Running too cold
The only defect on this site that can look good and be worthless.
Read this one differently. Every other guide here describes something you can see. A cold weld's defining feature is that its worst consequence — lack of fusion — is invisible from above. A skilled welder running too cold can lay a bead that looks tidy, even attractive, and is barely attached to the plate. This is the failure mode that a photograph, an app, and your own eyes are all worst at catching.
What it looks like — when it looks like anything
The visible signs, when they're present at all:
- A high, narrow, rounded bead that sits on the plate rather than flowing into it — see a ropey bead, which is the visible form of the same underlying problem.
- A steep angle where the bead meets the plate, instead of a smooth shallow transition. The single most reliable visible clue.
- Overlap (cold lap) — weld metal rolled over onto the parent plate with no bond beneath the lip. If you can get a thin blade under the edge of the bead, that's conclusive.
- Slag that won't lift on stick or flux-core, clinging in the toes rather than peeling off. Stubborn slag is usually a heat message.
- A stuttering, spitting, irregular arc rather than a steady crackle.
- Wire stubbing into the plate and pushing the gun back at you on MIG.
- A narrow heat-affected zone — very little discolouration either side.
And on a bad day, none of the above. A cold bead run with steady hands can have even ripples, consistent width and a clean surface. That is the trap.
What causes it
Amperage or voltage too low
The direct cause. Often the result of deliberately backing off after burning through something, or of settings copied from a chart written for different thickness. Worth checking against the manufacturer's range for your electrode diameter and material thickness before assuming technique.
Travel speed too fast
Correct machine settings, insufficient dwell. The arc passes over the joint too quickly to melt the parent metal, so filler is deposited onto steel that never reached melting point. This produces some of the most convincing-looking cold welds, because the bead itself can be perfectly regular.
Excessive stickout (MIG/flux-core)
Amperage in wire processes is a consequence of wire feed and arc length, not something you dial in directly. Long stickout raises resistance in the wire and drops the current actually reaching the arc, so the weld runs cold even though nothing on the machine changed. If your welds went cold when the joint got harder to reach, this is why.
Wrong joint access and gun angle
If the arc can't reach the root of the joint — poor prep, insufficient gap, an angle that buries the arc against one face — filler stacks up in a corner it never fused into. Common on fillet welds with a tight included angle.
Heat sinking into the parent metal
Thick section, cold ambient temperature, or clamping to a large steel bench all draw heat out of the joint. Settings proven on 6mm plate in summer can genuinely be too cold on 20mm in an unheated workshop in February. Thick material may require preheat as part of the procedure.
How to tell — properly
Since the visible signs are unreliable, use destructive testing on practice coupons. This is what practice coupons are for, and it is the only honest way to calibrate whether your settings are producing fusion:
- Bend test. The standard method, and the one used in certification testing. Lack of fusion opens up as the coupon bends.
- Break test. Put a fillet coupon in a vice and break the joint. Examine the fracture face: it should tear through weld and parent metal, not separate cleanly along the interface. A break that peels apart at the joint line with a smooth, shiny face is lack of fusion, and it is unmistakable once you've seen it once.
- Cut and etch. Section the coupon, polish the face and etch it. The fusion boundary and the actual penetration profile become directly visible. This is the single most instructive thing most practising welders never do.
Break one coupon from every session where you changed settings. It costs one plate and tells you what no photograph can.
Why this is the dangerous one
A weld that is too hot is ugly, and ugly gets fixed — someone sees it, it gets ground out and rerun. A weld that is too cold can pass a casual visual check, get painted, get installed, and hold nothing. The joint has a fraction of its designed strength and no outward sign of it.
This is also precisely why nobody should treat a photo-based score — from this app or any other — as evidence that a weld is sound. Fusion is the property that matters most and the one least visible from above.
What a photo can and can't tell you
A photograph can flag the correlates of a cold weld: a high narrow profile, a steep toe angle, visible overlap, clinging slag. Those are real signals and worth acting on when they appear.
It cannot detect lack of fusion, and no amount of image quality changes that — the defect is at the boundary between the bead and the plate, underneath metal that is in the way. A clean-looking cold weld will score well on surface appearance and be a bad weld. That is a limit of the method, not a bug to be fixed in a later version.
If the joint carries load, it needs a bend test, RT or UT, and a Certified Welding Inspector. Not a photo. Not a score. Not an app.
Related
- A ropey bead — the visible form of the same problem.
- Running too hot — the opposite failure, and the easier one to catch.
- Porosity — the other defect that hides below the surface.