Welding processes and equipment
The Welding processes and equipment domain checks whether you can select a process for a given joint and then set it up correctly - power-source characteristic, polarity, current, duty cycle and consumable handling. Expect questions that compare two processes on the same job rather than ask for a definition.
Process numbers and characteristics
- 111 (MMA/SMAW): drooping constant-current source, open-circuit voltage roughly 50–90 V, DCEN, DCEP or AC. Basic low-hydrogen electrodes need re-drying at about 300–350 °C and holding at 100–150 °C.
- 121 (SAW): constant-voltage source, 400–1000 A, granular flux; draughts destroy shielding and the arc is invisible.
- 131/135 (MIG/MAG): constant-voltage, wire-feed speed sets current; 135 uses CO2 or Ar/CO2 and transfers in short-circuit, globular or spray mode.
- 136 (FCAW): tubular wire, gas-shielded or self-shielded; high deposition, tolerant of site conditions when self-shielded.
- 141 (TIG): constant-current, DCEN for steel, AC for aluminium, separate filler.
- 311 (oxy-acetylene): neutral flame around 3100 °C; used for thin sheet, brazing and cutting rather than thick structural work.
Duty cycle
A set rated 300 A at 60% can deliver more at a lower duty and less continuously: current scales with the square root of the duty ratio, not linearly.
Worked example
A 300 A/60% machine must run continuously. I²t is constant, so I = 300 × √(60/100) = 232 A. At 80% duty: 300 × √(60/80) = 260 A. The reasoning is thermal: the permitted current falls as the arc-on fraction rises.
Traps that cost marks
Mixing constant-current and constant-voltage sources; claiming a high OCV is safer when it is more dangerous; comparing deposition rates without gas flow and stick-out; treating duty cycle as a simple linear ratio.