Material Grade: S690QL (according to the European standard EN 10025-6)
S690QL is a premium, high-strength, quenched and tempered structural steel. It is specifically engineered through advanced thermomechanical processing and precise heat treatment to deliver an outstanding combination of very high strength, good weldability (with proper procedures), and guaranteed toughness at low temperatures. It is a cornerstone material for modern, weight-optimized, and safety-critical heavy equipment.
Below is a comprehensive table detailing its key characteristics:
S690QL Steel Grade Specification Summary
| Category | Specification / Detail | Notes / Explanation |
|---|---|---|
| Standard & Designation | EN 10025-6: S690QL | The 'S' denotes structural steel, '690' the min. yield strength in MPa, 'Q' for quenched, and 'L' for low temperature impact properties. |
| Minimum Yield Strength (ReH) | 690 MPa (for thickness ≤ 50 mm) | This is the defining mechanical property. Strength decreases slightly for thicker sections as per the standard's tables. |
| Tensile Strength (Rm) | 770 - 940 MPa | Provides a high strength reserve. The specific range depends on product form and thickness. |
| Elongation at Fracture (A₅) | ≥ 14% (typical) | Indicates a degree of ductility despite the very high strength. |
| Impact Toughness (KV) | Minimum 40 Joules at -40°C or -60°C | The primary function of the 'L' designation. The exact test temp. (-40°C or -60°C) is defined by the ordered subgrade (e.g., L or L1). |
| Typical Subgrades (EN 10025-6) | S690QL, S690QL1 | The '1' suffix (e.g., S690QL1) explicitly guarantees 40 J at -40°C for all thicknesses up to the limit, a common requirement. |
| Delivery Condition | Quenched & Tempered (Q+T) | This heat treatment is essential for achieving the fine-grained microstructure that provides the strength and toughness. |
| Chemical Composition (Typical Wt. %) | C: ≤ 0.18, Mn: ≤ 1.70, Si: ≤ 0.80, P: ≤ 0.020, S: ≤ 0.010. Micro-alloyed with Nb, V, Ti, and elements like Cr, Ni, Mo, B for hardenability. | Low carbon and sulfur for weldability and toughness. Micro-alloying elements refine grains and enhance strength through precipitation. |
| Carbon Equivalent (CET/CEV) | Typically ~0.40 - 0.55 | A moderate value indicating that welding requires specific procedures (pre-heat, controlled heat input, matching high-strength consumables) but is routinely performed. |
| Primary Advantages | 1. Exceptional Strength-to-Weight Ratio 2. Good Low-Temperature Toughness 3. Good Fatigue Resistance 4. Commercial Availability |
Enables lighter, more powerful, and more fuel-efficient structures without compromising safety or performance. |
| Common Applications | • Mobile & Tower Cranes (booms, jibs) • Mining & Earth-Moving Equipment (truck frames, excavator arms) • Heavy Commercial Vehicles • Offshore & Lifting Structures • Advanced Military & Rescue Equipment |
Used wherever reducing structural weight is critical for payload, mobility, or energy efficiency. |
Key Distinction: While often compared to S890QL, S690QL offers a more favorable balance for many applications due to its slightly lower strength, which often translates to better weldability, formability, and sometimes lower cost, while still providing massive weight savings over conventional steels like S355.
What is the main difference between S690QL and the more common S355 steel?
The difference is profound. S690QL has roughly double the yield strength (690 MPa vs. 355 MPa). This allows designers to use thinner, lighter sections to carry the same load, or to greatly increase the load capacity of a similarly sized structure. S690QL is also quenched and tempered with guaranteed low-temperature toughness, whereas standard S355 is typically delivered as normalized or thermomechanically rolled with lower toughness specs.
Can S690QL be welded on-site, and what are the critical precautions?
Yes, but with rigorous controls. It is not like welding mild steel. Critical precautions include:
Procedure Qualification: A Welding Procedure Specification (WPS) qualified to a relevant standard (e.g., EN ISO 15614) is mandatory.
Pre-heat & Interpass Temperature: Strictly required to prevent hydrogen-induced cold cracking (HICC). Temperature depends on thickness and CEV.
Consumables: Must use matching high-strength, low-hydrogen electrodes or wires (e.g., classification matching 690 MPa strength).
Heat Input Control: Must stay within the qualified range to preserve the heat-affected zone (HAZ) properties.
Post-Weld Heat Treatment (PWHT): Often required for thick sections to relieve stresses and temper the HAZ.
How does S690QL compare to S890QL? When do you choose one over the other?
It is a trade-off between ultimate strength and fabricability/cost.
S890QL: Higher strength (890 MPa yield) for maximum weight saving. However, it generally has lower toughness, is more challenging to weld (higher CEV, stricter procedures), and is more expensive.
S690QL: The "sweet spot" for many applications. Offers massive weight savings over S355 with better overall weldability, toughness, and often better formability than S890QL at a lower cost.
Choice Driver: If the design is strength-limited and every kilogram saved is critical (e.g., a crane boom tip), S890QL may be chosen. If the design involves complex welding, high dynamic loads, or cost sensitivity, S690QL is often the preferred, more pragmatic solution.
What are the common subgrades of S690QL and what do they mean?
Subgrades define the impact test temperature:
S690QL: The base grade. The test temperature (often -40°C or -60°C) and energy must be agreed upon at the time of ordering.
S690QL1: A specific, common subgrade that explicitly guarantees a minimum of 40 Joules impact energy at -40°C for all thicknesses. This provides a clear, high-toughness specification.
Is S690QL corrosion resistant?
No. S690QL is not a weathering or stainless steel. Its corrosion resistance is similar to that of ordinary carbon steel. For corrosive environments (e.g., offshore), it must be protected by painting systems, galvanizing, or metal spraying. Using it in an unpainted condition will lead to rust