What's inside
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Heat tint on a TIG-welded corner joint is oxidation caused by heat and exposure to air. A faint straw color can be normal on stainless steel; dark blue, gray, or black tint usually means the joint stayed hot too long, shielding was poor, or both. The aim is not to make every weld look silver. It is to control the heat and shielding well enough to get the required weld quality without an oversized, heavily oxidized heat-affected zone.
Why corner joints tint
A corner joint concentrates heat where the two pieces meet. If you are welding stainless steel, excessive heat can thicken the oxide layer and reduce corrosion resistance near the weld. On mild steel, color is less useful as a quality measure, but overheating can still cause distortion, undercut, and a wide heat-affected zone.
Inside corners are particularly easy to overheat: the torch has limited access, the joint can trap heat, and the arc may reflect off both sides. A slow travel speed, long arc, oversized filler rod, or repeated stops add heat. Poor gas coverage can darken the weld even when the heat input is reasonable.
Prepare the joint before striking an arc
Remove oil, paint, marker, and oxide from both sides of the joint. For stainless steel, use a clean stainless-dedicated brush or abrasives that have not been used on carbon steel. Contamination can cause discoloration and corrosion later. Fit the parts consistently: a gap that varies along the joint makes it harder to keep penetration and bead size even.
Tack the pieces at intervals appropriate to their size, then check alignment. On thin sheet, tacks every 25–50 mm (about 1–2 in.) can help limit movement, though the right spacing depends on the part and restraint. Keep tacks small and fully fused; a cold, oversized tack can become a defect when you weld through it. Clamp only as much as needed—heavy restraint can reduce movement but may also increase stress or make access difficult.
Set up the welder and shielding
For stainless steel, use DC electrode negative (DCEN) and argon shielding gas. A common starting range is roughly 30–50 amps for 0.8–1.2 mm (0.030–0.047 in.) sheet, and 50–90 amps for 1.5–2 mm (0.060–0.080 in.) material. These are starting points, not universal settings: joint fit, torch angle, machine, and position all matter. Set enough current to establish a stable puddle promptly, then control heat with travel speed and the foot pedal or torch control rather than lingering at low current.
Start with about 7–10 L/min (15–21 CFH) of argon at the torch, then adjust for cup size, drafts, and torch position. More flow is not automatically better; excessive flow can create turbulence and pull air into the shielding zone. Use a gas lens and a larger cup if the torch can reach the corner. If the back of a stainless joint needs oxidation control, arrange a suitable purge; a front-side torch shield cannot protect the root by itself.
A TIG gas lens kit can improve coverage and allow a longer electrode extension, useful in a tight corner. It is not a substitute for correct flow or a wind-free work area. A basic collet body is fine for accessible joints where shielding is already reliable.
Weld with less heat input
Use a sharp tungsten and keep a short, steady arc—typically around 1–2 mm (1/16 in.) from the work. A long arc spreads heat, makes the puddle less predictable, and weakens gas coverage. Position the torch so the arc reaches the root without pointing into one face for too long. Add small amounts of filler at the leading edge of the puddle; feeding slowly while holding the arc stationary is a common cause of a wide, hot bead.
Move continuously once the puddle is established. Aim for a bead just large enough for the design, not a broad cosmetic cap. On thin material, a brief pedal release or a controlled pulse can limit heat, but very low current can make you dwell too long and create more total heat. If using pulse, begin with a modest peak current and a background current that keeps the puddle from freezing; tune it on scrap of the same thickness before welding the part.
For longer seams, weld short sections in a balanced sequence rather than running the entire joint in one pass. Let the work cool between sections. Do not quench hot stainless with water as a routine shortcut: rapid cooling can distort the part and does not correct a poor weld. Use a temperature indicator or contact probe if the procedure requires a maximum interpass temperature.
Read the color and diagnose the cause
| Appearance | Likely cause | What to check |
|---|---|---|
| Light straw or pale gold | Moderate oxidation; may be acceptable for some applications | Confirm the service requirements and inspect the root if it matters |
| Blue or purple band | High heat input, inadequate travel speed, or marginal shielding | Shorten arc, move faster, check gas flow and drafts |
| Gray or black weld | Severe oxidation or shielding failure | Check leaks, cup coverage, gas supply, and torch position; do not simply polish over it |
| Dark root on stainless | Unprotected backside oxidation | Use an appropriate purge or backing arrangement when required |
Color is a clue, not a complete inspection. A bright weld can still have lack of fusion, porosity, or an unsuitable profile. If the joint is for food equipment, corrosive service, pressure, or a code-regulated structure, follow the applicable procedure and acceptance criteria; visual color alone cannot establish suitability.
Clean up and protect the finished joint
Remove heat tint with a method approved for the alloy and application. Mechanical cleaning with dedicated stainless abrasives is often practical for small jobs, though it may leave a different finish. Pickling products can remove oxide more evenly but are hazardous and require strict label directions, chemical-resistant protection, and proper waste handling. Do not use an improvised acid mixture. A stainless-dedicated wire brush is a modest-cost option for light residue, not a cure for deep black oxidation or an unsuitable weld.
Wear a properly rated welding helmet, gloves, and protective clothing, and use local fume extraction. TIG does not make stainless fumes harmless. Keep your face out of the plume, particularly when welding chromium-containing alloys, and avoid cleaning chemicals near an active arc. If the tint is consistently heavy, correct the heat input or shielding before buying more cleanup tools.