Materials, Metal Preparation and Weldability
This domain focuses on what happens before the arc starts: cleaning, edge preparation, fit-up and contamination control. Poor preparation causes porosity, lack of fusion and cracking that no welding skill can repair.
Core rules
- Clean at least 25 mm each side of the joint. Remove oil, grease, paint, rust, scale and moisture. Use the correct solvent and allow it to dry.
- Prepare bevels, root faces and root gaps to the WPS. A V-groove included angle is the sum of both bevel angles. A root face supports the root pass; too little causes burn-through, too much causes lack of penetration.
- Use dedicated stainless and aluminium tools. Carbon-steel contamination causes corrosion and cracking in stainless steel.
- Check fit-up: alignment, gap, root face, bevel angle, tack welds and restraint. Tack welds must be clean and fused; defective tacks should be removed.
Worked example
A plate is bevelled 30° on each side with a 2 mm root face and a 2 mm root gap. The included angle is 30° + 30° = 60°. The root opening is the gap, 2 mm. If the gap closes to 0.5 mm, penetration may be incomplete; if it opens to 5 mm, burn-through and excess weld metal are likely. The reasoning is that the WPS values control both root access and heat balance.
Traps that cost marks
- Welding over paint or galvanising without removing it.
- Using the same grinding wheel for carbon steel and stainless steel.
- Setting a gap “by eye” instead of using a gauge.
- Leaving a cracked or undersized tack weld in the joint.