Welding is the permanent joining of metals by locally melting them, so that the weld pools of both parts flow together into one — usually with added filler metal and protected by a shielding gas or a slag layer that keeps the air out.
Unlike bolting or riveting, no mechanical coupling is formed but a continuous transition of material. That is why a sound weld is in principle as strong as the parent metal — and why a poor weld is immediately a structural problem.
The main processes
- MIG/MAG — a wire is fed continuously and melted under a shielding gas. Fast, easy to learn, the workhorse for structural steel, railings and sheet metal. MAG (active gas, usually a CO2 mix) for steel, MIG (inert argon) for aluminium and stainless.
- TIG — a non-consumable tungsten electrode carries the arc while filler is fed with the other hand. Slow, but the cleanest and most controllable process. The choice for thin-wall stainless, aluminium and visible work.
- Stick welding (MMA) — a coated rod electrode; the coating produces both gas and slag. Tolerant of wind and contamination, which keeps it popular outdoors on site and on thicker material.
Joint types
- Fillet weld — two parts at an angle, usually 90°. By far the most common in structural steelwork. Its size is the throat thickness: the height of the largest triangle that fits within the weld cross-section. That is also how it is called up on the drawing.
- Butt weld — two parts in line, usually with a prepared edge (V, X or U preparation) so the weld penetrates fully.
- Lap weld and plug weld on sheet assemblies.
Where it goes wrong
- Lack of fusion. The bead sits neatly on top but has not grown into the parent metal. Causes: current too low, travelling too fast, or a dirty surface.
- Slag inclusions in multi-pass welds, because the previous run was not fully chipped and brushed.
- Porosity from moisture, grease, paint, rust or a draught blowing the shielding gas away. Grind or brush the joint bright and weld out of the wind.
- Distortion and shrinkage. Weld metal shrinks as it cools and pulls the assembly out of square. Tack first, weld in stages, and alternate sides.
- Oversizing. A thicker weld is not automatically better: it puts in more heat, causes more distortion and costs more time. Stick to the throat thickness on the drawing.
Regulation and safety
- For load-bearing steelwork the European execution standard EN 1090 applies; depending on the execution class it requires a certified fabricator, qualified welders and documented welding procedures. On a structural steel frame this is not optional.
- Protective clothing and an auto-darkening helmet at the correct shade are essential; arc radiation causes arc eye within minutes and burns exposed skin.
- Extraction or ventilation. Welding fume is harmful; stainless steel releases chromium and nickel compounds with strict exposure limits.
- Fire watch and a clear working area. Sparks travel metres and stay hot for a long time. Remove combustibles and keep checking the area after you finish.
Related terms
- Welding on
- Welding hammer
- Laser cutting
- Reinforcing steel
- Corten steel
- Aluminium
- Cold-formed steel
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