What does TIG post-flow do?

TIG post-flow keeps shielding gas flowing after the arc stops so the hot tungsten, weld crater, and filler end can cool without oxygen contamination.

How do I know if TIG post-flow is too short?

Post-flow is too short if the tungsten turns black, blue, gray, or crusty after the weld, or if arc restarts become rough because the tungsten oxidized during cooling.

What is a good TIG post-flow starting point?

A common starting point is about 1 second of post-flow per 10 amps of welding current, then adjust based on tungsten color, torch heat, cup size, material, and gas coverage.

Tag: black tungsten

TIG Post-Flow Setting Troubleshooting: Black Tungsten, Porosity, Gas Waste, and Torch Cooling

TIG post-flow is the shielding gas that keeps flowing after the arc stops. If it is too short, the hot tungsten and cooling weld crater are exposed to air, causing black, blue, gray, or crusty tungsten, rough restarts, porosity, and contaminated weld starts. If post-flow is too long, weld quality may be fine, but argon usage goes up fast during tack welding or short beads.

Start by watching the tungsten after arc stop. If the tungsten is still glowing when argon shuts off, increase post-flow. If the tungsten stays clean but gas keeps flowing long after the torch cools, reduce post-flow in small steps. Do not fix black tungsten by only increasing flow rate; a cracked cup, leaking back cap O-ring, clogged gas lens, or loose torch fitting can still expose the electrode to oxygen.

Common Symptoms

Symptom	Likely Post-Flow Issue	First Check
Tungsten turns black after weld	Post-flow too short or gas leak	Increase post-flow and inspect gas path
Tungsten turns blue or gray	Hot tungsten exposed during cooling	Watch whether gas stops before glow is gone
Rough arc restart	Oxidized tungsten from previous stop	Regrind tungsten and extend post-flow
Porosity at crater or restart	Weld pool loses shielding while cooling	Hold torch over crater during post-flow
Argon bottle empties quickly	Post-flow too long for short welds	Reduce time gradually after tungsten stays clean

What Post-Flow Does

Post-flow protects three hot areas after the arc shuts off: the tungsten, the weld crater, and the end of the filler rod if it remains inside the gas envelope. Tungsten can oxidize after the bead looks finished because the electrode remains hot longer than many operators expect. The goal is enough shielding to let the tungsten cool without discoloration, not maximum gas flow for every weld.

Starting Point for Post-Flow

A common field rule is about 1 second of post-flow per 10 amps of welding current. Some Miller GTAW guidance also lists 10–15 seconds as a corrective range when inadequate post-flow is causing tungsten or arc problems. Use those as starting points, then tune by tungsten color, material, torch heat, tungsten size, and weld length.

Welding Current	Common Starting Range	What To Watch
50 amps	5 seconds	Tungsten should not color after gas stops
80 amps	8 seconds	Good range for many light TIG jobs
120 amps	12 seconds	Check torch heat and tungsten color
150 amps	15 seconds	Often needs longer protection on hot torch setups
200 amps	20 seconds	Verify torch rating and cooling; gas use increases quickly

Inspection Steps

Confirm the gas. Most TIG work uses 100% argon. Do not use MIG gas with CO2 or oxygen for TIG.
Watch tungsten color. Black, gray, blue, or crusted tungsten after arc stop points to oxygen exposure, contamination, or too little post-flow.
Hold the torch still. Keep the cup over the crater until post-flow ends. Moving away early defeats the setting.
Check flow at the cup. A regulator reading does not prove gas is reaching the tungsten.
Inspect the cup. Replace cracked, chipped, loose, or overheated cups.
Inspect the gas lens or collet body. Blocked screens or damaged gas passages can cause poor coverage even with long post-flow.
Check the back cap O-ring. A damaged O-ring can pull air into the torch and oxidize tungsten.
Check hoses and fittings. Use approved leak-check methods and repair leaks before welding.
Adjust gradually. Add or subtract a few seconds at a time, then retest on clean material.

