GTAW NC II common mistakes
GTAW NC II · Understand the traps and how to avoid them.
Setting amperage by copying the machine's front-panel sticker or another welder's setting instead of calculating from material thickness and joint type, then never adjusting once the arc starts.
Candidates memorize a single 'number for 6 mm steel' and treat it as universal, ignoring that amperage depends on thickness, position, joint configuration, travel speed, and whether filler is added. They also fear changing parameters mid-pass because they think the examiner will mark it as indecision.
What to do instead
Before striking an arc, state aloud the reasoning chain: thickness, current range (roughly 30–40 A per mm for carbon steel GTAW is a starting reference, not a rule), electrode diameter, gas flow, and position. Then treat the first 20 mm as a trial: observe the puddle width and bead profile and adjust current in 5–10 A steps. Practice writing three candidate parameter sets for one joint and justify each.
Reading a welding symbol on the drawing and welding on the arrow side by default, without checking which side of the reference line the symbol sits on or whether the tail carries a process specification.
Candidates learn 'arrow side = near side' but then stop reading the symbol as a whole. They skip the tail, ignore supplementary symbols (backing, spacer, consumable insert), and miss multiple reference lines showing a root pass and fill/cap sequence with different processes.
What to do instead
Adopt a fixed decoding order every time: identify the joint and arrow side, note symbol position relative to the reference line, read size and length, read the tail for process/spec, then read any second reference line. Sketch the symbol and annotate each element in words before touching the workpiece. Do ten symbols from a symbol chart and translate each into a one-sentence instruction.
Cleaning stainless steel with the same wire brush, grinding disc, or bench area used for carbon steel, and using a disc that has previously cut carbon steel.
Candidates see 'clean the joint' as a box to tick and grab whatever is nearest. They do not connect carbon-steel contamination to loss of corrosion resistance, and many have never been shown the colour of a contaminated stainless weld.
What to do instead
Establish a dedicated stainless kit: separate stainless-only wire brush, separate grinding discs reserved for stainless, separate gloves or at least clean ones, and a covered clean surface. Demonstrate the rule by explaining that embedded iron particles become corrosion initiation sites. State the contamination-control step aloud during the practical so the assessor sees the intent, not just the result.
Welding aluminium with the same AC balance, frequency, and cleaning action used for a previous job, or setting the machine to DCEN because 'that is what works on steel'.
Aluminium's oxide layer and high thermal conductivity require the cleaning half-cycle and higher current, but candidates treat the AC settings as decoration. They also fail to re-check settings after another candidate has used the machine.
What to do instead
Before each aluminium task, verify and state: AC mode, balance set toward more cleaning action for heavily oxidised or cast material, frequency appropriate to the joint, current increased relative to the equivalent steel thickness, and a fresh tungsten tip (balled or truncated depending on the machine's capability). Wipe the surface and remove oxide mechanically, then weld within a short window before re-oxidation.
Filling a pipe root by 'chasing' a keyhole that has already opened too wide, adding large dabs of filler to close it, and continuing without stopping.
Candidates fear leaving an incomplete pass and believe more filler fixes everything. They also lack the habit of watching the keyhole size as the primary feedback signal, so they react after the puddle has already dropped.
What to do instead
Drill the response sequence: if the keyhole grows beyond the intended size, reduce current slightly, increase travel speed, add smaller and more frequent filler dabs, and if the puddle still sags, stop, let it cool, and grind back to sound metal before restarting. Practise on scrap pipe with deliberate keyhole growth so the corrective action becomes automatic.
Leaving the gas pre-flow and post-flow at the machine's default, or removing the torch immediately after the arc extinguishes to inspect the bead.
Post-flow feels like wasted time to a candidate being timed, and the tungsten discolouration it prevents is invisible until the next arc is struck. Many candidates have never been told what a grey or blue tungsten means.
What to do instead
Set post-flow proportional to current and joint thickness (several seconds minimum on stainless and aluminium) and use the wait to observe the crater. Treat tungsten colour as a diagnostic: a shiny tip means gas coverage was adequate, discolouration means flow, post-flow, or draught control failed. Record the post-flow setting on the parameter sheet before welding so it is deliberate.
Reporting a weld as complete and acceptable without performing or recording the specified inspection steps, and describing defects in vague terms such as 'looks a bit rough'.
Candidates conflate 'I finished the pass' with 'the weld passed'. They have not practised naming defects precisely, so they avoid committing to a judgement, and they treat documentation as paperwork that happens after the real work.
What to do instead
Use a fixed inspection routine: visual examination of the cap and, where accessible, the root; check for undercut, porosity, incomplete fusion, crater cracks, and oxidation colour; measure leg length or reinforcement where specified. Name each defect using its standard term and its location. Practise writing one defect report per practice piece with defect type, location, likely cause, and corrective action.
Filling in the welding procedure or inspection record from memory at the end of the session, copying parameter values from the procedure sheet rather than recording what the machine actually displayed.
Candidates see the record as a formality and assume 'the procedure says 90 A, so I welded at 90 A'. They also do not realise that traceability is the purpose of the record, and that an inaccurate record is worse than none.
What to do instead
Record as you go: note actual current, voltage, travel speed, gas flow, filler classification, and interpass temperature at the time of welding, then transfer to the final record. Where a value deviates from the procedure, record the deviation and the reason. Treat the record as evidence that can be audited, and check that heat/lot numbers and welder identification are legible.
Starting work in a confined space or at height without confirming the permit, gas-cylinder restraint, fume extraction, fire watch, and PPE for the specific task.
Candidates treat OHS as a written test topic separate from the practical, so they skip the pre-task check under time pressure. They also assume someone else has already set up the area, and they underestimate fume and fire risk from TIG on stainless and aluminium.
What to do instead
Run a short verbal pre-start checklist every time: permit or authorisation, ventilation or extraction, cylinders secured upright and hoses inspected, fire extinguisher and fire watch in place, screens to protect others from arc radiation, correct shade lens, leathers, and respiratory protection where fume warrants it. Practise the checklist until it takes under a minute, and state it aloud so it is observable.
Preparing the joint by eye - bevelling and fit-up without checking bevel angle, root gap, root face, and alignment against the drawing, then discovering the gap is wrong only when the root pass will not penetrate.
Candidates rush preparation to spend time on the 'real' welding, and they trust visual judgement over measurement. They also do not link fit-up errors to specific defects such as incomplete penetration or burn-through.
What to do instead
Measure before welding: bevel angle with a gauge, root gap and root face with a rule or feeler gauge, misalignment with a straight edge, and tack placement so tacks do not sit in the root path. Record the measured values against the drawing tolerance. If the fit-up is out of tolerance, correct it before welding rather than compensating with current.