Materials
This domain tests basic weldability of carbon steel, stainless steel and aluminium, including carbon equivalent, preheat and contamination control. The exam expects you to link material composition to cracking risk.
Core rules
- Carbon steel weldability falls as carbon and alloy content rise. The IIW carbon equivalent formula is CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. Higher CE means greater hardenability and hydrogen-cracking risk, so preheat and low-hydrogen practice become more important.
- Stainless steel needs low heat input and clean, dedicated tools to avoid sensitisation and contamination. Do not use carbon-steel brushes or grinding wheels.
- Aluminium has a tough oxide layer and high thermal conductivity. Clean with a stainless wire brush and use the correct current and shielding gas.
- Moisture, oil, paint and rust can introduce hydrogen or porosity.
Worked example
A steel has C = 0.18%, Mn = 1.20%, and no significant Cr, Mo, V, Ni or Cu. CE = 0.18 + 1.20/6 = 0.38. This is below 0.40, so hardenability is moderate, but a thick section or high restraint can still require preheat as stated in the WPS. The reasoning is that CE estimates alloy effect, not section thickness alone.
Traps that cost marks
- Assuming low carbon always means no preheat; thickness and restraint matter.
- Calculating CE and forgetting Mn/6.
- Using a carbon-steel grinding disc on stainless steel.
- Ignoring aluminium oxide before welding.