Flame Cutting of Q355D S355J2 Heavy Steel Plates: Cutting Methods & Key Precautions for Different Pl

Pre-Amalysis: Core Cutting Risks of S355J2 Q355D

1. Oxygen purity ≥99.5%. Low oxygen purity leads to dross adhesion, incomplete cutting and local hardening cracks on cutting surfaces.
2. Strict neutral flame for cutting. Oxidizing flame melts and collapses the top edge; carburizing flame causes carburization and hardening on cutting edges. The flame core shall stay 1.5~2.5 mm above the plate surface.
3. Leave a 30~80 mm gap under the cutting table to ensure molten slag flows down freely and prevent back slag from hardening the base metal.
4. Remove oil stains, heavy scale and paint within 200 mm of the cutting line before cutting.
5. When ambient temperature is below 5℃, increase preheating temperature by 20~30℃ for all plate thicknesses.

I. Cutting Procedures, Parameters & Special Precautions by Plate Thickness

Category 1: Thin Plates (10~25 mm S355J2)

• Cutting oxygen pressure: 0.60~0.75 MPa
• Preheating pressure (propane/acetylene): 0.05~0.08 MPa
• Travel speed: 350~500 mm/min
• Preheating temperature: 80~100℃ (local preheating is acceptable at normal ambient temperature; full preheating is mandatory under low temperature)

1. Start cutting from plate edges to avoid piercing. If piercing is required, use a scrap lead piece to prevent cracking caused by concentrated high-temperature stress at a single point.
2. Keep the cutting nozzle perpendicular to the plate surface at all times. Thin plates dissipate heat fast; avoid prolonged fixed-point preheating to prevent melting collapse of the top edge.
3. Reduce travel speed to 70% of the standard speed when approaching the cutting end to guarantee full penetration at the bottom and minimize tail dross.

• Thin plates are prone to large deformation. For stacked multi-plate cutting, keep the gap between each layer ≤2 mm and fully support the bottom to avoid plate warping which blocks oxygen flow.
• No post-cut heat preservation required. Do not apply forced water cooling immediately; allow natural air cooling for 30 minutes before handling.
• Leave connecting bridges for batch small workpieces. Separate parts only after full cooling to release thermal stress.

Category 2: Medium Heavy Plates (26~50 mm S355J2, widely used for flanges and bearing base blanks)

• Cutting oxygen pressure: 0.70~0.85 MPa
• Preheating gas pressure: 0.07~0.10 MPa
• Travel speed: 220~340 mm/min
• Preheating temperature: 100~130℃. Evenly preheat a 200 mm wide area on both sides of the cutting path.

1. Direct piercing cutting is prohibited; use lead plates or extended lead lines. Perform circular preheating at the starting point until full plate thickness is heated through before turning on cutting oxygen.
2. Maintain constant travel speed. Vertical downward sparks indicate normal cutting; forward-tilting sparks mean excessive speed which easily causes layered cracks; backward-floating sparks mean insufficient speed resulting in top edge burning loss.
3. For long straight cutting, divide the path into segments with a 50 mm connecting bridge every 3 meters. Cut off bridges only after full cooling.

• Preheating is mandatory for plates ≥30 mm. Cracks along cutting edges within 24 hours are highly likely without preheating.
• If cutting surfaces require subsequent welding, slow cool the plates while retaining preheating temperature until temperature drops below 60℃ before stacking to prevent hydrogen-induced cracks.
• For plates with Z-direction property requirements (Z15/Z35), avoid prolonged concentrated heating on one side to prevent lamellar tearing.

Category 3: Heavy Plates (51~100 mm S355J2, for equipment bases, frames and large flanges)

• Cutting oxygen pressure: 0.85~1.0 MPa
• Preheating gas pressure: 0.10~0.14 MPa
• Travel speed: 120~210 mm/min
• Preheating temperature: 130~160℃. Large-area preheating for the entire cutting zone; maintain interpass temperature above 100℃.

1. Lead plate process for cutting start: Attach scrap S355J2 lead pieces tightly to the starting point. Pierce the lead piece first then cut into the base metal to eliminate layered cutting surfaces and cracks caused by temperature difference between top and bottom.
2. Tilt the cutting torch 5°~10° forward at cutting start until full thickness penetration, then adjust back to vertical.
3. Set deceleration section for CNC cutting (reduce speed by 30% for the last 20 mm) to avoid incomplete bottom cutting and root stress cracks.
4. Preserve heat between consecutive workpieces during batch cutting; prohibit rapid plate cooling.

