Which Polarity? And Why

matt1978 · Mar 2, 2009

GeorgeB said:
I've used the DC inverter arc welder on this so far, because arc starting is easier than with the AC machine. I prefer the DC inverter for tacks, whereas I still prefer the AC transformer for running a seam.

Are you running the Inverter on DC Positive or Negative George?

weldequip · Mar 2, 2009

That's rod holder Positive + socket & earth lead Negative - socket George

You should get much better/easier results all round on a DC machine, not just on the tacks.
weldequip

GeorgeB · Mar 2, 2009

matt1978 said:
Are you running the Inverter on DC Positive or Negative George?

I'm running the inverter as 'suggested' on the front panel, like we did at the arc weld meeting, i.e. rod holder +ve, as weldequip also confirmed. Is that best, would you say? Does the weld get warmer or cooler if you reverse it? I forget the theory and have never tried it out with rod holder -ve.

Everyone says DC is smoother and the same inverter did quite well at the arc weld meeting. But my retired friend the ex-welder is still against it for mild steel and I still seem to get a more consistent weld bead with the AC transformer, like with the Lidl rod I tried recently. But note these were (DC) tacks and short bursts on purpose to avoid blowing holes.

All these welds were either vertical or upside down, neither of which I'd tried before.

malcolm · Mar 2, 2009

For thin metal it might be worth trying what they call DCEN or DC- (DC Electrode Negative, which is plugging the rod holder into the negative terminal and the earth lead into the positive. That puts less heat into the metal you are trying to weld. AC will be in the midddle as it doesn't have a polarity.

This insight was brought to you by http://www.mig-welding.co.uk/forum/showthread.php?t=1463 (I'm going to bookmark it now as I'm forever being confused by all this). Also bare in mind that apparently some rods can be funny on one polarity but OK on the other. It might say on the box which one they like.

zzr1200 · Mar 2, 2009

malcolm said:
For thin metal it might be worth trying what they call DCEN or DC- (DC Electrode Negative, which is plugging the rod holder into the negative terminal and the earth lead into the positive. That puts less heat into the metal you are trying to weld. AC will be in the midddle as it doesn't have a polarity.

Malcom, if I remember correctly its Dc Electrode +ve that has less heat in the metal to be welded.

malcolm · Mar 2, 2009

Damn - have I got it wrong again? I've promised before that I'll write it down once someone tells me for sure in simple words.

zzr1200 · Mar 2, 2009

malcolm said:
Damn - have I got it wrong again? I've promised before that I'll write it down once someone tells me for sure in simple words.

DC> -ve 2000 deg......+ve 4000 deg............-ve Low Temp..........+ve High Temp...........total DC weld temp 6000 deg

AC> -ve 2000 deg......+ve 2000 deg........total AC weld temp 4000 deg

hth......

GeorgeB · Mar 2, 2009

malcolm said:
For thin metal it might be worth trying what they call DCEN or DC- (DC Electrode Negative, which is plugging the rod holder into the negative terminal and the earth lead into the positive. That puts less heat into the metal you are trying to weld.

Thanks for the suggestion. I'm over most of the thin metal stuff for this job but I'll see how it works on scrap metal and may use electrode -ve for thin metal in future. Isn't it a similar effect to simply turning down the amps, though? Don't answer, or this thread will become an extension of the other one.

Back to MOT arc welding... from my photos, is there anyone who thinks those welds could fail if left as they are?

matt1978 · Mar 2, 2009

Its about how the 'heat' is shared between the poles.......

DC+ has 2/3rd's of the heat

DC- has 1/3rd.

This is why the Tungsten is DC- on Tig welding, and its also why a small ball forms on AC tig, the ball is formed from the + cycle.

I would try DC- on MMA, you'll find the weld pool less hot and aggressive, as 2/3rd's of the heat will be on the welding return/work piece, if your using 6013's they should run just as well on DC-. Give it a try.

zzr1200 · Mar 2, 2009

Matt, when I was at college the instructer stated that total DC temp was between 6000 to 7000 degs and the difference was due to the electrode flux covering, which types of rod give the higher temp???

matt1978 · Mar 2, 2009

Cellulosic rods I should imagine.........Technic Al would be best bloke to answer though.

scarecrow · Mar 2, 2009

matt1978 said:
Its about how the 'heat' is shared between the poles.......

DC+ has 2/3rd's of the heat

DC- has 1/3rd.

This is why the Tungsten is DC- on Tig welding, and its also why a small ball forms on AC tig, the ball is formed from the + cycle.

I would try DC- on MMA, you'll find the weld pool less hot and aggressive, as 2/3rd's of the heat will be on the welding return/work piece, if your using 6013's they should run just as well on DC-. Give it a try.

zzr1200 said:
Matt, when I was at college the instructer stated that total DC temp was between 6000 to 7000 degs and the difference was due to the electrode flux covering, which types of rod give the higher temp???

