Welding Processes and Equipment
This domain tests process selection and safe equipment use: SMAW, MIG/MAG, TIG, FCAW and oxy-fuel welding. You should know polarity, shielding, duty cycle and typical applications.
Core rules
- SMAW uses a covered electrode and constant-current power source. DCEP gives deeper penetration on many electrodes; DCEN is used for some special electrodes. Electrode storage matters for low-hydrogen types.
- MIG/MAG uses a continuous wire and constant-voltage characteristic. MAG uses reactive gas or mixed gas; MIG uses inert gas. Gas flow must be set with a flowmeter, not guessed.
- TIG uses a non-consumable tungsten electrode and separate filler. DCEN is usual for steel; AC is used for aluminium to break the oxide.
- Duty cycle is based on a 10-minute period. A 60% duty cycle allows 6 minutes of welding at rated current.
Worked example
A machine is rated 200 A at 60% duty cycle. At 250 A, the allowable duty cycle is approximately 60 × (200/250)² = 38.4%. In a 10-minute period, that is about 3.8 minutes of welding. The reasoning is that heating rises roughly with current squared, so exceeding rated current must reduce on-time.
Traps that cost marks
- Treating duty cycle as continuous welding time.
- Using the wrong polarity for a process or electrode.
- Setting gas flow by sound rather than a flowmeter.
- Using damaged cables or an uninsulated holder.