1. Highest risk of cracks; both sufficient preheating and low travel speed are indispensable. Forbid forced air cooling or water spraying.
2. Apply post-heat slow cooling after cutting: Insulate at 200~250℃ for 20~40 minutes and cover cutting edges with fire-resistant cotton for gradual cooling to eliminate thermal hydrogen stress.
3. Hardened heat-affected zones easily form on cutting surfaces. Grind off 1~2 mm hardened layers before welding or machining.
4. Avoid single-point lifting at hot cutting edges during hoisting; high-temperature edges have low tensile strength and may tear easily.

Category 4: Extra-Heavy Plates (100~200 mm S355J2, for heavy construction machinery and pressure vessel bases)

• Cutting oxygen pressure: 1.0~1.2 MPa, equipped with high-pressure oxygen stabilizer tank
• Preheating gas pressure: 0.13~0.18 MPa, dual preheating flames for enhanced heat input
• Travel speed: 60~110 mm/min
• Preheating temperature: 160~200℃. Full-plate overall preheating with real-time temperature monitoring via thermal guns; stop cutting and reheat if temperature drops below 140℃.

1. Direct piercing is completely forbidden. External lead plates must be used. Apply multi-point circular preheating on the starting point for 3~5 minutes to ensure uniform temperature across full plate thickness.
2. Keep the torch fully vertical throughout cutting; avoid large-angle tilting. Pause cutting for 30 seconds every 500 mm to replenish heat and prevent local temperature drop and temperature differential stress.
3. Adopt segmented intermittent cutting for large contours; limit single cutting length ≤1.5 m with heat preservation intervals to avoid massive thermal accumulation and deformation.
4. Extend flame heat preservation for 10~20 seconds at cutting ends before closing cutting oxygen to eliminate stress concentration at cutting roots.

1. Mandatory hydrogen elimination & slow cooling after cutting: Insulate at 250~300℃ for more than 1 hour and fully cover with rock wool to cool naturally to room temperature; otherwise penetrating delayed cracks will occur.
2. Sufficient oxygen flow is required. Insufficient oxygen pressure causes incomplete fusion in the middle of cutting sections and internal lamellar tearing.
3. Extra-heavy plates generate wide heat-affected zones with increased hardness at cutting edges. Grinding or low-temperature tempering is required before finish machining or welding.
4. Stack cutting is prohibited; cut single plates separately with uniform bottom supports to prevent plate bending and tearing at cutting edges due to self-weight.

II. Universal Critical Control Points (Applicable to All Plate Thicknesses)

1. Crack Prevention (Top Priority for S355J2)

1. Root causes of delayed cracks: Excessively fast cooling rate, hydrogen accumulation and concentrated thermal stress.
2. Three lines of defense: Preheating before cutting, low constant travel speed during cutting, slow cooling / post-heat treatment after cutting.
3. Inspection: Perform penetrant testing (PT) on cutting edges after 48 hours of standing to detect microcracks.

2. Bevel Cutting Process (Pre-Welding Preparation)

1. Bevel angle: 30°~45°, torch tilt angle 25°~40°.
2. Increase preheating temperature by 20℃ for bevel cutting compared to flat plates of the same thickness.
3. Two-pass cutting for heavy plate bevels: Cut straight edges first then trim bevels in a second pass to avoid concentrated heat input and hardening from one-step forming.
4. Fully remove molten slag and oxide layers on bevel surfaces by grinding; residual impurities cause welding porosity and cold cracks.

3. Standardized Cutting Start & Termination Operations

• Cutting Start: Heat the plate surface to cherry red with neutral flame, slowly open cutting oxygen, and move the torch at constant speed only after continuous sparks drop vertically from the plate bottom.
• Cutting Termination: Reduce travel speed 15~20 mm ahead of the end point, lower cutting oxygen flow, hold flame for several seconds, then close cutting oxygen and preheating gas sequentially.

4. Post-Cutting Treatment Specifications

1. Cooling: Forbid water cooling or forced air cooling. Heavy & extra-heavy plates must be insulated for slow cooling.
2. Slag Removal: Remove slag only after plates cool below 50℃; chipping hot edges easily generates microcracks.
3. Grinding: Remove melted collapse, hardened layers and scale on top & bottom cutting edges to achieve surface roughness Ra ≤12.5 μm for welding compatibility.
4. Non-Destructive Testing: Penetrant testing (PT) is mandatory for load-bearing components with plate thickness ≥50 mm to confirm zero cracks before transferring to subsequent processes.

III. Common Defects, Causes & Targeted Solutions for S355J2

IV. Supplementary Rules for Low Ambient Temperature (Ambient Temperature ＜5℃)

1. Increase preheating temperature by 30℃ for all plate thicknesses.
2. Stop cutting immediately if plate temperature drops below preheating minimum limit during processing; resume cutting only after reheating to qualified temperature.
3. Extend slow cooling insulation time by 50% after cutting.
4. Outdoor open-air cutting is prohibited for S355J2 plates thicker than 30 mm.