1. Pre-Cutting Preparation
1.1 Plate Condition Inspection
- Remove rust, scale, oil stains and accumulated water on the steel plate surface. Heavy scale on extra-thick plates easily causes flame interruption and incomplete cutting.
- Pay close attention to low-alloy high-strength steel, pressure vessel steel and flaw-detection steel plates. Such plates feature large thickness and high hardenability; preheating shall be carried out when necessary. Preheating for ordinary Q235/S235/A36 thick plates can be determined according to site conditions.
- The steel plate shall be leveled and suspended, with a clearance of 80～150 mm reserved at the bottom to ensure smooth outflow of molten slag. Do not place the plate close to the ground or cutting platform; accumulated slag will stick firmly and result in incomplete kerf penetration.
1.2 Nozzle Selection (Core Item)
- An equal-pressure heavy-duty cutting torch and large annular cutting nozzle must be adopted for extra-thick plate cutting. Small torches for thin plates are prohibited.
- Follow the nozzle manufacturer’s specifications: the thicker the plate, the larger the nozzle model and the higher the oxygen pressure. Never force cutting thick plates with undersized nozzles.
- Inspect nozzles: free of deformation, pores and blockages; inner and outer nozzles must be concentric. Clear minor blockages with a special cleaning needle in the original direction; avoid hard poking and scraping.
1.3 Reference Gas Pressure Settings (Final values subject to trial cutting on site)
Parameters for reference only and shall be adjusted according to pipeline length and ambient temperature
- Fuel gas (Acetylene / Propane): 0.05～0.08 MPa
- High-pressure cutting oxygen: 50～100 mm plate: 0.60～0.75 MPa 100～200 mm plate: 0.75～0.90 MPa Plate thickness ＞200 mm: 0.9～1.1 MPa
- Pressure stabilization suggestion: Install pressure reducing valves and buffer tanks at the pipeline end to avoid pressure fluctuation during cutting.
1.4 Preheating Treatment (Critical for Preventing Cutting Cracks)
- Carbon steel Q235/S235, Q355/S355 thick plates: Preheat to 80～150 ℃ in low-temperature winter conditions or when plate thickness ＞120 mm.
- Alloy steel plates (16MnDR, NM450, 45# steel, quenched & tempered plates): Mandatory preheating to 150～250 ℃.
- Preheating range: Heating width on each side of the kerf shall be no less than the plate thickness. Do not only heat along the cutting line.
2. Key Points During Cutting Operation
2.1 Ignition and Cutting Initiation
- Ignite the preheating flame first and adjust to neutral flame (optimal condition). Avoid carburizing flame (causes carburization, hardens cut surface and impedes subsequent machining) and oxidizing flame (melts and collapses the top edge).
- Vertical direct piercing is strictly prohibited for extra-thick plates. Prioritize lead-in cutting outside the plate. If piercing inside the plate is unavoidable, extend preheating time. Open cutting oxygen slowly only after the piercing center is fully melted through to prevent high-pressure oxygen splashing and flashback.
2.2 Travel Speed Control (Frequent Cause of Scrap Parts)
- The cutting speed for thick plates is far lower than thin plates. Excessively fast speed: incomplete penetration at plate bottom, serrated cut and heavy dross adhesion. Excessively slow speed: melting and collapse of top edge, wider kerf and dimensional deviation.
- Judgment criterion: Molten slag flows continuously and smoothly from the plate bottom with bright yellow color, without intermittent black slag retention.
- CNC cutting machines shall run at constant low speed and minimize start-stop actions. Avoid pausing manually operated cutting as much as possible.
2.3 Torch Posture and Flame Height
- The cutting nozzle shall be perpendicular to the plate surface; do not tilt the torch during straight cutting.
- The tip of the flame core shall be 3～6 mm above the plate surface. Do not lift the nozzle too high for extra-thick plates, otherwise insufficient preheating energy occurs.
2.4 Handling Abnormal Conditions Mid-Cutting
- Incomplete cutting: Do not blindly speed up. Pause cutting immediately, enhance preheating, moderately increase cutting oxygen pressure, and re-cut from the melted-through section if necessary.
- Flashback emergency: Close cutting oxygen → preheating oxygen → fuel gas in sequence. Do not reverse the order in panic. Cool down the torch and troubleshoot blockages before reuse.
- Keep observing both sides of the kerf. Slightly reduce travel speed if local heavy slag accumulation appears.
3. Cutting Termination & Post-Blanking Control
3.1 Termination Operation
Slightly reduce travel speed when approaching the cutting end to guarantee full penetration at the workpiece tail, preventing adhesion, tearing and notches at the end.
3.2 Slow Cooling for Crack Prevention (Critical for Alloy Thick Plates)
- Never splash water for rapid cooling immediately after cutting alloy extra-thick plates, otherwise cold cracks easily form along cut edges.
- Cover workpieces with asbestos cloth for slow cooling if available. Crack risks rise sharply for outdoor operation in winter.
3.3 Slag Removal
Remove slag with air chisels or grinders after cooling. The hardened layer along cut edges is relatively thick; grinding to remove the hardened layer is recommended before subsequent machining.
4. Dimensional and Cut Surface Quality Control
- Extra-thick plates feature wide kerfs. Kerf compensation must be reserved during CNC programming and marking. The thicker the plate, the larger the compensation value, which shall be measured and confirmed via pre-trial cutting.
- Causes of common defects:
- Top edge collapse: Excessive flame power, low cutting speed, oxidizing flame
- Heavy bottom dross: Excessively fast cutting speed, insufficient oxygen pressure, poor ventilation under the plate
- Slanted and uneven kerf: Non-concentric nozzle, unstable travel movement
- Cutting cracks: Missing preheating, rapid cooling after cutting, high carbon equivalent of steel
5. Mandatory Safety Regulations
- Fire blankets and fire extinguishers shall be available on site. No combustible materials are allowed below falling molten slag. Extra-thick plate cutting generates massive high-temperature molten slag.
- Acetylene cylinders shall stand upright and stay away from hot slag. The distance between gas cylinders ≥5 m; the distance from cylinders to open flame ≥10 m.
- Flame cutting on pressure vessels or sealed tanks is forbidden.
- Continuous long-time cutting leads to torch overheating. Stop periodically for cooling to avoid nozzle deformation and subsequent flashback.
