S690 Steel Plate: What the Grade Covers
S690 is a high strength quenched and tempered structural steel grade defined in EN 10025-6. The number 690 is the minimum specified yield strength in megapascals, and the letter Q again denotes the quenched and tempered delivery condition. Two toughness qualities are regularly supplied: S690Q, impact tested at minus 20 degrees Celsius, and S690QL, impact tested at minus 40 degrees Celsius for service where brittle fracture resistance at low temperature is critical.
These grades are selected for highly stressed structures where weight reduction has direct economic value: the higher working stress allows thinner plate, lower dead load and smaller weld volumes than a conventional S355 design at the same load.
Yield Strength of S690 Steel Plate by Thickness
| Nominal thickness | Minimum yield strength ReH (MPa) | Tensile strength Rm (MPa) | Minimum elongation A (%) |
|---|---|---|---|
| 3 to 50 mm | 690 | 770 to 940 | 14 |
| Over 50 to 100 mm | 650 | 760 to 930 | 14 |
| Over 100 to 150 mm | 630 | 710 to 900 | 14 |
The tabulated values are minimum requirements of EN 10025-6. They are the figures to be used for design verification, because a certificate that reports a higher measured yield strength gives no additional design allowance.
Toughness and Impact Testing
Brittle fracture resistance is the property that separates the two qualities. For S690Q the specified impact test temperature is minus 20 degrees Celsius; for S690QL it is minus 40 degrees Celsius. Charpy V notch specimens are taken in the longitudinal direction and the minimum average energy requirement of the standard applies at the stated temperature.
The practical consequence is a documented service temperature limit rather than a change in strength: both qualities share the same yield and tensile requirements, but S690QL is the correct choice for structures exposed to arctic, offshore or refrigerated conditions, while S690Q is normally sufficient for temperate climate cranes, quarry plant and general heavy machinery.
Chemical Composition of S690QL
| Element | Maximum (%) | Element | Maximum (%) |
|---|---|---|---|
| C | 0.20 | Cr | 1.50 |
| Si | 0.80 | Ni | 2.00 |
| Mn | 1.70 | Mo | 0.70 |
| P | 0.025 | Nb | 0.06 |
| S | 0.015 | V | 0.12 |
The alloy design combines low carbon with nickel, chromium and molybdenum for hardenability, and micro additions of niobium and vanadium for precipitation strengthening. Boron is limited to a very low maximum. Typical hardness of S690QL plate is around 414 HBW; hardness should be treated as an indicative figure, since the mechanical requirement of the standard is expressed as strength and impact energy, not hardness.
Available Sizes, Certificates and Equivalent Grades
EN 10025-6 tabulates properties for thicknesses from 3 mm to 150 mm. Plates are commonly rolled in widths of about 1500 mm to 2500 mm and lengths of about 6000 mm to 12000 mm, with heavier and wider formats subject to agreement for a specific order. Inspection documents are issued to EN 10204 type 2.2 or type 3.1, and ultrasonic testing to EN 10160 is often specified for plate above 50 mm.
| Reference system | Comparable designation | Note |
|---|---|---|
| ASTM | A709 Grade 100 | bridge steel, similar strength level |
| ISO | E690 | strength class designation |
| AFNOR | E690T | French designation of the same strength class |
Equivalent grades indicate a similar strength and toughness class, not an automatic substitution. Chemistry limits, impact test temperature, tolerances and inspection requirements differ between standards and must be compared before a swap is approved.
Welding and Fabrication Considerations
Welding of S690QL follows the principles applied to all high strength quenched and tempered steels: a qualified welding procedure, controlled heat input, low hydrogen consumables such as E9018-G or ER110S-G type electrodes and wires, and preheating of roughly 100 to 150 degrees Celsius for thicker sections. Hydrogen control and interpass temperature limits protect the heat affected zone toughness, and post weld heat treatment is specified only where the design requires it, because excessive tempering reduces strength.
Frequently Asked Questions
Q: What is the yield strength of S690 steel plate?
A: The minimum yield strength is 690 MPa for thicknesses from 3 to 50 mm, 650 MPa for over 50 to 100 mm, and 630 MPa for over 100 to 150 mm, per EN 10025-6.
Q: What does the QL suffix mean?
A: The letter L denotes low temperature toughness; S690QL is impact tested at minus 40 degrees Celsius, while S690Q is tested at minus 20 degrees Celsius.
Q: What is the tensile strength of S690QL?
A: EN 10025-6 specifies 770 to 940 MPa for 3 to 50 mm, 760 to 930 MPa for over 50 to 100 mm, and 710 to 900 MPa for over 100 to 150 mm.
Q: What is the minimum elongation of S690QL?
A: 14 % in the tensile test for all thickness bands covered by the standard.
Q: Which applications suit S690QL plate?
A: Mobile cranes and lifting equipment, offshore structures and platforms, bridges and long span frames, mining and earth moving machinery, pressure retaining structures, rail and transport equipment, and heavy power generation components.
Q: What certificate should be requested for S690QL?
A: EN 10204 type 3.1 is the usual requirement for structural and offshore work, and type 2.2 where full heat traceability is not specified.
