The weld pool
A finished weld tells you what happened. The pool tells you what is happening — and it is the only indicator that works in real time. Machine settings are an approximation; the confirmation is what you can see under the arc.
What it actually is
The pool is the metal that is molten at one moment: the edges of the parent material and the filler together. Surface tension and arc pressure hold it in place, and its size follows from the balance between heat going in and heat conducted away into the material.
Everything else follows from that. Anything that shifts the balance — current, travel speed, thickness, the starting temperature of the part — changes the pool at once, in front of the welder, and does so many seconds before the consequences show in the finished weld.
How to read it
| What you see | What it means | What to change |
|---|---|---|
| Oval, following the arc evenly | The balance is right | Nothing |
| Round and growing under the electrode | Too much heat in one spot | Speed up or drop the current |
| Narrow, tearing away behind the arc | Too little heat | Slow down or add current |
| Darkening and thickening as you go | The part has cooled, or the current has fallen | Check the earth and the leads |
| Starting to sag downwards | Volume beyond what surface tension holds | Thinner bead, less current |
| Slag running ahead of the pool | Too slow, or too flat an electrode angle | Speed up, raise the angle |
The last row is the important one. Slag that has washed ahead of the arc stays under the liquid metal and comes out as an inclusion (301) — and it is visible while you are still welding, while there is still something to be done about it.
Why position changes everything
Flat, the pool is held by the bottom of the groove. Vertical and overhead, it is held by surface tension alone, and surface tension supports a limited volume of liquid metal — several times less than the current you weld comfortably with downhand would produce.
That is the origin of every positional rule that otherwise looks arbitrary: current down by 10–20 %, electrode one size thinner, weave instead of a straight run. Each of them does the same thing — reduces the volume of the pool at any one moment. Details in welding positions.
To read it you first have to see it
It sounds obvious, and it is the commonest reason a welder does not read the pool at all: a filter darker than necessary hides its edges. You see a bright arc and darkness around it, so the electrode is guided from memory and the bead wanders off the groove.
The shade is chosen for the current, not for habit — the DIN shade calculator. The other half of the problem is the cover lens: a film of spatter costs more visibility than one shade number, and it is replaced in a minute.
What next
Matching current to electrode diameter — the welding current calculator. What happens when the pool never reached the groove face — lack of fusion and penetration. How to prepare a groove so the pool has room to work — joint preparation.
Frequently asked questions
What should a correct pool look like?
Like an oval, slightly elongated drop that follows the arc at the same speed as your hand, and whose trailing edge freezes into an even pattern of ripples. A round pool growing under the electrode means you are dwelling; a narrow one tearing away behind the arc means you are moving too fast or running too little current.
Why does the pool run in the vertical position?
Because gravity acts on liquid metal and surface tension only holds a small amount of it. That is where the whole of positional technique comes from: current down by 10–20 %, a thinner electrode, and a movement that gives the pool time to freeze — several narrow beads instead of one wide one.
Can you see the pool the same way through an auto-darkening helmet?
You can, provided the shade suits the current. Too dark a filter hides the edges of the pool and you end up welding from memory — the bead wanders off the groove because you cannot see where the liquid metal ends. The shade is picked by the DIN shade calculator.
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