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TIG preflow and postflow are small settings with a large effect on weld quality. Preflow protects the tungsten and weld area before the arc starts. Postflow keeps shielding gas over the hot tungsten and weld pool after the arc stops. The correct times depend mainly on tungsten size, torch size, amperage, gas flow, and how long the weld remains hot—not simply on the diameter of the torch body.
What Preflow and Postflow Do
Preflow starts the argon before the arc is established. This clears oxygen and nitrogen from the cup and creates a protective envelope around the tungsten. Without enough preflow, the first part of the weld can turn gray, porous, or contaminated. The tungsten may also develop a dirty or irregular tip.
Postflow keeps argon flowing after you release the torch trigger or foot pedal. It protects the cooling tungsten from oxidation and shields the final section of the weld while it is still hot. If postflow is too short, the tungsten can discolor blue, purple, or gray. Repeated overheating can cause the tip to crack, ball excessively, or shed contamination into the weld.
These settings are separate from gas flow rate. Gas flow is normally adjusted in cubic feet per hour (CFH), while preflow and postflow are measured in seconds. A larger torch often needs more gas flow, but it does not automatically require dramatically longer preflow or postflow.
Starting Settings by Torch Size
The table below gives practical starting points for air-cooled and water-cooled torches. Adjust from these numbers after observing the tungsten and the start and finish of the weld.
| Torch type and typical use | Argon flow | Preflow | Postflow |
|---|---|---|---|
| Small air-cooled torch, 80–125 amps | 12–18 CFH | 0.3–0.5 seconds | 5–7 seconds |
| Medium air-cooled torch, 125–180 amps | 15–22 CFH | 0.5–0.8 seconds | 6–10 seconds |
| Large air-cooled torch, 180–250 amps | 18–25 CFH | 0.7–1.0 seconds | 8–12 seconds |
| Water-cooled torch, 250 amps and above | 20–30 CFH | 0.8–1.5 seconds | 10–15 seconds |
These are starting values for a normal gas lens or standard collet body in a draft-free shop. A small torch welding 1/8-inch stainless steel may use less postflow than a large torch welding thick aluminum at high amperage. The workpiece and tungsten heat matter as much as the torch rating.
How to Set Preflow Correctly
Start with about 0.5 seconds on a small or medium torch. Hold the torch in position, initiate the arc, and inspect the start of the bead. A clean start with no black specks, pinholes, or gray halo usually means the setting is adequate.
Increase preflow to about 0.8 or 1 second when using a large cup, a gas lens, long torch leads, high gas flow, or a remote torch valve positioned far from the machine. Some systems have a noticeable delay before gas reaches the cup. Long hoses and restrictive fittings can make the programmed time less effective.
Do not solve every gas problem by adding preflow. If the start is still contaminated after two seconds, check for an empty or contaminated cylinder, leaks, a clogged cup, a loose gas fitting, or a poor torch angle. Excessive preflow wastes argon and can slow production without improving the weld.
For small sheet-metal work, a cheaper TIG gas flow meter can be useful for verifying actual flow at the torch. The regulator gauge alone may not show what is happening at the cup.
How to Set Postflow Correctly
A common starting rule is one second of postflow for every 10 amps of welding current, with practical limits of roughly 5 to 15 seconds. For example, a 120-amp weld may need 8–10 seconds, while a 200-amp weld may need 10–12 seconds. This rule is not exact, but it is a useful way to avoid setting postflow too low.
Watch the tungsten immediately after the arc stops. It should remain covered by gas until it is no longer visibly glowing. If the tip changes color or develops a rough surface, add two or three seconds. If the tungsten stays clean but the gas cylinder is being consumed quickly, reduce postflow one second at a time.
A large cup does not necessarily require more postflow. Cup size affects gas coverage during welding; tungsten diameter and amperage affect how much cooling protection is needed afterward. A 1/16-inch tungsten in a small torch generally needs less postflow than a 1/8-inch tungsten driven near its rated current, even if both use the same ceramic cup.
Torch Size, Tungsten, and Gas Coverage
Choose the torch based on amperage and duty cycle, not just convenience. A small air-cooled torch is lighter and cheaper, and it is a good choice for thin steel, stainless, and occasional aluminum. It becomes uncomfortable or overheated when operated continuously near its maximum rating.
A water-cooled torch costs more and requires a cooler, but it stays comfortable during long, high-current welds. It is worthwhile for production aluminum, heavy stainless, or repeated welds above roughly 180–200 amps. For occasional repair work, the cheaper air-cooled option is often sufficient if you stay within its duty cycle.
Use a properly sized TIG gas lens kit when you need a smoother, wider shielding envelope or a longer tungsten stickout. A gas lens may allow lower flow than a conventional collet body, but drafts can still disturb the shield. More flow is not always better: excessive flow can create turbulence and draw surrounding air into the arc.
Troubleshooting Common Flow Problems
Black or gray starts usually point to insufficient preflow, poor gas coverage, contamination, or a dirty joint. A discolored tungsten after shutdown usually points to insufficient postflow, an oversized electrode for the job, or inadequate gas reaching the torch.
A wandering arc and a crater-like finish can result from turbulent gas. Reduce flow slightly, shorten the tungsten stickout, and keep the cup close to the work. Weld in a sheltered area; even a shop fan or open garage door can overwhelm a carefully chosen setting.
Finally, keep the torch moving during the last part of the weld and taper current gradually if your machine supports downslope. Postflow protects the tungsten, but it cannot repair a crater caused by stopping abruptly or leaving the weld pool unfilled.