A Butt Joint Set Up With a Kinked Liner and a Worn Contact Tip
A welder sets up a butt joint in 6 mm mild-steel plate using 1.2 mm ER70S-6 wire and 75% Ar/25% CO₂. The machine is set to 21 V, 170 A, wire feed 6.5 m/min, gas flow 12 L/min measured at the regulator, stick-out about 18 mm. The plates were stored outdoors; the joint faces show light rust and a film of cutting oil. Fit-up is square with a 1 mm root gap and two tacks. The wire spool brake drags, the liner has a kink near the feeder, and the contact tip bore is worn to about 1.8 mm. The first weld shows intermittent crackling, scattered porosity, and lack of fusion at the root. The welder blames the gas cylinder and wants to increase gas flow to 25 L/min.
1. Rank the likely causes of the poor weld and explain which evidence supports each.
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Rank by evidence:
- Wire-feed instability - highest. The dragging spool brake, kinked liner and worn 1.8 mm contact tip all point to erratic wire speed and poor current transfer. Crackling and porosity follow when the wire stutters or arcs inside the tip.
- Worn contact tip. A 1.2 mm wire in a 1.8 mm bore loses electrical contact, so current fluctuates, the arc wanders and fusion becomes unreliable.
- Contamination. Rust and cutting oil on the joint faces generate hydrogen and gas porosity; oil also causes smoke and an unstable arc. Cleanliness is a WPS requirement, not optional.
- Gas delivery. 12 L/min at the regulator is not the same as flow at the nozzle. A leak, restriction or draft can reduce actual shielding. Raising to 25 L/min can create turbulence and entrain air - it is not automatically a fix.
- Parameters/stick-out. 21 V/170 A with 1.2 mm wire is near the short-circuit/globular transition and can be unstable; 18 mm stick-out is long and reduces effective current and shielding.
The cylinder is low on the list unless a leak test or contents check proves otherwise.
2. State the corrective setup sequence before the next weld, including specific checks and values.
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Correct in this order:
- Wire path: replace the kinked liner, blow it out with dry air, fit the correct 1.2 mm contact tip (bore about 1.2–1.4 mm), set the spool brake so the wire cannot overrun but the drive rolls do not slip, and set drive-roll tension correctly.
- Gas: leak-test from regulator to torch with soap solution; set flow at the nozzle with a flow tester to the WPS value (commonly 12–18 L/min for 75/25, but follow the WPS); remove drafts or add a shield.
- Material: degrease before grinding - oil first, then mechanical cleaning - and clean at least 25 mm each side to bright metal; do not smear oil into the joint.
- Fit-up: maintain the specified root gap; re-tack if the gap has closed.
- Parameters: set voltage and WFS from the WPS, not by guesswork. For 6 mm butt with 1.2 mm wire, a short-arc setting may be around 18–20 V and a spray setting around 26–28 V; 21 V is a transition point. Set stick-out at 10–15 mm.
- Proof: run a test bead on scrap and check penetration, bead shape and porosity before welding the joint.
3. The welder insists that more gas flow will solve the porosity. Explain why 25 L/min can make it worse and how you would decide whether the cylinder is actually at fault.
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Increasing gas flow is not a cure and can backfire. Above the recommended range the gas leaves the nozzle as a turbulent jet; turbulence draws surrounding air into the shielding envelope, so porosity can increase even though the regulator gauge shows more flow. High flow also wastes gas and can chill the weld or disturb the arc. The real question is whether the gas reaching the arc is clean and continuous: check for a leaking hose, cracked torch body, blocked diffuser, loose nozzle or draft. Measure flow at the nozzle with a flow tester, not by reading the regulator. If flow is already correct at the nozzle, the porosity is coming from contamination, wire feed or arc instability. The cylinder is only suspect after a leak test and contents check; changing flow without diagnosis just moves the problem.