S690QL is a high-yield strength structural steel (HSLA) known for its exceptional strength (minimum 690 MPa yield) and toughness, produced to the EN 10025-6 standard using a quench and temper heat treatment, allowing for lighter, more efficient designs in heavy-duty applications like cranes, bridges, and earthmoving equipment, with excellent weldability and formability despite needing careful handling due to its strength. Its name signifies "Structural" (S), "690 MPa" minimum yield, "Quenching" (Q), and "Low" temperature toughness (L).
Key Characteristics
High Strength: Minimum yield strength of 690 MPa (for thinner sections), enabling lighter structures and increased payload capacity.
Quenched & Tempered (Q+T): Heat treatment provides superior strength, toughness, and durability.
Excellent Toughness: Good impact strength even at low temperatures (indicated by 'L').
Good Weldability & Formability: Allows for complex fabrications when proper procedures are followed.
Composition (Typical Max Values)
Carbon (C): ~0.20%
Manganese (Mn): ~1.70%
Silicon (Si): ~0.80%
Chromium (Cr): ~1.50%
Molybdenum (Mo): ~0.70%
Nickel (Ni): ~2.0%
Grain Refiners: Contains V, Nb, Ti, Zr (min 0.015%) for fine grain structure.
Applications
Mobile cranes and lifting equipment
Bridges and heavy-duty construction
Earthmoving and mining machinery (dump trucks, drilling rigs)
Wind turbine towers
Welding S690QL requires a strict, controlled procedure to preserve its high strength and guaranteed low-temperature toughness (down to -40°C) while preventing defects, particularly hydrogen-induced cold cracking. A qualified Welding Procedure Specification (WPS) is mandatory.
Here is a concise overview of the essential requirements:
1. Key Pre-Weld Principles
Preheat & Interpass Temperature Control: This is critical. Preheat (typically 100–150°C / 212–302°F) is required to slow cooling, reduce hydrogen diffusion, and prevent martensite hardening. The exact temperature depends on thickness and the specific CEV (Carbon Equivalent Value). Interpass temperature should be maintained but generally not exceed 200–250°C.
Stringent Cleanliness: All moisture, rust, oil, paint, and mill scale must be removed from the joint area (≥20 mm from edges).
Joint Design: Use well-prepared, smooth bevels to avoid stress concentrations.
2. Filler Metal Selection
Low-Hydrogen Practice is Essential: Use only low-hydrogen (basic-coated) electrodes (SMAW), low-hydrogen flux-cored wires (FCAW), or solid wires (GMAW/SAW) classified for high-strength steels.
Strength Matching/Undermatching: Filler metals are typically chosen to match (69 MPa yield) or, more commonly, slightly undermatch (e.g., 62 MPa yield) the base metal. This improves weld metal toughness and reduces cracking sensitivity. Common designations:
EN ISO 16834-A: G 69 6 M21 4Ni2.5CrMo
AWS A5.28: ER110S-G / E110C-G
Storage & Handling: Electrodes/flux must be stored in a drying oven (80–120°C) after opening to prevent hydrogen pickup.
3. Welding Process & Parameters
Suitable Processes: SMAW (111), GMAW (135), FCAW (136), SAW (12), GTAW (141).
Heat Input Control: Must be strictly regulated within a defined window (e.g., 0.5–1.5 kJ/mm).
Too low: Risk of hard, brittle heat-affected zone (HAZ).
Too high: Excessive softening of the HAZ and grain growth, degrading toughness.
Technique: Use stringer beads or a slight weave. Avoid long arc lengths and ensure proper fusion at the toes.
4. Post-Weld Requirements
Post-Weld Heat Treatment (PWHT): Often required for thick sections or highly restrained joints to relieve residual stresses and temper the HAZ. Common practice is stress relieving at 550–600°C.
Slow Cooling: Insulate the weldment after completion to allow gradual cooling, especially in cold ambient conditions.
5. Mandatory Qualification & Documentation
WPS Qualification: The procedure must be qualified by testing according to a standard (e.g., EN ISO 15614-1), including mechanical (tensile, impact at -40°C) and hardness tests.
Welder Qualification: Welders must be certified for the specific WPS.
Inspection: NDT (e.g., UT, MT, RT) is typically required for critical joints.
1. What is S690QL steel?
S690QL is a high-strength, quenched and tempered structural steel grade with a minimum yield strength of 690 MPa, specifically optimized for low-temperature toughness down to -40°C.
2. What does the "L" stand for in S690QL?
The "L" stands for "Low temperature," indicating the steel is certified for enhanced impact toughness at temperatures as low as -40°C.
3. What are the typical applications of S690QL?
It is commonly used in extreme environments such as Arctic construction, offshore platforms, high-latitude mining equipment, and mobile cranes operating in freezing conditions.
4. How does S690QL differ from S690Q?
The key difference is that S690QL guarantees higher impact toughness at -40°C, while standard S690Q is typically tested at 0°C, making S690QL better suited for freezing climates.
5. What is the chemical composition of S690QL?
It is a low-alloy steel containing carbon, manganese, silicon, and micro-alloys like niobium, vanadium, and titanium, with strict control of phosphorus and sulfur to ensure weldability and toughness.
6. What welding procedures are required for S690QL?
Welding requires low-hydrogen electrodes, strict preheating (often 100-150°C), controlled heat input, and often post-weld stress relief to maintain toughness and prevent cracking.
7. Is S690QL corrosion resistant?
No, S690QL is not inherently corrosion-resistant. Like other high-strength structural steels, it requires protective coatings (e.g., painting or galvanizing) for use in corrosive environments.
8. What standards cover S690QL material?
It is primarily defined by the European standard EN 10025-6, with equivalent specifications in other systems (e.g., ASTM, ISO).
9. Can S690QL be machined easily?
Machining S690QL is challenging due to its high strength and hardness; it requires robust tooling, adequate cooling, and optimized cutting parameters to avoid excessive tool wear.
Full specification and details are available on request. The above information is provided for guidance purposes only. For specific design requirements please contact our technical sales staff.
