1. Standard & Scope
Standard: EN 10025-6 (High-strength quenched and tempered steels).
Grade: S690QL ("S" for structural steel, "690" for minimum yield strength in MPa, "Q" for quenched, "L" for low-temperature impact toughness).
Applications: Heavy-duty machinery, mining equipment, crane booms, offshore structures, and high-stress components requiring extreme strength and toughness.
2. Chemical Composition
| Element | Content (%) | Role & Impact |
|---|---|---|
| Carbon (C) | ≤0.20 | Balances strength and weldability. |
| Manganese (Mn) | ≤1.70 | Enhances hardenability and strength. |
| Silicon (Si) | ≤0.80 | Improves deoxidation and strength. |
| Phosphorus (P) | ≤0.025 | Strictly controlled to prevent brittleness. |
| Sulfur (S) | ≤0.015 | Minimized to avoid hot cracking. |
| Microalloys | Nb, V, Ti, B (trace) | Refine grain structure and enhance toughness. |
Notes:
Microalloying with niobium (Nb) and vanadium (V) improves grain refinement during quenching.
Boron (B) is optionally added to boost hardenability.
3. Mechanical Properties
3.1 Key Mechanical Values
| Property | Requirement | Test Standard |
|---|---|---|
| Yield Strength (Reh) | ≥690 MPa | EN ISO 6892-1 (tensile test) |
| Tensile Strength (Rm) | 770–940 MPa | EN ISO 6892-1 |
| Elongation (A5) | ≥14% | 5.65√S₀ gauge length |
| Impact Toughness | ≥40J @ -40°C | EN ISO 148-1 (Charpy V-notch) |
| Hardness | 240–300 HB | Brinell Hardness (HBW 10/3000) |
3.2 Heat Treatment
Quenching & Tempering (Q&T):
Quenching: Rapid cooling from 880–950°C in water or oil.
Tempering: Heated to 550–660°C to achieve optimal toughness and stress relief.
Delivery Condition: Supplied in quenched and tempered state.
4. Weldability & Fabrication
4.1 Welding Guidelines
| Parameter | Details |
|---|---|
| Carbon Equivalent (CEV) | ≤0.47 (CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) |
| Preheating | 100–200°C (depending on thickness and hydrogen content of electrodes). |
| Recommended Methods | SMAW (shielded metal arc), SAW (submerged arc), GMAW (gas metal arc). |
| Post-Weld Treatment | Stress-relief annealing (optional for thick sections >30 mm). |
Electrode Selection: Use low-hydrogen electrodes (e.g., EN ISO 16834-A T69 4 P M3 M H5).
4.2 Machinability
Cutting: Plasma or laser cutting preferred; flame cutting requires preheating.
Forming: Cold forming possible but limited due to high strength; hot forming at 850–1100°C.
5. Corrosion Resistance & Protection
Inherent Resistance: Limited; S690QL is not stainless.
Protection Methods:
Galvanizing: Hot-dip galvanizing (post-fabrication).
Coatings: Epoxy, polyurethane, or zinc-rich primers for harsh environments.
6. Comparison with Similar High-Strength Steels
| Property | S690QL (EN 10025-6) | ASTM A514 (USA) | Hardox 600 (SSAB) |
|---|---|---|---|
| Yield Strength | ≥690 MPa | ≥690 MPa | ≥700 MPa (wear-resistant) |
| Impact Toughness | ≥40J @ -40°C | ≥34J @ -32°C | Not standardized |
| Primary Use | Structural components | Crane booms, military vehicles | Abrasion-resistant parts |
Advantage of S690QL: Superior low-temperature toughness compared to ASTM A514.
7. Applications
Mining Equipment: Bucket teeth, excavator arms, and crusher frames.
Crane Components: Boom sections, slewing rings.
Offshore Structures: Jacket legs, platform connectors.
Wind Energy: Tower flanges, foundation components.
S690QL is a premium choice for engineers prioritizing high strength-to-weight ratios and extreme toughness in demanding environments. For projects requiring even higher strength, consider grades like S890QL or S960QL. Let me know if you need further details!
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