Post-Flow Too Short vs Too Long

Condition	Result	Corrective Action
Too short	Black tungsten, rough restarts, crater oxidation	Increase time and hold torch over weld
Too long	High argon consumption with no quality gain	Reduce time after tungsten remains clean
Correct time but black tungsten	Leak, cracked cup, bad O-ring, dirty gas lens	Inspect torch and gas path
Correct time but porosity	Draft, contamination, wrong cup, no purge	Check shielding coverage and base-metal prep

Field Fix vs Proper Fix

Problem	Field Fix	Proper Fix
Tungsten blackens after stop	Add post-flow time	Set time by amps and repair leaks or worn torch parts
Gas wastes during tacks	Lower post-flow slightly	Use a repeatable tack schedule that still protects tungsten
Crater porosity	Hold torch over crater longer	Correct post-flow, torch angle, cup size, and cleanliness
Blue tungsten on aluminum	Add post-flow	Check AC heat, torch cooling, gas lens, and cup size
Soot remains after increasing post-flow	Clean cup and tungsten	Fix gas coverage, contaminated material, or wrong gas

Common Wrong-Part Mistakes

Replacing tungsten repeatedly while ignoring a leaking back cap O-ring.
Using a cracked cup and trying to compensate with longer post-flow.
Installing gas lens parts that do not match the torch series or cup setup.
Using a collet that does not match tungsten diameter, causing poor alignment and overheating.
Turning gas flow too high and creating turbulence instead of fixing post-flow time.

Compatibility Notes

Post-flow is a machine setting, but the correct result depends on torch family, cup size, gas lens or standard collet body, tungsten diameter, amperage, material, and torch cooling. Consumables for WP-9/20-style torches and WP-17/18/26-style torches are not automatically interchangeable. Verify torch series and tungsten diameter before replacing cups, collets, gas lenses, or back caps.

Related Failure Paths

Black tungsten from oxygen exposure after arc stop.
Rough arc starts from oxidized tungsten.
TIG porosity at crater or restart.
Sooty TIG welds caused by poor gas coverage.
Cracked cups or clogged gas lenses mistaken for bad post-flow.
High argon use from excessive post-flow during tack welding.

Safety Notes

Let tungsten, cups, and torch parts cool before handling.
Secure argon cylinders upright and protect regulators from impact.
Argon can displace oxygen in confined areas; use ventilation and confined-space controls where required.
Use eye protection when grinding tungsten.
Do not weld through suspected gas leaks or damaged hoses.

Sources Checked

Weld Support Parts TIG tungsten discoloration support page.
Weld Support Parts TIG porosity and soot troubleshooting pages.
Weld Support Parts TIG cup size and gas lens support page.
CK Worldwide TIG troubleshooting and gas shielding guidance.
Miller GTAW troubleshooting guidance.

May 18, 2026

Lincoln Square Wave 205 TIG Tungsten Contamination Troubleshooting

If the tungsten on a Lincoln Square Wave 205 turns black, balls unevenly, grows a dirty tip, spits into the puddle, or makes the TIG arc wander, stop and correct contamination before continuing. Tungsten contamination usually comes from dipping the electrode, touching filler metal, poor argon shielding, too little post-flow, a cracked cup, a leaking torch connection, dirty base metal, or the wrong tungsten size/prep for the amperage.

The Square Wave 205 is an AC/DC TIG and Stick machine with pulse, AC frequency, AC balance, and post-flow control. Those controls help, but they do not fix a contaminated electrode. If the tungsten is dirty, cut or grind back to clean material, correct the shielding or torch issue, then restart the weld.

Common Symptoms

Black tungsten: Hot tungsten is being exposed to oxygen, contamination, or poor post-flow.
Green/gray dusty tip: Oxidation, gas coverage loss, or contaminated argon path.
Arc wandering: Dipped tungsten, poor grind direction, oversized tungsten, or bad work return.
Arc splits or flutters: Dirty tungsten, wrong diameter for amperage, or damaged cup/collet setup.
Metal sticks to tungsten: Electrode touched the puddle or filler wire.
Aluminum puddle gets dirty fast: Oxide, wrong AC balance, poor cleaning, or weak gas shielding.
Tungsten keeps overheating: Amperage too high for tungsten size, too little stickout control, or inadequate torch cooling.

What Tungsten Contamination Means

TIG welding uses a non-consumable tungsten electrode to carry the arc while argon shielding protects the tungsten and weld puddle. When the tungsten touches molten metal, filler wire, oil, oxide, or air while hot, it becomes contaminated. Once contaminated, the arc becomes unstable and can transfer contamination into the weld.