Is the not the same thing

when welding electrode negative the electrode has one 1/3 of the heat and the work piece has 2/3 : So less filler metal and more penetration (welds will look very flat)

When welding electrode positive the electrode has 2/3 and the work piece 1/3 : So you get more filler metal and less penetration (welds will look high)

But I could be wrong need to write this one down myself as I tend to get mixed up might be best to wait for Technic Al

zzr1200 · Mar 2, 2009

scarecrow said:
Is the not the same thing

when welding electrode negative the electrode has one 1/3 of the heat and the work piece has 2/3 : So less filler metal and more penetration (welds will look very flat)

When welding electrode positive the electrode has 2/3 and the work piece 1/3 : So you get more filler metal and less penetration (welds will look high)

But I could be wrong need to write this one down myself as I tend to get mixed up might be best to wait for Technic Al

Yep...pretty much....

Its also why DC electrode +ve is usually used for over head stick...

matt1978 · Mar 3, 2009

Rods will typically burn around 50% faster when welded on the DC+ pole, the arc is hotter and more aggressive, thats why I would suggest George trying to run on DC- pole. Esecially considering he is only using Rutile electrodes.

There approx 10,000 Deg C in total, and that is roughly shared 2/3rd's on Positive pole (6000-7000) and the remainding 1/3rd on the Negative pole.

TechnicAl · Mar 3, 2009

I have a mental block with bloody positive and negative. I remember by "positive gives positive penetration".

You also have to take into account the Energy, so the Amps and Volts are important. Different rods give different Arc Voltages, so for the same current, same polarity, some rods will give better penetration because they have higher voltages.

Some of the "soft" rutiles can be as low as 24 volts whilst the Cellulosics can be up near 38 volts. LoH Hys are somewhere in between.

Upto a point the welder can influence the arc voltage by regulating the arc length. Some rods can be "touched" or you can hold an arc. Holding the arc will give higher arc voltage = higher energy = better pen/ faster speed.

It is actually a very complex subject.

zzr1200 · Mar 3, 2009

TechnicAl said:
I have a mental block with bloody positive and negative. I remember by "positive gives positive penetration".

You also have to take into account the Energy, so the Amps and Volts are important. Different rods give different Arc Voltages, so for the same current, same polarity, some rods will give better penetration because they have higher voltages.

Some of the "soft" rutiles can be as low as 24 volts whilst the Cellulosics can be up near 38 volts. LoH Hys are somewhere in between.

Upto a point the welder can influence the arc voltage by regulating the arc length. Some rods can be "touched" or you can hold an arc. Holding the arc will give higher arc voltage = higher energy = better pen/ faster speed.

It is actually a very complex subject.

.....

.....

rtbcomp · Mar 3, 2009

zzr1200 said:
DC> -ve 2000 deg......+ve 4000 deg............-ve Low Temp..........+ve High Temp...........total DC weld temp 6000 deg

AC> -ve 2000 deg......+ve 2000 deg........total AC weld temp 4000 deg

hth......

Now that confuses me! How can 2000 + 2000 deg give a total of 4000?

You can't add two temperatures

malcolm · Mar 3, 2009

This could be a good thread to write things down in and link from a helpful information page - I've moved posts from from the original thread to make them easier to refer to later on.

But I'm confused. I like "Positive gives positive penetration".

For thinner metal that might suggest putting the arc rod holder into the positive terminal (DCEP) would end up with more holes and blow through.

For thicker metal puttting it into the negative terminal (DCEN) might not give the penetration you might have wanted.

I can imagine it's even more complicated than that. I understand that if the rod is positive it will get 2/3 of the heat of the whole weld. But if that extra heat is transferred to the metal by way of a stronger arc the thing you were trying to weld will end up hotter than if you'd made the rod negative.

Is that a reasonable summary or am I still confused?

GeorgeB · Mar 3, 2009

Thank you for opening this new thread - much better.

I think the best way I'm going to understand what's going on here is to try electrode +ve vs electrode -ve in practice, to see if I notice must difference. You get less heat and penetration by turning the current down (electrode +ve) so what does swapping the electrode over, offer as an alternative approach to current adjustment?

shenion · Mar 3, 2009

There was a description in one of my welding references of how the electrons/ions slam into the surface to increase heat.

However consider MIG. Gasless is DCEN with more penetration than DCEP.

Arc seems to be the other way.

I think the rod type has a lot to do with it. The flux must effect the heat in the arc. There are a lot of factors like heat of gasses breaking down and recombining (like CO2 to CO + O.) There are deep penetration rods (6010/11) that really only differ in the flux.

Really I have not found all that much a difference. When running DCEN I have to increase the current as the slag get too cool real fast. So you have less penetration, but need to run hotter. It seems to only be an issue when running rods at the lowest currents.

I have still been playing with polarity. Really have seen only a small difference. After you adjust the current for a nice clean weld, the difference seems small.

Really, the best is to choose the right rod. I have some small 7014's that can run 25A on thin metal easily. Problem is they are expensive; as they are in small tubes or large boxes. So you end up buying several 1/2 lb tubes or spend a fortune on a 30lb box.

Which Polarity? And Why

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