Q690 vs S690 High-Strength Steel Plate: Material Differences & Key Precautions for Cutting & Machini

1. Standards and Grade Definitions

2. Delivery State Differences (Critical)

1. Q690 Two supply forms: ① TMCP thermo-mechanical rolled (low cost, slightly lower hardening tendency); ② Q&T quenched and tempered (high toughness, primary choice for thick plates). Domestic steel mills can flexibly switch between rolling and quenching & tempering processes.
2. Full S690 Series Mandatorily delivered in Q&T quenched & tempered condition (uniform quenching + high-temperature tempering after production); TMCP rolling supply is prohibited. S690 features stable microstructure and highly consistent low-temperature toughness, mandatory base material for overseas engineering projects.

3. Chemical Composition & Carbon Equivalent

• S690 adopts stricter control on alloy ratios with lower limits of P/S impurities (S ≤ 0.015%), featuring superior resistance to lamellar tearing and fatigue performance compared with ordinary domestic Q690.
• Partial TMCP rolled Q690 allows higher upper limit of Mn content with lower material cost.

4. Mechanical Property Comparison (Plate thickness t ≤ 50mm)

5. Typical Application Scenarios

• Q690: Domestic crane booms, pump truck outriggers, mining truck carriages, hydraulic supports and lightweight bridge components.
• S690: Export construction machinery, European wind turbine towers, offshore platforms, polar equipment and EU-certified special equipment.

II. Common Thermal Cutting Risks (Applicable to Both Q690 & S690)

1. Cold cracks on cutting edges (highly likely for thick plates processed at low ambient temperature);
2. Hardened cutting surface with hardness up to HV380~450, causing severe tool sticking and poor machinability in subsequent processing;
3. Stress-induced deformation, including warpage and dimensional out-of-tolerance for large-size cut parts.

III. Precautions for Cutting & Machining by Process

(1) Flame Cutting (Preferred for plates ≥25mm, extra-thick plates 50~200mm)

1. Mandatory Preheating (Top Priority)
   • Plate thickness ≤50mm: Preheat to 120~150℃;
   • Plate thickness ＞50mm: Uniformly bake the whole plate to 180~250℃; local spot heating is forbidden;
   • If ambient temperature ＜5℃, raise preheating temperature by 50℃ to avoid cracking caused by temperature drop during cutting.
2. Cutting Parameter Control
   • Oxygen pressure: 0.7~1.1MPa; adopt large-flow diffusion cutting nozzles to ensure complete penetration of cutting kerf;
   • Maintain uniform and slower cutting speed (30% slower than ordinary Q355 steel). Fast torch travel that triggers rapid cooling of edges is prohibited;
   • Tilt the cutting nozzle reversely at the end of cutting to prevent tearing caused by concentrated stress at the terminal section.
3. Slow Cooling Treatment after Cutting Immediately fully cover cut plates with fire-resistant asbestos cloth after cutting, and cool naturally to room temperature for at least 4 hours. Forced air cooling or water quenching is strictly forbidden. For plates ≥80mm thick, perform low-temperature hydrogen removal baking (200℃, 2h) within 1 hour after cutting.
4. Machining Allowance & Post-Treatment The carburized hardened layer formed by flame cutting ranges 0.8~1.5mm deep. For load-bearing welded components, reserve a machining or grinding allowance of 3~5mm to completely remove the hardened layer. Flat grinding without allowance is only acceptable for non-load-bearing blanks.

(2) Plasma Cutting (Medium & Thin Plates 6~30mm)

1. Advantages: Low heat input, narrow heat-affected zone and thin hardened layer, with lower cracking risk than flame cutting.
2. Process Key Points:
   • Adopt high-purity oxygen as cutting gas; match current with plate thickness to reduce accumulated heat;
   • Preheat to 80~100℃ for plates thicker than 20mm;
   • Reserve a 1~2mm processing allowance; light grinding can eliminate the hardened layer.

(3) Laser Cutting (Thin plates ≤12mm for high-precision parts)

1. Ideal for flanges, small cut pieces and precision blanks with minimal deformation.
2. Notes: Instant ultra-high temperature from high-power laser still generates a thin hardened microstructure on cutting edges.
3. Pre-treatment: Remove rust and oil stains within 20mm of cutting paths to avoid light reflection and dross adhesion on kerfs.
4. Cutting Path Planning: Cut small holes and special-shaped blanks first, then outer contours to release internal stress and reduce plate warpage.

(4) Thermal-Stress-Free Cutting (First Choice for Ultra-High Precision / Fatigue-Critical Parts)

IV. Unique Machining Restrictions for S690 (Distinct from Q690)

1. S690 is delivered in quenched & tempered state. Overall high-temperature tempering or re-quenching after cutting is forbidden, which will destroy the original quenched-tempered microstructure and drastically reduce strength and low-temperature toughness. If stress relief is required, only low-temperature baking at 200~250℃ is allowed.
2. EU engineering projects impose hardness inspection standards on S690 cutting workpieces: Edge hardness of heat-affected zone shall not exceed HV380; flaw detection is mandatory after grinding off hardened layers.
3. Cold bending of S690 is not recommended; bending radius shall be no less than 5 times plate thickness, otherwise microcracks are highly likely to form.

V. Exclusive Machining Guidelines for Q690

1. TMCP rolled Q690 allows 30~50℃ lower preheating temperature, with lower cracking risk compared with quenched & tempered S690.
2. Impurity content fluctuates among domestic steel mills for Q690; 100% visual inspection for cracks is required after cutting thick plates.
3. Domestic equipment has no strict temperature limit for stress relief; short-time stress relief heating at 300℃ will not impair base material performance.

VI. General Specifications for Post-Cutting Processing

1. Grinding Requirements: All thermal-cut edges must be ground to metallic luster before welding to eliminate hardened layers, microcracks and dross.
2. Machining Rules: Turning, milling or drilling is forbidden before complete removal of hardened layers, which will lead to severe tool wear.
3. Welding Coordination: Unify preheating temperature for cutting and welding to eliminate secondary heating procedures.
4. Storage Protection: Avoid outdoor low-temperature stacking of semi-finished cut blanks; excessive temperature difference may induce delayed edge cracking.