What's inside
As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.
A square corner is one of the easiest TIG joints to overheat. Three surfaces meet in a small area, the inside corner traps heat, and the joint often needs enough filler to prevent undercut or a sunken corner. The result can be warping, excessive penetration, a wide discolored zone, or a corner that collapses before the weld is complete.
The cure is not simply turning the amperage down. Use the right joint preparation, control the arc length, add filler consistently, and let the workpiece shed heat between short welds.
Prepare the Square Corner Before Striking an Arc
Clean both sides of the joint to bright metal. Remove mill scale, paint, oil, and oxide with a dedicated stainless brush or abrasive wheel. Wipe the parts with acetone or an appropriate metal cleaner, then allow them to dry. Contamination makes the arc unstable and encourages you to linger in one spot while trying to correct the puddle.
For a 90-degree outside corner, fit the parts tightly but do not force them together with heavy clamps. A small, even gap can improve penetration, but a large gap demands more filler and increases heat input. On material around 1/8 inch thick, a gap of roughly 1/32 inch is usually manageable. For thin sheet, aim for a close fit with no visible daylight.
Use several small, evenly spaced tacks rather than one long tack at each end. Alternate sides as you tack so the joint does not pull out of square. Let each tack solidify before placing the next one. If the parts move during tacking, stop and correct the fit; welding a distorted corner usually requires more heat and makes the problem worse.
Choose Starting Settings That Limit Heat
Set the machine for the actual joint, not just the thickness of the thickest piece. As a starting point for mild steel with a sharp 2% lanthanated tungsten, use about 1 amp per 0.001 inch of thickness, then reduce that number when the corner is exposed on two or three sides. A 1/8-inch corner may begin near 90 to 110 amps, while 16-gauge sheet may need only 45 to 70 amps depending on fit and position.
Use a foot pedal or fingertip control if available. Start with enough current to establish a small puddle, then ease off as the surrounding metal becomes hot. Pulse can help, but it is not a substitute for torch control. A useful starting pulse might be 1 to 2 pulses per second with the peak current limited to 60–75 percent of the machine’s available setting and a background current around 30–40 percent. Adjust by watching the puddle rather than following those numbers rigidly.
| Adjustment | Heat-control benefit | Trade-off |
|---|---|---|
| Lower amperage | Reduces penetration and distortion | May cause lack of fusion if you move too slowly |
| Shorter weld segments | Lets the joint cool between passes | Creates more starts and stops to blend |
| Pulsed TIG | Controls average heat while retaining peak penetration | Requires coordinated travel and filler timing |
| Larger copper heat sink | Pulls heat from thin material | Can chill the puddle and reduce penetration |
For DC TIG on mild or stainless steel, set post-flow long enough to protect the tungsten as it cools. Around 1 second per 10 amps is a reasonable starting point. Excessively long post-flow wastes gas, but too little can turn the tungsten blue or gray and contaminate the next start. A 2% lanthanated tungsten electrode is a practical general-purpose choice.
Control the Torch and Filler
Grind the tungsten lengthwise to a clean point and keep the arc short, generally 1/16 to 1/8 inch. A long arc spreads heat, reduces shielding, and makes it harder to direct the puddle into the root of the corner. Hold the torch around 70 to 80 degrees to the work and point the tungsten at the joint bisector rather than favoring one leg.
Use a gas cup large enough to provide consistent coverage without making access awkward. A standard #7 or #8 cup with about 15 to 20 cubic feet per hour of argon is often sufficient indoors. Excessive gas flow can create turbulence and draw air into the shield. If the weld becomes gray, sooty, or peppered with pinholes, check leaks, drafts, dirty material, and cup position before simply increasing flow. A TIG gas lens kit can improve coverage when the corner is difficult to reach.
Establish a small puddle on both sides of the corner, then add filler at the leading edge. Do not use the filler rod to bridge a cold joint. Dip the rod decisively, withdraw it, and keep the tip inside the argon shield. For a 1/8-inch steel corner, 3/32-inch filler is often easier to control than 1/16-inch wire, which melts too quickly for some operators.
Weld in Short, Alternating Sections
Instead of running the entire corner in one pass, weld 1/2- to 1-inch sections. Move to the opposite end or another side of the assembly after each section. This distributes heat and limits the time any one area spends above its distortion point. On long corners, a skip sequence—such as alternating sections several inches apart—usually controls pull better than simply welding from one end to the other.
Stop while the puddle is still clean and controlled. Taper off current with the pedal, add a final small dab of filler if needed, and hold the torch over the crater during post-flow. Do not snap the torch away immediately; that can leave a crater crack or oxidized end. Allow the work to cool until it is warm rather than too hot to touch with a gloved hand before continuing. For thin stainless, use compressed air only cautiously; sudden quenching can increase distortion and residual stress.
Recognize Heat-Related Failure Modes
If the corner turns wide and dull, your arc is probably too long, travel is too slow, or the current is too high. If one leg shows undercut, direct slightly more of the arc toward that leg, add filler sooner, or reduce travel speed without dwelling. If the root is not fused, improve fit-up and torch angle before raising amperage.
For thin sheet, clamp a clean copper bar tightly behind the joint when access allows. Copper will not fuse readily to steel and acts as a heat sink, but it can also chill the puddle enough to cause lack of fusion. Use it with a small gap-free fit and test on scrap first. A basic copper welding heat sink is worthwhile for repeated thin-gauge work; for one small repair, a clean piece of thick copper may be enough.
Finally, protect yourself from the heat you are trying to control. Use a properly rated low-amp TIG welding helmet, fire-resistant clothing, gloves, and ventilation. A cheaper helmet can be fine for occasional welding if its low-current response, optical rating, and fit are dependable. The important part is seeing the puddle clearly enough to move continuously instead of stopping to inspect it.