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.
Related TIG checks include why TIG tungsten turns black, TIG porosity troubleshooting, sooty TIG weld gas coverage fixes, and TIG cup size and gas lens selection.
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.
