Which Polarity? And Why

[clem dog]

Thats good because i started to get alittle out of my depth there!
Now i know that i actually only thought i could weld. Would polarity effect hard facing rods (600's i think) as i need to reface a toe edge on a digger bucket? I have used them before with a small 200 amp inverter without giving much regard to polarity.

 

[madkayaker]

the box will tell you electrode positive or electrode negative

 

[clem dog]

Thanks MK!!

 

[olek]

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.

DCEN is quiter, used to avoid making holes in the first root pass for instance and up to a certain gauge (about 4mm I think ) for sheet metal

DCEP with 6013 put more heat, so it may eventually favor penetration on first pass if those electrodes (usually DCEN) are used . (Information from the electrode maker Selectarc, discussions with the engineer that formulates the rods

 

[olek]

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

Yes there is anther effect with DCEP, as the electrode is boosted the metal is projected with much force, and as a result one can obtain more penetration (with certain electrode)

 

[skotl]

Holy thread resurrection, Batman! Eight years has to be a record, right?

And welcome to the forum, by the way

 

[olek]

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.

ARe you sure ? theoretically you have more heat in the metal with DCEN , 70%, if that put less heat it is because there is more material to cool the heat , and because there is a secondary effect when using DCEP that create finally more heat, theoretically you have only 30% of the heat in the metal with DCEP

A little mysterious anyway !

 

[olek]

Holy thread resurrection, Batman! Eight years has to be a record, right?

And welcome to the forum, by the way

Thanks Kamerad Those threads are not located clearly in space time so they can revive from their slag easily

 

[Liquid Metal]

Right then, like a few others, the physics of it boggle my mind a little tbh.
4000deg c here, 2000deg c there? 1/3 vs 2/3 of the heat one side or the other?

My personal take and approach on it is:
When welding pipe work, full lowhy (7016) butts, neg root followed by pos fill and cap.
When using fleet 6010s, and rutile caps, pos all the way out. Thats when butts and fillets are rolled on a spinner. Neg fill and cap when welded in position.

Structural steel, fillets (ie web of beam to end plate) and butts pos. Except outside corner joints ie the welds on the outside of a 12mm thick end plate, these are welded neg polarity. Gives more control over the pool and stops the edges of the "prep" being obliterated with a 4 or 5mm lowhy sometimes rutile rod (depending on spec).

End of the day though, what is king really as far as polarities and amperages are concerned is the procedure youre working to when this sort of work is concerned.

With odds and sods at home, and bits and pieces made for beer tokens, those that i do stick weld, that are just ott to tig, most of the time will hook up DCEN, due to one factor: less spatter to clean up after.

 

[olek]

Hello, Liquid metal, thank you for sharing your knowledge & experience here.

I may say that it is interesting, at my "forever advanced beginner" level, to try to get the underlying causes of those choices.
It may appear more evident for the professionals but if you don't dare giving a little more to the 'why's", I would appreciate it greatly (you really said yet a lot, I may say)

For instance the change between pipe on spin and in position.

That said, as a puzzle it is fun yet to think about.

I believe that the coating of the rods play an important role in the fusion of both metals, asking for more energy for some so the final slag is cooked enough, or allowing a thicker one to help support the bead, etc...
Something strange happens with the arc noise when using DCEN with 7018 as it cracks and sound as a motor that hikes whole the arc is also quiter, and the sound is more consistent with DCEP..
Do you notice the slag coming off a little less easy with DCEN then, or do the amps need to be raised up (for instance)
Sorry to be long winding, I think much before doing and after, I hope it helps too in learning.

Regards, forgive my English please

 

[Liquid Metal]

I cant for the life of me remember it thoroughly enough to try to explain the physics of it ie pos vs neg, is quite a complex subject that touched on at college years ago now lol.
All i know is its to do with electron flow across the arc, but also has to do with ion flow.
When amperage is left alone, simply changing polarity:
With rutiles, low hys and iron powders, you get a calmer quieter arc with dcen, less spatter, and can make it more controllable if welding near an edge. With some 6010 cellulose rods on the other hand ie licoln fleetweld 5p+ or murex celtian, they both have DECP only on the pack. Although with amperage like for like, when run on DCEN they are as fierce as hell.

Your question about the slag coming of harder? I dont really notice much of a difference with either DCEP or DCEN as to which comes off easier tbh, but do know that i find slag comes off easier when welding with DC than with AC which it needs a bit more persuaion. With DC if youve got cleanish steel, nice tight arc with good travel angle and dont have the amperage too high it should come of nice and easy anyway.
Where i mentioned about not obliterating an edge, if you imagine a flat outside corner, of say 12mm plates, when it comes to putting the capping passes along the edges of the prep, i find with DCEN it helps stop the beads widening too much so helps build up a little higher and generally a bit more controllable.

As far as the stick work we do on pipe, if they are butts welding a flange onto a length, or the weld between an elbow onto one length, for example, we fit the welds together, then lay the whole thing in rollers and spin with one hand whilst welding with the other. Its a speed thing as well as making life a bit easier lol. Dont have to contort yourself around the work so is easier on your back and neck.
Obviously though with the elbow for example, having ie 3m of 8" dia pipe with a flange on the end on one side, and say 2m on the other side also with a flange on the end, the most can be rolled but the second side of the elbow has to be welded in position ie 5G, and thats where the positional welding comes into it.
As far as polarity and amperage are concerned with some of the work we do are concerned though, we have weld procedures that have to be followed especially the pipe work, so its a case of follow the procedure rather than "ooh ill try it negative instead" or "ill turn the amps down a touch".

Yes different rod types play a major part on amperage settings just as much as diameter, and the flux coating thicknesses are different, but relative i guess to the amperage demands for the rod? Dont really know tbh, just one of them things lol
You might whilst running a flat fillet joint on 10mm steel.... with a 3.2 for example:
With a 6013 rutile run it in at 110 amps, with a 7018 lowhy run it at 120, but a 3.2 cellulosic be at 85 amps, or at 145 with a 3.2 iron powder (off the top of my head).

 

[Liquid Metal]

Hope thats helped a little?? This has definitely sparked a curiosity in me though as to exactly why there is a marked difference when running a rod on DC at a set amperage but with different polarities. So will have a read up and try and find something with a bit of detail that i actually fully understand lmao.
Yes its all well and good knowing which will work better for a given application. But the same as with all sorts of things, in any part of life, there is a part of me that always wants to know "why?" with most things