| Charpy V Notch | ≥30J at -20°C; ≥27J at -50°C (QL1 grade) |
| Cev Max | 0.65 for t≤50mm; 0.75 for t>50mm |
Physical Properties
| Melting Point | 1400-1520 °C |
| Thermal Conductivity | 42 W/m·K at 20°C |
| Electrical Resistivity | 0.0000200 Ω·cm |
| Specific Heat | 460 J/kg·K |
| Coefficient Of Expansion | 12 µm/m·°C (20-100°C) |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| ASTM A514 Grade E | Compare |
| NAXTRA 700 (Dillinger) | Compare |
| Optim 700 (SSAB) | Compare |
| Weldox 700 (SSAB) | Compare |
Welding & Fabrication
| Preheat | 50-150°C depending on thickness and restraint (EN 1011-2 recommended values) |
| Filler Metal | Matching: EN ISO 16834 G69 6M21Mn3NiCr; Undermatching: G55 4M21Mn for reduced HAZ softening |
| Interpass Temp | Max 150-200°C |
| Pwht | Not normally required; may be needed for thickness >40mm in highly restrained joints |
| Weldability Rating | Moderate — requires qualified WPS, controlled heat input, and matching filler metal |
Related Materials
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Frequently Asked Questions
Can S690QL be welded with the same procedures as S355?
No. S690QL requires welding procedure qualification (WPQR) per EN 15614 with specific controls: (1) preheat of 50-150°C depending on thickness and joint restraint (S355 typically requires no preheat), (2) controlled heat input of 10-35 kJ/cm to avoid HAZ softening or excessive grain coarsening, (3) interpass temperature limited to 150-200°C (lower than S355's 315°C), (4) matching or undermatching filler metal (EN ISO 16834, yield ≥680 MPa). The HAZ of S690QL is the critical concern: excessive heat input causes a softened zone (strength reduction of 10-20%) adjacent to the weld. Proper WPS control keeps softening below 10%, which is acceptable per EN 1993-1-12 design rules.
When does the weight savings of S690QL justify the higher material cost?
S690QL costs approximately 2-3× more per kg than S355. However, because 40-50% less steel is needed for equivalent structural capacity, the raw material cost is roughly comparable. S690QL becomes clearly justified when: (1) weight drives operational cost — crane booms (lighter = higher lift capacity), transport trailers (lighter = higher payload), offshore platforms (lighter topside = smaller crane for installation), (2) fabrication savings offset material premium — less welding, smaller weld volumes, lighter handling during assembly, (3) space is constrained — urban bridge reconstruction with limited clearance requires shallower beams that only high-strength steel can provide. For statically-loaded structures where weight is not critical (building columns, foundations), S355 remains more economical.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu
Specialty Metals — Engineering Reference
Non-ferrous metals — aluminum, copper, titanium, zinc, magnesium — serve applications where steel cannot: electrical conductivity, thermal management, weight reduction, corrosion resistance in specific chemical environments. Each metal family has its own classification system and selection logic.
ASTM B209/B221 (Al), ASTM B152/B187 (Cu), ASTM B265/B348 (Ti), ASTM B86 (Zn), ASTM B90/B91 (Mg)
Electrical wiring and busbars (Cu), aircraft structures and automotive bodies (Al), medical implants and aerospace fasteners (Ti), die-cast consumer products (Zn), lightweight electronic enclosures (Mg)
Galvanic corrosion is the #1 failure mode in multi-metal assemblies. When joining dissimilar metals, consult the galvanic series: the more anodic metal will corrode preferentially. Use isolating washers, protective coatings, or select metals close together on the galvanic series.