The difference between S690Q and S690QL is subtle in name but significant in terms of required impact toughness and the specific standard that defines them.
Here's the clear breakdown:
Core Difference: The "L" denotes a Mandatory Minimum Low-Temperature Impact Toughness.
S690Q: The "Q" stands for Quenched. This grade is defined in the older standard EN 10025-5 (which is now withdrawn/superseded). It specified a minimum impact toughness at room temperature (+20°C).
S690QL: The "QL" stands for Quenched and Tempered. This grade is defined in the current standard EN 10025-6. The "L" suffix specifically mandates a much higher impact toughness at low temperatures.
Detailed Comparison Table
| Feature | S690Q (Historical / Superseded) | S690QL (Current Standard) |
|---|---|---|
| Governing Standard | EN 10025-5 (Withdrawn) | EN 10025-6 (Current) |
| Designation Meaning | Q = Quenched | QL = Quenched and Tempered |
| Key Mechanical Difference | Impact toughness tested at +20°C. | Impact toughness tested at -40°C (for S690QL) or -60°C (for S690QL1). |
| Minimum Charpy V-Notch Impact Value | Typically 30-40 J at +20°C (as per old specs). | 27 J at -40°C (S690QL) or -60°C (S690QL1). This is a vastly more demanding requirement. |
| Application Implication | Suitable for high-strength applications where low-temperature service is not a concern. | Mandatory for applications where the structure may face sub-zero temperatures, such as offshore environments, winter construction, or mobile equipment in cold climates. It guarantees resistance to brittle fracture. |
| Metallurgical Refinement | The quenching process was primary. Tempering might not have been as rigorously specified. | The Quenching and Tempering process is explicitly controlled to achieve the required combination of high strength and low-temperature toughness. |
Practical Implications for Engineers and Purchasers
S690Q is Largely Obsolete: You should not specify or procure "S690Q" for new designs. The standard that defined it (EN 10025-5) has been replaced. Modern procurement and design codes reference EN 10025-6.
S690QL is the Correct, Modern Designation: When you need a 690 MPa yield strength quenched and tempered steel, you specify S690QL (for -40°C) or S690QL1 (for -60°C). The "L" is not optional; it's an integral part of the grade defining its enhanced toughness.
Safety-Critical Difference: Using an old stock of "S690Q" in a structure destined for a cold environment could be catastrophic. The material might lack the necessary ductility at low temperatures, leading to a risk of brittle fracture.
Certification: Mill certificates for S690QL will explicitly show impact test results at -40°C, proving compliance with the more stringent standard.
Why the Change? Standardization and Safety
The evolution from EN 10025-5 to EN 10025-6 consolidated and rationalized the standards for high-strength steels. The key improvement was explicitly linking the grade designation to a guaranteed low-temperature service capability, which is essential for safe design.
In summary:
Think of S690Q as the "1st generation" – very strong, but with toughness only guaranteed for mild conditions.
Think of S690QL as the "2nd generation" – equally strong, but with its toughness engineered and guaranteed to perform in harsh, cold environments. It is the only grade you should specify today.