Two Grades, One Steel Family: S690Q (1.8931) and S690QL (1.8928)
S690Q and S690QL are the two most widely used high-strength plate grades of EN 10025-6. Both are quenched and tempered, both are delivered at a minimum yield strength of 690 MPa, and both are specified where a heavy mechanical or structural load has to be carried by a relatively thin section. The difference between them lies in one letter: the L in S690QL stands for low-temperature toughness, and it moves the guaranteed Charpy V-notch test temperature down to −40 °C.
The material numbers also differ. Under EN 10027-2, S690QL is registered as 1.8928 and S690Q as 1.8931. Those numbers are the unambiguous identifiers used on drawings, mill certificates and purchase orders, because a written grade designation alone can be misread a lot more easily than a five-digit number.
Grade Designation Decoded
| Element | S690Q | S690QL | Meaning |
|---|---|---|---|
| Material number | 1.8931 | 1.8928 | Unique identifier in the European material numbering system |
| S | yes | yes | Structural steel |
| 690 | yes | yes | Minimum yield strength of 690 MPa for thickness ≤ 16 mm |
| Q | yes | yes | Quenched and tempered delivery condition |
| L | no | yes | Low-temperature toughness class |
The Decisive Difference: Low-Temperature Impact Toughness
The impact test temperature of S690QL is 20 °C lower than that of S690Q, and the minimum absorbed energy is higher as well: 40 J against 30 J in the EN 10025-6 impact test. That combination is what allows S690QL to keep resisting brittle fracture when the structure is cold.
| Requirement | S690Q (1.8931) | S690QL (1.8928) |
|---|---|---|
| Charpy V-notch test temperature | −20 °C | −40 °C |
| Minimum impact energy | 30 J | 40 J |
| Typical service climate | Temperate | Arctic and sub-zero |
| Relative plate cost | Reference | Approximately 5 to 10 % higher |
In Chinese practice the two grades are usually compared with the quenched and tempered plate series of GB/T 16270: S690QL corresponds to Q690E and S690Q corresponds to Q690D, following the same logic in which the last letter marks the impact test temperature class.
Mechanical Properties and Delivery Condition
| Property | Value |
|---|---|
| Delivery condition | Quenched and tempered |
| Minimum yield strength | 690 MPa at thickness up to 50 mm; lower values apply as thickness increases |
| Tensile strength | 770–940 MPa |
| Density | 7.84 g/cm³ at 20 °C in the quenched and tempered condition |
| Impact class | −20 °C / 30 J for S690Q, −40 °C / 40 J for S690QL |
Both grades are produced with a tight composition window and a closely controlled quenching and tempering cycle, because at this strength level even small variations in carbon equivalent or tempering temperature shift the balance between strength and toughness. In ASTM and ASME terms the closest family is the quenched and tempered A514 / SA514 high-strength plate series rather than the much lower strength Q345 grade; the material number and the required impact temperature, not a single foreign grade name, remain the reliable way to specify the plate.
Welding, Processing and Grade Selection
Both grades require a low-hydrogen welding procedure. A preheat of roughly 80 to 150 °C is normally applied according to plate thickness and joint restraint, and the interpass temperature should be kept below 200 °C so that the tempered structure of the parent plate is not degraded. Because S690QL has a slightly lower carbon equivalent, it welds somewhat more readily, and its heat-affected zone retains better toughness when welding has to be done at low ambient temperature.
Selection follows the lowest service temperature the structure will actually see:
Choose S690Q when the service temperature does not fall below −20 °C and the structure is not exposed to extreme cold, when a cost-sensitive project still needs 690 MPa strength, and when no special low-temperature toughness class is specified.
Choose S690QL when the service temperature may drop below −20 °C, when the project is located in an extremely cold region, when low-temperature pressure vessels or piping are involved, or when the specification demands a higher safety margin for a large, highly loaded structure.
Typical duties for S690Q are cranes and construction machinery in temperate climates, general building structures and bridges, and equipment supports outside cold environments. S690QL is the usual choice for cold-region pipelines and mining equipment, offshore platforms and ship structures in cold seas, LNG storage tanks and cryogenic equipment, long-span bridges and high-rise buildings, and mining drills, electric shovels and haul trucks that work across a wide temperature range. The price premium of the L grade reflects tighter composition control, a more demanding heat treatment route and the additional low-temperature impact testing on every plate.
Frequently Asked Questions
Q: What is the difference between S690Q and S690QL?
The only difference is the impact toughness class. S690Q is tested with a Charpy V-notch specimen at −20 °C with a minimum of 30 J, while S690QL is tested at −40 °C with a minimum of 40 J, so S690QL is the grade for extremely cold service.
Q: What do the material numbers 1.8931 and 1.8928 mean?
They are the EN 10027-2 material numbers of the two grades: 1.8931 identifies S690Q and 1.8928 identifies S690QL. Both grades also share the designation S690, Q for quenched and tempered.
Q: What is the Chinese equivalent of S690QL and S690Q?
Under GB/T 16270, S690QL corresponds to Q690E and S690Q corresponds to Q690D, because the final letter marks the low-temperature impact class in both standards.
Q: What is the ASTM equivalent of S690QL?
The closest North American family is the quenched and tempered high-strength plate series ASTM A514 / ASME SA514, which covers a comparable strength level. Q345 is not an equivalent grade, since it is a much lower strength structural steel.
Q: What are the yield and tensile strength of S690QL?
S690QL has a minimum yield strength of 690 MPa for thickness up to 50 mm, with lower guaranteed values as thickness increases, and a tensile strength range of 770 to 940 MPa.
Q: What welding precautions are needed for S690Q and S690QL?
Both require low-hydrogen consumables, a preheat of about 80 to 150 °C depending on thickness and joint restraint, an interpass temperature below 200 °C, and a qualified welding procedure that protects the toughness of the heat-affected zone.
Q: What is the density of S690QL steel?
In the quenched and tempered condition the plate has a density of 7.84 g/cm³ at about 20 °C, which is used for weight calculations in design.
Q: How much more expensive is S690QL than S690Q?
S690QL is typically about 5 to 10 % more expensive than S690Q, reflecting stricter composition control, a more complex heat treatment cycle and the additional −40 °C impact testing required on each plate.
