The relationship between thickness and mechanical properties in S690QL is not linear; it is governed by the fundamental metallurgical constraints of the Quenched & Tempered (Q&T) process. Understanding this relationship is essential for safe and efficient design, as the guaranteed properties on a mill certificate are valid only for a specific thickness range.
Here is a detailed research-based analysis of this relationship.
1. The Core Metallurgical Principle: Hardenability & Cooling Rate
S690QL gains its properties through a rapid quench from the austenitizing temperature (~900°C), forming a hard martensitic/bainitic microstructure, which is then tempered for toughness.
Thin Plate: Cools rapidly and uniformly throughout its cross-section. This results in a fully martensitic microstructure after quenching, which after tempering yields maximum strength and optimal toughness.
Thick Plate: The core (center) cools significantly slower than the surface during quenching. This slower cooling rate can lead to the formation of softer transformation products (like ferrite or pearlite) in the core, creating a through-thickness property gradient. The tempering process also becomes less uniform.
Result: As thickness increases, the guaranteed minimum properties decrease.
2. Quantitative Analysis: How Properties Degrade with Thickness
The standard EN 10025-6 formally recognizes this by grouping plate thicknesses and assigning reduced property values for thicker groups.
Example from EN 10025-6 (S690QL):
| Product Thickness *t* (mm) | Yield Strength ReH (MPa) min | Tensile Strength Rm (MPa) | Elongation A (%) min | Impact Energy KV (J) min |
|---|---|---|---|---|
| t ≤ 30 | 690 | 770 - 940 | 14 | 30 J at -40°C (L) / -60°C (L1) |
| *30 < t ≤ 50* | 670 | 770 - 940 | 14 | Same |
| *50 < t ≤ 100* | 650 | 750 - 930 | 14 | Same |
| *100 < t ≤ 150* | 630 | 730 - 910 | 13 | Same (Note 1) |
| *150 < t ≤ 250* | 600 | 710 - 890 | 13 | Same (Note 1) |
*(Note 1: For thicknesses >100mm, impact test location (surface vs. 1/4 thickness) and sample orientation become critical, and the test temperature may be subject to agreement.)*
Key Observations:
Yield Strength: Drops by ~13% when moving from ≤30mm to 150-250mm plate (690 → 600 MPa). A designer using the nominal "690" value for a 200mm thick component would be 13% overstressed.
Tensile Strength: The range shifts downward but maintains a significant overlap, indicating the core strength mechanism is still active but less effective.
Elongation: Slightly reduced in very thick plates, indicating a small loss in uniform ductility.
Impact Toughness: The minimum energy value (e.g., 30 J) is maintained, but this is measured on a standard 10x10mm sample taken from a specific location (often the 1/4 thickness point). The through-thickness toughness, especially at the centerline, can be significantly lower due to segregation and microstructural changes.
3. Mechanisms Driving Property Reduction per Thickness
| Property | Primary Mechanism of Degradation with Thickness | Engineering Implication |
|---|---|---|
| Yield & Tensile Strength | Decreased Cooling Rate in Core: Leads to formation of non-martensitic phases (e.g., bainite, ferrite) which have lower strength. Also, tempering may be less effective in the core if the plate does not reach uniform temperature. | Design must use the derated strength value for the specific thickness. The "S690" name is misleading for thick plates. |
| Toughness (Fracture Resistance) | 1. Microstructural Coarsening: Slower cooling leads to coarser prior austenite grains and microstructural constituents. 2. Centerline Segregation: Impurities (P, S) and alloying elements (Mn) segregate to the mid-thickness during ingot solidification, creating a continuous band of potentially brittle material. |
Increased risk of brittle fracture initiation at the core, especially under through-thickness stress. Mandates strict control of sulfur (Z-quality steel) and may require additional testing (e.g., CTOD tests at depth). |
| Through-Thickness (Z-direction) Properties | Elongated Inclusions: Sulfide and oxide inclusions elongate in the rolling direction. In thick plates, this creates planar weaknesses perpendicular to the short-transverse (Z) direction. | High risk of lamellar tearing under welding-induced shrinkage stresses. For thicknesses >30mm in welded construction, specifying S690QL with Z-quality (e.g., Z35) is mandatory. |
| Weld HAZ Properties | Increased Restraint & Residual Stress: Thicker plates create higher levels of triaxial restraint during welding, locking in higher residual stresses and increasing cold cracking susceptibility. The width of the softened HAZ also increases. | Requires more stringent welding procedures (higher pre-heat, controlled interpass temp, possibly PWHT). The softened HAZ becomes a larger and more critical weak link. |
4. Research-Based Design & Procurement Guidelines
A. For Designers:
Never Use Nominal Grade Strength: Always obtain the guaranteed mechanical properties for the exact thickness range from the standard or mill certificate. Perform calculations using the derated values (e.g., 650 MPa for t=65mm).
Thickness is a Design Parameter: Consider splitting a thick member into two thinner plates welded or bolted together. The gain in material strength (from 650 back to 690 MPa) may outweigh the cost of an extra weld.
Prioritize Toughness for Thick Sections: For fracture-critical applications (e.g., offshore nodes, heavy crane booms), specify the highest subgrade (S690QL1 for -60°C) and consider additional fracture mechanics testing (CTOD) for thicknesses >50mm.
B. For Fabricators & Procurement:
Mandate Z-Quality for Welded Thick Plates: For any plate >30mm that will be welded, especially in T-joints or corner joints, S690QL1 Z35 should be the default specification. This ensures a minimum 35% reduction of area in the through-thickness tensile test and ultra-low sulfur.
Request Additional Testing: For very thick plates (>100mm), it is prudent to request additional Charpy tests at the 1/2 thickness (centerline) to confirm toughness uniformity.
Understand the Mill's Capability: The ability to deliver consistent properties in thick S690QL plates varies by mill. Inquire about their quenching system (high-pressure water jets vs. bath) and tempering furnace uniformity.
5. Summary: The Thickness-Performance Trade-Off Curve
The relationship can be visualized as a set of declining curves:
Strength vs. Thickness: A steep initial decline that gradually flattens. Most significant derating occurs in the first 50mm.
Toughness (Uniformity) vs. Thickness: A more complex curve. Surface toughness remains high, but centerline toughness can drop precipitously if segregation is poor and plate is very thick.
Fabrication Risk vs. Thickness: An exponentially rising curve. Risk of lamellar tearing, distortion, welding cracks, and residual stresses escalates rapidly with thickness.
Conclusion
For S690QL, thickness is the primary determinant of its deliverable mechanical properties. The nominal "690" is a theoretical maximum only achievable in thinner sections.
Successful engineering with thick S690QL requires a dual strategy:
Metallurgical Realism: Accept and design with the derated properties stipulated by the standard for the thickness group.
Proactive Mitigation: Specify enhanced quality levels (Z-quality, QL1 subgrade) and plan for more sophisticated fabrication and inspection regimes to counteract the inherent risks introduced by thickness.
Ignoring this relationship is a direct path to overestimation of capacity and underestimation of risk, particularly the risk of brittle fracture originating from the core of a thick, highly stressed component. The material's legendary performance is strictly conditional on its cross-sectional dimensions.