How much oxidation is a problem?

[Luke'sScreenName]

Im a very inexperienced mig welder so hopefully this not an incredibly stupid question. I know that having an uncontaminated joint is important for a good weld. I spent several hours today cutting, grinding, and preping a project. Had to quit early and won't get back to it for 5 days.
I'm sure there will be some oxitation of the metal before I can weld. Most likely just a slight dulling of the shine. Is this a problem? Do I need to re-grind all my surfaces?

 

[Teachu2]

If mild steel, just weld it up. If aluminum, you'll need to clean again immediately before you weld.

 

[Shield Arc]

If it's not outside in the weather, I wouldn't worry to much about it. Lincoln's L-56 Mig wire is pretty impressive running over a little mill scale, or oxidation.

Just remember, when Mig welding run as hot as you can control the puddle.

 

[Stimw]

If it's not critical welding you can still weld steel with mild rust on it.
The trick is to go a little slower and let the puddle flow forward. It will burn off the impurities and you will see them on top of the weld when done. It will look like the flux on a stick weld.

 

[Luke'sScreenName]

If it's not outside in the weather, I wouldn't worry to much about it. Lincoln's L-56 Mig wire is pretty impressive running over a little mill scale, or oxidation.

Just remember, when Mig welding run as hot as you can control the puddle.

Could you elaborate on "contol the puddle?"

 

[Shield Arc]

Could you elaborate on "contol the puddle?"

Ummmmm, boy that's a hard one to put into words:confused2:! The tricks to welding is to know how to read the puddle. Control the puddle. An anticipate what the puddle is going to do, so you can make corrections on the fly.

It's like when guys first start to learn to weld vertical up hill. Human nature is to run cold. That is dead wrong!:thumbdown: You want to run hot! Right on the edge of the puddle boiling out and falling on your feet.

Mig welding is notorious for cold lap / lack of penetration. So you want to run Mig as hot as you can. Right on the edge of not being able to control the puddle!

Now is that clear as mud?

 

[Shield Arc]

OK, here we go I found some pictures.
See how the weld in the first picture was ran so cold. This is very bad business!:thumbdown:

Now look at the second picture. Can you see the difference in the two welds?

 

[BukitCase]

Carl, are you cheating again and comparing short circuit to spray transfer? ... Steve

 

[CalG]

I'm Not sure I can really add to anything regarding "control the puddle", but my coupons did pass. (and I stayed at.......;-)

For my own purposes, if I watch the puddle, and it looks smooth and full when wet and hot, it will look good and be good when it cools off.
A puddle out of control is when severe undercutting along the weld results, or having the puddle fall through or fall away at the ends.
A cold puddle might not look out of control, but if you observe carefully, you will see a puddle on top of cold metal. That too is "out of control".

If you watch the puddle carefully, any of these faults are readily seen with enough warning to do something with the heat to allow a return to controlled shape and penetration if the amperage is somewhere near. There are a lot of stick/filler rod manipulations that can work wonders in coping with less than optimal machine settings and workpiece configurations.

Setting up the complete work environment to allow a good look at the entire weld is the real art! It's too easy to be "blinded by the light".

 

[Shield Arc]

Carl, are you cheating again and comparing short circuit to spray transfer? ... Steve

Well yeah. I'm trying to drive home the point to run Mig hot!
Here are a couple short circuit Mig welds that don't look to bad.