What S960QL Designates
S960QL is a quenched and tempered structural steel defined by EN 10025-6, the European standard for technical delivery conditions of flat products of high yield strength structural steels in the quenched and tempered condition. Every element of the designation carries information: S for structural steel, 960 for the minimum yield strength in megapascals in the reference thickness range, Q for the quenched and tempered delivery condition, and L for the impact quality class, which specifies Charpy V-notch testing at -40 degrees C.
This is the top of the structural steel strength range in common commercial use. It is a low-carbon, low-alloy steel that is water quenched and then tempered to produce tempered martensite, giving an unusual combination of very high strength and dependable notch toughness at sub-zero temperature.
Mechanical Properties to EN 10025-6
The property table below reflects the guaranteed minimum values that a purchaser can rely on for the S960Q and S960QL grades. Values are stated by thickness because the attainable strength falls once the cooling rate through the core of the plate slows.
| Property | Guaranteed value | Applicable thickness |
|---|---|---|
| Minimum yield strength ReH | 960 MPa | Up to 50 mm |
| Tensile strength Rm | 980-1150 MPa | Up to 50 mm |
| Impact test temperature | -40 degrees C | Quality class L |
| Delivery condition | Quenched and tempered | All thicknesses |
Because the gap between the minimum yield strength and the lower end of the tensile band is narrow, the material is sensitive to anything that reduces section thickness locally, such as grinding marks or a weld that undermines the plate. Wall thickness reduction during fabrication should be treated as a design issue rather than a cosmetic one.
Comparison with Other Quenched and Tempered Grades
The EN 10025-6 series is progressive, and the choice between grades is a weight-against-fabrication-cost decision.
| Grade | Minimum ReH (MPa) | Tensile strength Rm (MPa) | Impact class suffix |
|---|---|---|---|
| S690Q / S690QL | 690 | 770-940 | Q, L or L1 |
| S890Q / S890QL | 890 | 940-1100 | Q, L or L1 |
| S960Q / S960QL | 960 | 980-1150 | Q, L or L1 |
Moving up the series reduces plate thickness for a given load but increases the demand on welding procedures, cut-edge preparation and inspection. The L1 class extends the impact requirement to an even lower test temperature than the L class, and is chosen for the most severe service conditions.
Manufacturing Route and Chemistry
S960QL is produced by a low-carbon, micro-alloyed route followed by water quenching and tempering. Alloying elements such as nickel, chromium, molybdenum and boron are used at levels that deliver hardenability without excessive carbon, so that the heat-affected zone formed during welding retains useful toughness. The standard limits the carbon equivalent value to keep the material weldable with qualified procedures, and sulphur and phosphorus are held at low levels to protect toughness and to reduce the risk of lamellar tearing in thick joints.
The tempering treatment must be controlled to balance strength against toughness. Any subsequent thermal process, including stress relief or hot forming, changes that balance and should be treated as a departure from the delivery condition requiring agreement.
Fabrication and Welding Practice
Fabrication requirements for S960QL are stricter than for conventional structural steel. Recommended controls include low-hydrogen consumables matched to the base metal, controlled preheat matched to thickness and carbon equivalent value, an upper limit on heat input, an upper limit on interpass temperature, and inspection of the heat-affected zone by hardness and impact testing. Heat input limits exist because toughness in the coarse-grained heat-affected zone degrades when the weld pool is allowed to cool too slowly.
Design practice for this grade uses the strength of the welded joint rather than the parent plate, because a softened zone forms adjacent to the fusion line. Cutting is normally done by waterjet, plasma or laser, and the cut edge should be dressed where it will carry fluctuating load.
Typical Applications
S960QL is applied where payload and weight are in direct competition: mobile cranes, aerial work platforms, concrete pump booms, mining and earthmoving equipment, heavy transport frames, and specialised bridge or offshore hardware. In most cases the weight saved against a 690 MPa grade justifies the additional welding qualification effort.
Frequently Asked Questions
Q: What does the L mean in S960QL?
A: It identifies the impact quality class, requiring Charpy V-notch testing at -40 degrees C in addition to the strength requirement. The Q in the same designation confirms the quenched and tempered delivery condition.
Q: What is the minimum yield strength of S960QL?
A: 960 MPa for thickness up to 50 mm, with a corresponding tensile strength range of 980-1150 MPa guaranteed by EN 10025-6.
Q: Can S960QL be stress relieved after welding?
A: Post-weld heat treatment must be considered carefully because it tempers the material further and reduces strength. If it is required, the effect on the delivery condition and on properties should be agreed in advance and verified by test.
Q: Why is heat input limited during welding?
A: A slow cooling thermal cycle coarsens the heat-affected zone and lowers its toughness. Limiting heat input keeps the coarse-grained zone fine enough to satisfy the impact requirement.
Q: How does S960QL differ from S690QL?
A: Both are quenched and tempered structural steels with the same impact class designations, but S960QL guarantees 960 MPa minimum yield strength and a 980-1150 MPa tensile range, while S690QL guarantees 690 MPa and 770-940 MPa.
Q: Is S960QL used for cold formed sections?
A: Its high yield strength and low elongation limit cold forming, and bends require generous radii. Cold formed sections are usually produced from lower strength grades, with S960QL reserved for flat plate applications.