Master the challenges of welding thick steel plate. Learn about joint preparation, multi-pass welding, pre-heating, and post-weld treatment for thick materials.
Introduction
Welding thick plate (6 mm and above) requires specialized techniques, equipment, and procedures. At Fulei Metal, we weld thick plate components for structural and heavy machinery applications.
Challenges of Thick Plate Welding
Heat Management
Thick plates require high heat input for penetration. Excess heat causes distortion and residual stress. Pre-heating may be required. Inter-pass temperature control is essential.
Joint Preparation
Thick plates need beveled joints. Proper groove design is critical. Root gap and face must be correct. Fit-up affects weld quality.
Multi-Pass Welding
Thick joints require multiple weld passes. Root pass for penetration. Fill passes for volume. Cap pass for finish. Each pass must be properly executed.
Residual Stress
Thick welds create high residual stress. Can cause cracking. Stress relief may be required. PWHT may be necessary for some applications.
Joint Preparation
Groove Types
V-groove for 6-12 mm. Double V for 12-20 mm. U-groove for 20 mm+. Bevel angle 60 degrees. Root gap 2-3 mm. Root face 1-2 mm.
Preparation Methods
Laser cutting for precise bevels. Milling for thick plate. Grinding for small jobs. Plasma for thick structural. Ensure clean, oxide-free edges.
Pre-Heating
When Required
Carbon steel above 25 mm. High-carbon or low-alloy steel. Low ambient temperature. High-hydrogen processes. Codes and standards may require it.
Temperature
Typically 100-200 degrees C. Measured with temperature sticks or pyrometer. Maintain until welding is complete.
Multi-Pass Technique
Root Pass
First pass in the joint. Provides penetration. Uses lower amperage. Must fuse to root faces. Critical for weld quality.
Hot Pass
Second pass after root. Burns out slag. Improves root penetration. Slightly higher amperage.
Fill Passes
Build up the joint. Multiple passes as needed. Clean slag between passes. Stagger start/stop points. Control inter-pass temperature.
Cap Pass
Final pass. Provides weld profile. Must meet size requirements. Smooth, uniform appearance. No undercut.
Parameters
Amperage
Higher than sheet metal. 150-300 amps depending on thickness and process. Adjust for each pass type.
Electrode/Wire
MIG: 1.0-1.2 mm wire. Stick: 3.2-4.0 mm electrodes. Filler matching base material. Low-hydrogen consumables for thick steel.
Inter-Pass Temperature
Monitor between passes. Typical: 150-250 degrees C maximum. Too high: grain growth, distortion. Too low: cracking risk. Use temp sticks.
Post-Weld Treatment
Slag Removal
Remove all slag between passes. Chip, brush, grind as needed. Slag inclusions cause defects.
Stress Relief
Heat to 550-650 degrees C. Hold per thickness. Slow cool. Reduces residual stress. Prevents cracking.
Inspection
Visual: bead shape, size, undercut. Radiography: internal defects. Ultrasonic: thick section defects. Magnetic particle: surface cracks.
Conclusion
Thick plate welding requires proper preparation, multi-pass technique, and heat management. At Fulei Metal, our equipment and procedures ensure quality thick plate welds for structural applications.