What the S690QL Designation Covers
S690QL belongs to the quenched and tempered structural steel grades of EN 10025-6, which covers flat products with high yield strength in the quenched and tempered condition. The code is read in four blocks. "S" indicates structural steel, "690" indicates a minimum yield strength of 690 MPa in the reference thickness band, "Q" indicates quenching and tempering, and "L" indicates the low temperature impact class assigned to the grade. EN 10025-6 works together with EN 10025-1, which carries the general technical delivery conditions of the structural steel series.
Strength is obtained by alloying with combinations of chromium, molybdenum, nickel, boron and micro-alloying elements, then austenitising, water quenching and tempering. The tempered martensitic or bainitic structure delivers a very high yield strength while retaining enough toughness and ductility for welded fabrication, which is what separates the grade from a merely hard wear plate.
Mechanical Properties and Thickness Limits
| Property | Requirement | Reference |
|---|---|---|
| Minimum yield strength (ReH), t up to 50 mm | 690 MPa | EN 10025-6 |
| Minimum yield strength, t over 50 mm to 100 mm | 650 MPa | EN 10025-6 |
| Minimum yield strength, t over 100 mm to 150 mm | 630 MPa | EN 10025-6 |
| Tensile strength range (Rm), t up to 50 mm | 770-940 MPa | EN 10025-6 |
| Charpy V-notch impact energy | 27 J minimum at minus 40 degrees C | EN 10025-6, L class |
| Delivery condition | Quenched and tempered | EN 10025-6 |
The thickness step-downs are explicitly part of the specification: the 690 MPa figure applies to the 3-50 mm band, and heavier plate is designed on the reduced values. Using the thin-plate value for a 120 mm section is a common and expensive design error.
Toughness Class and Sub-Zero Service
The impact class is the difference between the grades of this family. An S690Q plate carries no sub-zero impact class in its designation, whereas S690QL is qualified by a mandatory Charpy V-notch test at minus 40 degrees C, and the QL1 class moves the test further down the temperature scale. The distinction matters most in the weld, where the heat-affected zone can be more brittle than the parent plate. For a structure in cold service the purchaser should state the impact class, the test temperature and the minimum energy in the order, and require the welding procedure qualification to be tested at the same temperature.
Because the grade is sensitive to hydrogen, the material condition at the plate surface is also relevant. Plates are normally ordered with ultrasonic testing to EN 10160 at an agreed class and with certificates to EN 10204 type 3.1 for structural use, and the surface is inspected against EN 10163 for plates. These checks are about detection of laminations and repair areas that could act as crack initiators in a highly stressed joint.
Welding, Forming and Fabrication
Welding of S690QL follows a qualified procedure with matched high-strength consumables, low-hydrogen practice, controlled preheat and controlled interpass temperature. Heat input is usually limited to a middle band: too much heat softens the heat-affected zone above its design allowance, while too little increases the cooling rate and the risk of hydrogen cracking. Moisture control on electrodes and flux, clean joint faces and prompt baking are practical steps that matter more here than with mild steel.
Cold forming is possible with generous bend radii and is usually performed transverse to the rolling direction where the design allows. Thermal cutting leaves a heat-affected edge that is removed by grinding or machining when the edge is a fatigue-critical detail. Hot forming and post-weld heat treatment are avoided unless the supplier has confirmed the resulting property loss and the design has been adjusted for it, because tempering the plate above its original tempering temperature permanently lowers the yield strength.
Applications and Weight Economics
S690QL is selected where strength is used to save weight and where brittle fracture must be excluded at the same time: mobile crane booms and outriggers, concrete pump arms, excavator and mining equipment structures, offshore platform components, wind energy hardware, heavy transport equipment and high-pressure structural assemblies. The design decision is an economic one as well as a technical one, because the higher price per tonne is offset by a lower steel weight, lower welding volume and reduced transport and lifting cost. The comparison should be made on the finished assembly, including the extra inspection and procedure qualification costs that the higher strength grade requires.
Frequently Asked Questions
Q: What is S690QL steel?
A: It is a quenched and tempered ultra high strength structural steel plate to EN 10025-6, with a minimum yield strength of 690 MPa up to 50 mm thickness and a tensile range of 770-940 MPa.
Q: What does the L in S690QL mean?
A: The L denotes the low temperature impact class: the grade is tested with a mandatory Charpy V-notch requirement of 27 J minimum at minus 40 degrees C.
Q: Is S690QL weldable?
A: Yes, with a qualified procedure, matched consumables, low-hydrogen practice and controlled heat input, preheat and interpass temperature. Procedure qualification is normally impact tested at the service temperature.
Q: Can S690QL be bent or formed?
A: It can be cold formed with larger bend radii than mild steel, and the bend direction is usually arranged transverse to the rolling direction. Hot forming is avoided because it reduces the delivered strength.
Q: What is the difference between S690Q and S690QL?
A: The strength levels are the same, but S690QL carries a mandatory sub-zero impact class at minus 40 degrees C, while S690Q has no sub-zero impact class in its designation.
Q: Which thicknesses are available in S690QL?
A: EN 10025-6 covers plate from about 3 mm up to 150 mm; the applicable minimum yield strength steps down at 50 mm and at 100 mm, and the design values must follow the ordered thickness.
