For thick plates of S690QL, any form of flame cutting and plasma cutting is absolutely prohibited due to the catastrophic and irreversible thermal damage these processes inflict on the steel's sophisticated, heat-treated microstructure. This prohibition is a non-negotiable safety and quality mandate in high-integrity fabrication.
Here is a detailed breakdown of the metallurgical reasons and consequences:
Core Reason: Destruction of the Quenched & Tempered (Q&T) Microstructure
S690QL derives its exceptional strength and toughness from a precise quenching and tempering process, which creates a uniform, fine-grained microstructure of tempered martensite/bainite. Thermal cutting processes destroy this engineered state in the heat-affected zone (HAZ).
1. Formation of an Unacceptable Hard and Brittle Zone (Re-Hardening)
Process: The intense, localized heat of flame (>3000°C) or plasma (up to 20,000°C) instantly heats a narrow band of material above its austenitization temperature (Ac3).
Metallurgical Transformation: The surrounding cold, massive plate acts as an ultra-fast quench medium, causing this heated zone to transform into hard, untempered (fresh) martensite.
Result: This creates an extremely hard (often 650-800 HV), brittle "white layer" along the cut edge. This layer is:
Highly crack-sensitive: Micro-cracks can form spontaneously due to thermal stress alone.
A perfect fatigue crack initiator: Under dynamic loads (crane booms, mining equipment), cracks will nucleate here first, leading to premature failure.
Susceptible to Hydrogen Cracking (HIC): Flame cutting introduces hydrogen from combustion, which can diffuse into this brittle zone and cause delayed cracking.
2. Creation of a Softened (Over-Tempered) Zone
Adjacent to the re-hardened zone, the steel is heated to a temperature range high enough to over-temper the base metal.
This creates a soft band where the yield and tensile strength can plummet by 30-50% or more. For S690QL, this zone's strength may drop to levels equivalent to S355 steel, creating a critical weak link.
3. Introduction of Severe Tensile Residual Stresses
The extreme thermal gradient locks in high tensile residual stresses at the cut edge, which algebraically add to applied service stresses. This can:
Promote stress corrosion cracking (SCC).
Reduce fatigue life dramatically.
Cause distortion in the plate.
Why These Defects Are Unacceptable and Unrepairable for S690QL
The Damaged Zone is Deep: In thick plates (e.g., >50mm), the thermal gradient penetrates deeply. The brittle/softened zone can extend 3-10mm or more below the surface.
Grinding is Not a Reliable Solution:
Removing the entire affected zone by grinding would require machining away 5-15mm of material from the entire cut edge-a costly, time-consuming process that alters dimensions and is impossible to verify fully.
Hardness testing cannot guarantee the complete removal of the affected microstructure gradient. A remaining microscopic brittle layer is enough to initiate a crack.
The Risk is Catastrophic: Components made from S690QL are used in safety-critical, high-energy applications (crane booms under full load, excavator arms). A brittle fracture originating from a cut edge could lead to sudden, catastrophic failure with immense risk to life and property.
The Mandated Alternative: Cold Cutting Methods
For S690QL, fabrication codes and material specifications mandate cold cutting processes that introduce no significant heat:
| Permitted Process | Mechanism | Key Advantage for S690QL |
|---|---|---|
| Abrasive Waterjet Cutting | Ultra-high-pressure water + abrasive grit erosion. | Zero Heat-Affected Zone (HAZ). Perfectly preserves the Q&T microstructure. The only method for complex shapes in thick plate. |
| Milling / Machining | Mechanical material removal with coolant. | Produces a pristine, ready-to-weld edge with negligible thermal input. Ideal for preparing weld preparations. |
| Sawing (Carbide or Abrasive) | Mechanical shearing/abrasion. | Suitable for straight cuts. Must be controlled to avoid excessive work-hardening or burrs. |
Conclusion: The Absolute Prohibition is Rooted in Risk Management
The ban on flame/plasma cutting is not a preference but a fundamental requirement of the material science. For S690QL:
Thermal Cutting = Guaranteed Introduction of a Critical Defect.
Cold Cutting = Guaranteed Preservation of Material Integrity.
The premium paid for S690QL's performance is utterly negated by thermal cutting. Therefore, all relevant fabrication standards (EN 1090, ISO 9013), manufacturer guidelines, and engineering specifications for quenched and tempered steels explicitly prohibit these processes. It is the first and most critical rule in fabricating this elite material.