S690Q and S690QL are high-strength quenched and tempered structural steel plates specified in EN 10025-6, and they are among the most commonly used materials for heavy mechanical projects. In the EN 10027-2 designation system, S690QL is also identified as 1.8928 and S690Q as 1.8931. Both grades are supplied in the quenched and tempered condition, and both guarantee a minimum yield strength of 690 MPa for thickness up to 50 mm. The practical difference between them is the level of low-temperature toughness guaranteed by the specification.
Chemical Composition Compared
The two grades share very similar maximum limits for most elements, including carbon of 0.20 percent, manganese of 1.7 percent, silicon of 0.8 percent, copper of 0.5 percent, chromium of 1.5 percent, nickel of 2.0 percent, and molybdenum of 0.7 percent. The main difference is in phosphorus and sulfur, which are held to 0.020 and 0.010 percent for S690QL versus 0.025 and 0.015 percent for S690Q. The tighter limits on S690QL support its better low-temperature toughness and through-thickness properties.
Mechanical Properties Compared
For thickness up to 50 mm, both S690Q and S690QL provide a minimum yield strength of 690 MPa and a tensile strength of 770 to 940 MPa. The difference appears in the impact test temperature: S690Q is tested at -20 deg C, while S690QL is tested at -40 deg C. For applications requiring even lower temperatures, the L1 variants of the S690 family extend the guarantee to -60 deg C.
The Core Difference: Low-Temperature Toughness
The letter L in S690QL stands for low-temperature impact properties. In the EN 10025-6 specification, S690Q must meet Charpy V-notch impact requirements at -20 deg C, typically an average of 30 J, while S690QL must meet a higher requirement of at least 40 J at -40 deg C. In practice this means S690QL is certified for Arctic pipelines, offshore platforms, and mining vehicles that operate in cold regions, whereas S690Q is sufficient for cranes and quarry equipment in temperate climates.
Processing and Weldability Differences
S690QL also carries stricter supplementary requirements in typical supply practice. Through-thickness or Z-direction testing, which verifies resistance to lamellar tearing, is commonly specified for S690QL and is often optional for S690Q. The carbon equivalent CET is also controlled to a lower value for S690QL, typically about 0.45 versus 0.47 for S690Q, which improves weldability by reducing the risk of hydrogen cracking in thick sections.
Typical Applications
S690Q is used for crane structures in temperate climates, quarry equipment, and general heavy machinery where -20 deg C toughness is adequate. S690QL is selected for offshore wind turbine foundations, Arctic and sub-zero structures, and critical lifting equipment where the material must keep its toughness at -40 deg C and below. For the same design, the choice between the two grades is driven by the minimum service temperature and the weldability requirements of the project.
Frequently Asked Questions
What is the difference between S690Q and S690QL? Both have 690 MPa minimum yield strength, but S690QL guarantees impact toughness at -40 deg C while S690Q is tested at -20 deg C.
What do the designations 1.8928 and 1.8931 mean? They are the material numbers assigned to S690QL and S690Q respectively in the EN 10027-2 designation system.
Are S690Q and S690QL quenched and tempered? Yes, both grades are supplied in the quenched and tempered condition to EN 10025-6.
Which grade should be used for offshore applications? S690QL is normally specified for offshore and Arctic service because it guarantees toughness at -40 deg C and below.
What is the carbon equivalent of S690QL? The CET carbon equivalent of S690QL is typically controlled to about 0.45, which improves weldability.
What are the Chinese equivalents of S690Q and S690QL? S690Q is broadly comparable to Q690D and S690QL to Q690E in the GB/T 16270 series, subject to design confirmation.