Running too hot
Wide, flat, thin at the edges — and taking the plate with it.
The short answer. Too much heat is going into the joint per unit length. That's amperage or voltage too high, travel too slow, or — the one people forget — heat accumulating in the plate over successive passes. A setting that was correct on your first bead can be far too hot by your fourth on the same coupon.
What it looks like
- A wide, flat bead, noticeably broader than the joint needed, sitting low with very little crown.
- Undercut along one or both toes — heat is the most common cause of it, so the two travel together.
- Heavy spatter scattered well beyond the weld.
- A wide heat-affected zone — the discoloured band either side of the bead is broad, with pronounced blue and straw temper colours running well back from the weld.
- Burn-through, or a puddle that keeps trying to fall through the joint and needs constant chasing.
- Excessive penetration — an oversized bead hanging off the back of the joint, sometimes called suck-through.
- Distortion. The plate pulls, cups, or bows noticeably as it cools.
- On thin sheet, the puddle simply drops out and leaves a hole.
With TIG, running hot also shows in the tungsten: excessive current balls or degrades the tip, and a tungsten that keeps needing regrinding is telling you something about your amps before the bead does.
What causes it
Amperage or voltage set too high
The obvious one, and worth ruling out first because it's a two-second test. Rod and wire manufacturers publish ranges for a reason; if you're near the top of the range for your diameter and thickness, come down. Note that rod diameter is part of this — a 1/8" rod at its correct current is simply too much heat for thin plate no matter how carefully you set the machine, and the answer is a smaller rod, not a lower setting.
Travel speed too slow
Heat input depends on how long the arc dwells over each part of the joint. Move slowly and you pour more energy into the same length of steel, regardless of the machine setting. If your amps are within range and the bead is still wide, flat and undercut, you're lingering.
Interpass heat build-up
The one that catches people out on multi-pass work and on test coupons. Each pass leaves the plate hotter than it found it. By the third or fourth pass the parent metal is no longer drawing heat away, so the same settings that were correct on a cold plate are now dumping heat into a joint that can't shed it. Symptoms appear progressively down the coupon rather than from the first bead — that progression is the diagnosis.
Codes for real work set maximum interpass temperatures for this reason, because excessive heat input changes the metallurgy of the weld and the surrounding metal, not just its appearance. For practice, the useful habit is simply to let the coupon cool between passes rather than racing through it.
Position and gravity
Settings that work flat are often too hot for vertical or overhead, where there's nothing holding a large fluid puddle in place. If your flat beads look right and your vertical-ups sag and run, that's not a technique failure alone — the heat needs to come down for the position too.
How to work out which one is yours
- Hot from the very first bead on a cold plate — it's the machine setting. Drop it 10% and rerun.
- First pass fine, getting worse each pass — interpass heat. Let it cool.
- Amps within the recommended range but the bead is still wide and flat — travel speed.
- Fine flat, sagging vertical or overhead — the position needs less heat than the flat setting.
- Burn-through on thin material only — you likely need a smaller electrode diameter, not just fewer amps.
- Wide bead but no undercut and no distortion — you might not actually be too hot. Check the joint prep and the weave width first.
Don't over-correct. The standard mistake after diagnosing "too hot" is to drop the amps hard and speed up at the same time, which lands you straight into a ropey bead or, worse, a cold weld with no fusion. Too hot is visibly ugly; too cold can be invisibly dangerous. Move in small steps and read each bead.
What a photo can and can't tell you
Running hot is the most photogenic of the failure modes. Bead width relative to the joint, a flat profile, undercut at the toes, spatter density, the width of the temper colours and outright burn-through are all surface-visible, and with a coin in frame for scale the proportions can be assessed rather than guessed at.
What a photograph cannot assess is what excessive heat input did to the metal. Grain growth in the heat-affected zone, reduced toughness, and altered mechanical properties are metallurgical outcomes — invisible on the surface, and not recoverable by grinding the bead back. Nor can a photo see excess penetration on the far side of the joint, which is often the clearest evidence of all and is simply not in the frame. Surface visual only; internal and metallurgical assessment needs proper testing and a qualified inspector.
Related
- Undercut — the defect heat causes most often.
- Running too cold — the opposite failure, and the more dangerous one.
- A ropey bead — where over-correcting lands you.