Burn-Through, Porosity and a Coiled Extension Reel
A Level-2 trainee must butt-weld 3 mm thick mild steel sheet, 300 mm long, using a 200 A single-phase inverter arc welder rated for a 230 V supply. The nearest socket is 25 m away, so he uses a 25 m, 1.5 mm² extension cable still wound on its reel, plugging into a 6 A socket through a multi-plug adaptor. He fits a 3.15 mm E6013 electrode, sets 90 A, and finds the electrode sticking and the arc wandering. Halfway along the seam the sheet edges open up and he burns through, so he turns the current up to 120 A to 'get penetration'. The finished joint shows porosity, undercut and a 4 mm bow along its length.
1. Explain, with numbers, why the extension arrangement is a serious electrical and welding problem, and specify what should replace it.
Read the worked answer
Voltage drop: 25 m of 1.5 mm² copper has resistance R = ρL/A = 0.0175 × 25 ÷ 1.5 ≈ 0.29 Ω per conductor, so about 0.58 Ω in the loop. At a welding current of 90 A that is roughly 52 V dropped in the lead - more than the arc voltage itself. The inverter cannot maintain its arc characteristic, so the arc wanders, the electrode sticks, and penetration becomes erratic.
Rating: a 200 A inverter needs a supply and cable sized for its input current (typically 25–32 A at 230 V for a 200 A set); a 6 A socket through a multi-plug adaptor is overloaded from the first arc.
Coiled reel: a cable carrying current while wound cannot dissipate heat, so it derates heavily - insulation softens, conductors short, and the reel becomes a fire risk.
Correct replacement: take the machine to within 5–10 m of the work, or use a fully unwound 4 mm² cable of the shortest practical length on a proper industrial plug and dedicated circuit. Check the voltage at the machine terminals under load; a drop beyond about 10 % of 230 V means the supply is inadequate.
2. Recommend the electrode size, current and joint preparation for 3 mm sheet, and justify each choice.
Read the worked answer
Electrode: 2.5 mm E6013 is the right general-purpose choice for 3 mm sheet. The rule of thumb is roughly 30–40 A per mm of electrode diameter, so 2.5 mm runs around 70–90 A; 3.15 mm at 90–120 A puts too much heat into a 3 mm edge and is the direct cause of the burn-through.
Current: set about 70–85 A for a 2.5 mm electrode on 3 mm sheet, and hold it constant along the seam rather than chasing penetration with the current knob.
Preparation and technique:
- Square butt with a 1.5–2 mm root gap and a 1 mm land; too wide a gap at 3 mm thickness is what opens up under heat.
- Tack every 50 mm before welding, and clamp to a flat backing bar.
- Use a copper backing bar or a removable ceramic/steel backing to support the root and stop burn-through.
- Maintain a short arc, about 2–3 mm, with a 60–70° work angle, and a steady travel speed; a slight weave is better than a fast straight run on thin sheet.
Why not just raise the current: more current increases heat input, widens the HAZ and shrinkage, and makes burn-through and bowing worse, not better.
3. The seam finishes with a 4 mm bow and porosity. Diagnose both and give the correction for the next plate.
Read the worked answer
Porosity comes from gas trapped in the weld: a long, unstable arc from the voltage-starved supply, a damp or damaged E6013 coating (cellulosic/rutile coatings pick up moisture), dirty or oily sheet edges, and no back purging on a gapped joint. Correction: dry the electrodes (E6013 is generally used as supplied, so discard damp stock), clean the edges to bare metal for 20 mm each side, run a short arc at the correct current, and never weld over mill scale and oil.
The 4 mm bow is shrinkage from unbalanced, excessive heat input - 120 A, a 3.15 mm electrode, and welding continuously from one end. Correction:
- Back-step welding from the middle outwards, or a balanced sequence alternating sides of the joint.
- Tack and clamp to a jig or backing bar so the shrinkage fights the restraint, not the plate.
- Keep the current at 70–85 A with a fast, even travel; let the plate cool between runs; skip-weld long seams.
- After welding, check straightness on a surface plate; correct small bows by controlled local heating, not by force.
Finally, record the parameters on a small welding procedure note so the next plate starts from known values rather than guesswork.