What the S690QL Designation Covers
S690QL is a quenched and tempered structural steel plate grade defined in EN 10025-6, the part of the European structural steel series that covers flat products with high yield strength in the quenched and tempered condition. The designation is read in four blocks: S for structural steel, 690 for the minimum yield strength in megapascals within the reference thickness band, Q for quenching and tempering, and L for the low temperature impact class. EN 10025-6 is applied together with EN 10025-1, which carries the general technical delivery conditions for the series.
The strength is obtained by alloying with balanced additions of chromium, molybdenum, nickel and boron together with micro-alloying elements, followed by austenitising, water quenching and tempering. The tempered martensitic or bainitic microstructure delivers a very high yield strength while keeping 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, 3 mm to 50 mm | 690 MPa | EN 10025-6 |
| Minimum yield strength ReH, over 50 mm to 100 mm | 650 MPa | EN 10025-6 |
| Minimum yield strength ReH, over 100 mm to 150 mm | 630 MPa | EN 10025-6 |
| Tensile strength range Rm, 3 mm to 50 mm | 770 MPa to 940 MPa | EN 10025-6 |
| Charpy V-notch impact energy, L class | 27 J minimum at -40 C | EN 10025-6 |
| Delivery condition | Quenched and tempered | EN 10025-6 |
The thickness step-down is an explicit part of the specification. The 690 MPa figure applies to the 3 mm to 50 mm band, while heavier plate is designed on the reduced values of 650 MPa and 630 MPa. Applying the thin plate value to a 120 mm section is a common and expensive design error, and it is the reason the ordered thickness has to be fixed before the strength check is closed.
Toughness Class and Sub-Zero Service
The impact class is the clearest 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 -40 C, and the QL1 class moves the test temperature further down the scale. The distinction matters most in the weld, where the heat-affected zone can be more brittle than the parent plate.
For cold service the purchaser should state the impact class, the test temperature and the minimum absorbed energy in the order, and require the welding procedure qualification to be tested at the same temperature as the design case. Because the grade is sensitive to hydrogen, the condition of the plate surface is also relevant: plates are normally ordered with ultrasonic testing to EN 10160 at an agreed class and with inspection documents to EN 10204 type 3.1 for structural use, while surface condition is inspected to EN 10163. These checks are aimed at laminations and repair areas that could act as crack initiators in a highly stressed joint.
Welding, Forming and Cutting Practice
Welding follows a qualified procedure with matched high strength consumables, low hydrogen practice, controlled preheat and controlled interpass temperature. Heat input is normally held inside a middle band: too much heat softens the heat-affected zone beyond its design allowance, while too little raises the cooling rate and the risk of hydrogen assisted cracking. Moisture control on electrodes and flux, clean joint faces and correct baking practice matter far more here than on mild steel.
Cold forming is possible with generous bend radii and is usually performed transverse to the rolling direction where the design permits.
Thermal cutting leaves a heat-affected edge that is ground or machined away when the edge is a fatigue critical detail.
Hot forming and post-weld heat treatment are avoided unless the resulting property loss has been confirmed and the design adjusted for it, because tempering above the original tempering temperature permanently lowers the yield strength.
Edge preparation, fit-up tolerance and weld toe profile deserve close attention, since the fatigue life of a high strength detail is governed by geometry as much as by plate strength.
Applications and Weight Economics
S690QL is chosen where strength is used to remove 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 decision is economic as well as technical. The higher price per tonne is offset by lower steel weight, lower welding volume and reduced transport and lifting cost, and the comparison is best made on the finished assembly, including the extra inspection and procedure qualification cost that the higher strength grade requires.
Frequently Asked Questions
Q: What is S690QL steel plate?
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 strength range of 770 MPa to 940 MPa.
Q: What does the L in S690QL mean?
The L denotes the low temperature impact class. The grade must pass a Charpy V-notch test with a minimum absorbed energy of 27 J at -40 C.
Q: Is S690QL weldable?
Yes, under a qualified procedure with 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?
It can be cold formed with larger bend radii than mild steel, and the bend 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?
The strength levels are the same, but S690QL carries a mandatory sub-zero impact requirement at -40 C, while S690Q has no sub-zero impact class in its designation.
Q: Which thicknesses are available in S690QL?
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 design values must follow the ordered thickness.
Q: Why is S690QL used instead of a lower strength plate?
Because the higher strength allows section weight, welding volume and transport cost to be cut in mobile equipment and lifting structures, provided the extra inspection and procedure qualification cost is included in the comparison.
