Fillet welds are the most important form of welds in T-shaped joints, lap joints, and corner joints. They are widely used in steel structures, equipment racks, pipeline supports and hangers, and engineering machinery components. The quality of the weld depends on three key points: the weld toes on both sides are fully fused, the weld leg size is uniform, and there are no pores, slag inclusions, undercut cracks. This article details the practical points from six modules: pre-welding preparation, three-position welding essentials, multi-layer and multi-pass processes, parameter selection, troubleshooting, and intelligent welding.
Key Skills of Pre-Welding Pre-Treatment
Thoroughly remove rust, paint, oil, oxide scale and galvanized layer within 20mm of the front and back sides of the weld. Oil, sewage, and vapor are the primary cause of pores. Special stainless steel grinding wheels must be used for stainless steel. It is prohibited to mix carbon steel grinding wheels to cause carburization corrosion.
Conventional manual arc welding sets have a gap of 0 to 1.5mm, and gaps >2mm require pre-wire plugging; the amount of misalignment is ≤ 1/3 of the thickness of the base metal. If the misalignment exceeds the standard, it is easy to have one side not fused. Carbon dioxide gas shielded welding can adapt to 0~2mm gaps, and robot intelligent welding can adapt to 0~3mm assembly errors.
J422 acidic welding rods are stored at room temperature; J506/J507 alkaline welding rods are dried at 350°C to 400°C for 60 minutes and stored in an insulation barrel at a constant temperature of 120°C to 150°C. Secondly, make sure the welding wire is moisture-proof when it is unsealed. The damp welding wire will spatter a lot and the weld seam will have dense pores.
When the wind speed of open-air secondary welding is >2m/s, a windproof shed must be erected. The airflow destroys the protective gas cover and directly creates pores. The suspended parts of the workpiece should be rigidly fixed to reduce welding deformation.
Practical Points for Flat-Angle Welds (Most Frequently Used)
The angle between the welding rod and the vertical plate is 38° to 45°, the welding advance inclination angle is 80° to 85°, and the angle between the second welding gun and the welding gun is 40° to 50°. If the angle is too large, the vertical plate will be burned through; if the angle is too small, the penetration of the flat plate will be insufficient, directly causing the welding feet to be larger on one side and smaller on the other.
Moving the gun in a straight line at a constant speed, with a small and slight swing, using right-direction welding (forward welding) to control the volume of the molten pool to prevent falling weld flakes.
The narrow strip in the base layer is moved in a straight line, the filling layer is jagged and swings, and the cover surface is widened and swung. The slag is thoroughly cleaned after each layer of welding, and all slag inclusions are cleared before the next pass.
| Method | Specification | Current | Voltage |
|---|---|---|---|
| Manual Welding | Φ3.2 electrode | 90 ~ 125A | — |
| Manual Welding | Φ4.0 electrode | 150 ~ 185A | — |
| CO₂ Gas Shielded | Sheet | 95 ~ 130A | 18.5 ~ 22V |
When the gap becomes larger, reduce the walking speed and slightly increase the current to fill the deposited metal.
Key Points of Practical Control of Vertical Angle Welds
For thin plates of 3 to 5 mm, vertical downward welding is used, and the strips are transported quickly with low current to achieve smooth formation. For medium and thick plates above 6 mm, vertical upward welding is used, and the saw teeth are swung in sections.
The arc stays at the upper and lower welding toes for 0.5 to 1 second to ensure fusion on both sides; the arc quickly passes through the molten pool in the middle to prevent the molten iron from falling due to gravity to form weld nodules. The overall current of vertical welding is 10% to 15% smaller than that of flat welding with the same specifications.
The vertical fillet welding of thick plates is arranged in layers, and the swing width is narrowed layer by layer. The swing width of the cover surface is strictly controlled to prevent undercutting at the upper edge.
Key Point of Transverse Fillet Welding Operation
Tilt the welding gun downward by 5° to 10° as a whole, and use the arc to hold the molten iron to prevent the molten iron from flowing down, causing undercutting on the upper plate and accumulation of excess weld meat on the lower plate. Large leg transverse fillet welding prohibits single-pass wide swing, and uses multiple passes of parallel welding, welding from bottom to top. The subsequent weld bead presses 1/3 of the width of the previous weld bead, and the layers are formed sequentially.
Causes of Common Defects and Prevention & Control Measures
One Side Not Fused
Cause: The angle of the welding gun is offset, the arc is long-term biased towards a single plate, and the swing is not in place.
Fix: Adjust the angle so that the arc balances the two base materials.
Dense Pores
Cause: Base metal contains oily sewage vapor, welding rod is damp, secondary gas pressure is insufficient, and there is draft on site.
Fix: Strictly remove rust and oil + standardize the drying of the welding rod + check the gas supply and wind protection.
Uneven Welding Feet
Cause: The walking speed is fast and slow, and the swing amplitude is irregular.
Fix: Move at a constant speed and maintain a fixed swing amplitude.
Undercut
Cause: The current is too large, the residence time on both sides is too short, and the swing exceeds the limit.
Fix: Reduce the current and stay appropriately at the welding toe.
Manual welding relies on the welder’s technique to control the forming quality, while intelligent welding relies on visual sensing for adaptive adjustment. By thoroughly understanding the above points, you can stably produce fillet welds with qualified appearance and up to standard mechanical properties.
Post time: Jun-03-2026