Practical Welding and Process Operation
This module tests whether you can set, hold and justify process parameters, and diagnose defects from the way the arc behaves.
The parameter set
Current controls penetration and deposition; voltage controls arc length and bead width; travel speed controls bead section and heat input; stick-out and torch angle control penetration and gas coverage; gas flow rate must be enough to shield without turbulence.
Heat input
Q = k · U · I · 60 / (1000 · v) kJ/mm, with v in mm/min and k the thermal efficiency: about 0.8 for MMAW and MAG, about 0.6 for TIG, higher for SAW. Heat input governs cooling rate, HAZ hardness and distortion.
Worked example
MAG welding at 200 A, 24 V, 250 mm/min, k = 0.8: Q = 0.8 × 24 × 200 × 60 / (1000 × 250) = 230 400 / 250 000 ≈ 0.92 kJ/mm. If the WPS allows a maximum of 1.5 kJ/mm, the run is acceptable at that speed; slowing to 150 mm/min would give 1.54 kJ/mm and exceed the WPS, requiring a procedure review rather than an on-the-floor decision.
Technique and defect diagnosis
- Porosity: damp electrodes, loss of gas coverage, draughts, dirty plate, excessive flow causing turbulence.
- Lack of fusion: current too low, travel too fast, wrong electrode angle, bead too convex.
- Undercut: current or voltage too high, travel too fast, poor manipulation at the toe.
- Root defects: wrong root gap or land, insufficient purge on stainless, keyhole collapse.
Traps
- Omitting k, or mixing mm/min with mm/s in the heat input formula.
- Forgetting that a double-sided weld deposits heat on both sides and may need the heat input checked per pass, not per joint.
- Adjusting parameters outside the WPS without notifying welding coordination.