Welding processes and equipment
This domain tests whether you can match a process to a joint, set its main variables, and explain the equipment limits behind the choice. Expect questions on power-source characteristics, metal transfer, duty cycle and the process variables that control penetration, dilution and deposition rate.
Process families and characteristics
- 111 SMAW: drooping constant-current source, open-circuit voltage typically 50–90 V, arc voltage 20–30 V; basic-coated electrodes need baking and holding ovens and give low hydrogen.
- 135 GMAW: flat characteristic gives a self-regulating arc; short-circuit, globular, spray and pulsed transfer. Argon-rich mixtures favour spray; CO2 gives good penetration but more spatter.
- 141 GTAW: constant current; DCEN for steels and most metals, AC for aluminium to break the oxide; HF or lift-arc ignition.
- 121 SAW: granular flux, high current, deep penetration, very high deposition; unsuitable for thin sheet and out-of-position work.
- 136 FCAW: flux-cored wire, gas-shielded or self-shielded; high deposition in flat and horizontal positions.
Duty cycle
Duty cycle is declared over a 10-minute period at a rated current, for example 60% at 350 A. Heating scales with current squared, so the permitted duty at another current is D2 = D1 x (I1/I2) squared.
Worked example
A 350 A, 60% set is to be used at 300 A. D = 0.60 x (350/300) squared = 0.60 x 1.361 = 0.82, so about 82% duty, or 8.2 minutes of arcing in each 10 minutes. The reasoning: lower current gives lower I squared R heating, so more on-time is permissible; the 60% figure cannot simply be transferred to a different current.
Traps that cost marks
- Quoting the rated duty cycle without recalculating for the actual current.
- Mixing polarity: DCEN in GTAW puts more heat into the work; DCEP puts it into the electrode.
- Assuming a constant-voltage GMAW set behaves like a drooping SMAW set; the static characteristics differ.
- Forgetting that SAW needs a flux layer and is position-limited.