Square Wave 205 Compatibility Notes

The Lincoln Square Wave 205 is sold as an AC/DC TIG and Stick welder with adjustable AC frequency, AC balance, pulse, and post-flow features. Lincoln literature describes AC frequency control for bead width and AC balance for cleaning/penetration control on aluminum. Use those settings after the torch, tungsten, gas, and work preparation are correct.

For machine-family context, see the Lincoln Electric Square Wave 205 overview. For related TIG support, see why TIG tungsten turns black, unstable TIG arc from poor tungsten prep, TIG torch support, and TIG collet support.

Fast Checks Before Regrinding Again

Confirm 100% argon for TIG welding.
Check that the cylinder is not empty and the flowmeter is stable.
Inspect the cup for cracks, chips, or spatter.
Inspect the collet and collet body for poor grip, heat damage, or gas leakage.
Check the back cap O-ring and torch head connection.
Clean the base metal and filler rod before welding.
Set enough post-flow to keep the tungsten shielded until it cools.
Cut off dipped tungsten instead of grinding only the surface stain.

Diagnosis Table

Symptom	Likely Cause	First Check
Tungsten turns black after stopping	Post-flow too short or torch leak	Increase post-flow and inspect back cap/cup
Tungsten balls unevenly on AC	Wrong prep, too much heat, contamination	Regrind/cut back and verify tungsten size
Arc wanders	Dipped tungsten or poor grind direction	Grind lengthwise on a dedicated wheel
Tip melts back	Too much amperage for tungsten size	Increase tungsten diameter or reduce current
Puddle gets gray/dirty	Gas coverage loss or dirty material	Check cup, flow, stickout, and cleaning
Contamination repeats immediately	Leaking torch or contaminated gas path	Check torch seals, hose, regulator, and fittings

What Wears Out First

The tungsten is the visible failure, but the cause is often the torch front end. A worn collet may not grip the electrode. A damaged collet body or gas lens can disrupt shielding. A cracked alumina cup can pull air into the gas envelope. A dried or missing back-cap O-ring can leak shielding gas before it reaches the cup.

AC Aluminum Contamination Checks

On aluminum, clean the oxide layer and remove oil before welding. If the Square Wave 205 AC balance is set for too much penetration and not enough cleaning, the puddle may look dirty even with good tungsten prep. If AC balance is set for excessive cleaning, the tungsten may run hotter. Start from a conservative setup, verify clean argon coverage, and adjust balance only after contamination sources are controlled.

DC Steel and Stainless Contamination Checks

For DC TIG on steel or stainless, tungsten contamination is commonly caused by dipping the puddle, touching filler wire to the electrode, grinding tungsten on a dirty wheel, using too long of an arc, or welding over oil, mill scale, paint, or solvent residue. Keep filler wire out of the arc cone until it enters the leading edge of the puddle.

Common Wrong-Setup Mistakes

Regrinding the tungsten without fixing gas coverage.
Using a cracked cup or worn collet body.
Letting post-flow stop while the tungsten is still hot.
Grinding tungsten across the electrode instead of lengthwise.
Using the same grinding wheel for tungsten and dirty steel.
Running too much stickout without a gas lens or larger cup.
Trying to weld aluminum without removing oxide and oil first.
Assuming AC balance will fix dirty base metal or a gas leak.

Test Procedure

Remove the tungsten and cut off any dipped or balled contaminated end.
Grind a fresh point lengthwise on a clean, dedicated wheel.
Install the tungsten in a matching collet and verify it does not slip.
Install a clean cup or gas lens setup that matches the torch series.
Set argon flow and post-flow for the cup size and amperage.
Run a bead on clean scrap without filler. Watch whether the tungsten stays clean.
Add clean filler rod and repeat the test.
If contamination returns without dipping, isolate gas leaks and torch consumables.

Field Fix vs Proper Fix

Field fix: Cut back the contaminated tungsten, regrind lengthwise, clean the cup, increase post-flow slightly, and test on clean scrap.

Proper fix: Replace worn collets, damaged collet bodies, cracked cups, bad O-rings, leaking hoses, or contaminated tungsten. Then document the tungsten size, cup size, argon flow, AC balance, AC frequency, amperage, and post-flow that keep the tungsten clean.

Safety Notes

Disconnect power before torch service.
Use eye and respiratory protection when grinding tungsten.
Do not grind radioactive thoriated tungsten without proper dust control and shop policy approval.
Keep solvent, oil, and unknown coatings away from TIG welding heat.
Use ventilation and keep your head out of fumes.

May 18, 2026