Which Polarity? And Why

[scarecrow]

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?

Okay I put my hand up I was wrong
gave it a try on some 3mm Box DCEP just burnt holds
DCEN putdown a nice weld

DC current or direct current only flows one way that is from positive to negative' I think. so I presume the difference is where the heat is generated in the arc whether it's on + OR - side of the arc

 

[matt1978]

If you welded with 2 rods.............both on 100 amps, one on DC- and the other on DC+, you will find that the rod on DC+ will burn faster and with more agression, not really an issue on thicker sections, but the rod on DC- should be a 'tamer' arc and be easier to control where blow thru is a problem.

Why not just give it a try??

 

[zzr1200]

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

Rtb, I can understand why your , I went to college years ago and so was I, but I will try and explain as far as I can remember.

Electrons travel from the -ve electrode to the +ve electrode.

DC Welding

The arc temperature is 2000 degrees at the -ve as the electrons travel across to the +ve electrode their temperature rises to 4000 degrees, it appears to be standard practise to add the two temperatures together.

ie: 2000 deg + 4000 deg = 6000 deg total temp for DC welding

AC welding

In an AC Arc the electrons change directional flow within the arc as the -ve becomes +ve and then returns to -ve.

So it seems that the standard practise is to accept that the -ve is 2000 deg and then as the electron flow changes the +ve also becomes
-ve/cools to 2000 deg.

ie: 2000 deg + 2000 deg = 4000 deg total temp for AC welding.

There probably some cleaver bod somewhere that has measured the temps at the electrodes but I'm just trying to remember what our instructor said.

hth

 

[rtbcomp]

I understand the electrics, it's just adding the two temperatures together that seems odd.

 

[zzr1200]

I understand the electrics, it's just adding the two temperatures together that seems odd.

Maybe someone with much more knowledge, could enlighten both of us....

 

[TechnicAl]

If anyone can be bothered to look I think there is an explanation in here
http://books.google.co.uk/books?id=...X&oi=book_result&resnum=11&ct=result#PPA10,M1

 

[asmeix6g]

AHHHH polarity question is back!!! From what i understood DCEP had more heat at the workpiece and less at the rod BUT DCEN had more heat in the rod and less at the workpiece.
To me this makes sense BUT for TIG it makes sense to be the other way round??
From what i understand when rooting with stick, you used dcen because the rod has most heat therefore you dont burn a great hole in the root face also the rod can melt easier and allow you to push the root through.
BUT dcen doesnt allow enough heat into the workpiece for any other work other than thin sections. thats why dcep is used for virtually everything else it allows the workpiece to get more heat and therefore more fusion over a larger area??

Its such a confusing subject! i know when i use dcen or dcep and why i use it BUT how it works i thought i knew untill the subject was bought up on UKwelder! oh well eh

 

[rtbcomp]

At least with AC you're right half the time.

 

[asmeix6g]

HAHA nice one RTB

 

[GeorgeB]

From what i understand when rooting with stick, you used dcen because the rod has most heat therefore you dont burn a great hole in the root face also the rod can melt easier and allow you to push the root through. BUT dcen doesnt allow enough heat into the workpiece for any other work other than thin sections. thats why dcep is used for virtually everything else it allows the workpiece to get more heat and therefore more fusion over a larger area?

I will try DCEN on thin metal but the professional on the following welding site seems to be saying it's the other way around (i.e. DCEN is more likely to blow holes) so I'm still confused:

http://www.markthewelder.co.uk/id25.html

 

[irondarren]

http://www.thefabricator.com/ArcWelding/ArcWelding_Article.cfm?ID=584
the so called professionals on here think DCEP is hotter. one of them is wrong. To be honest i think mark the welder might have it wrong and has his DCEP and DCEN mixed up. some what like G brown got his figures for economic growth mixed up

 

[GeorgeB]

http://www.thefabricator.com/ArcWelding/ArcWelding_Article.cfm?ID=584
the so called professionals on here think DCEP is hotter.

Most interestingly, thefabricator says:

"Right about now, some of you are hauling to your email to tell me that I'm wrong about the penetration. Some journals say that DCEN gives better penetration. I had to look through three different journals before I found one that agreed with me. All agree that DCEP has the best weld characteristics. It also has cleaning action that DCEN doesn't have."

 

[TechnicAl]

Remember that the DC+ gives better pen when stick welding but when TIG welding DC+ will overheat the tungsten so effectively you will reduce penetration.

 

[TechnicAl]

Ive been doing a bit of reading. It really is a very complex subject but basically the only processes where you have a real choice of DC- or DC+ is Stick and Sub Arc. In both TIG and MIG using both poles is not practical.

I have found an isothermal map across an arc between a tungsten and a water cooled copper block at 200 amps and a 5mm gap. The temperature ranges from 18000 oC in the centre to 10000 oC on the outside.

The rest of it does my head in. The elenbaas-heller equation is often mentioned (whatever that is)

My view is to forget trying to work out the theory, accept the practical knowledge and dont bother trying to analyse it.

 

[malcolm]

I've had a read through the thread in an attempt to figure out what the practical knowledge was. Here's my guess:

(I'm assuming here that DCEP, DC+, or DC electrode positive is where the MMA rod, the TIG torch, or the MIG torch is connected to the terminal marked + positive. Working on that basis DCEN, DC- or DC electrode negative would be the other way around.)

MMA

We do seem to have come to some agreement about what works with arc welders. We've got no control of parameters in MMA apart from current and arc length, so the behaviour of the arc is important.

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.

gave it a try on some 3mm Box DCEP just burnt holds DCEN putdown a nice weld

AHHHH polarity question is back!!! BUT dcen doesnt allow enough heat into the workpiece for any other work other than thin sections. thats why dcep is used for virtually everything else it allows the workpiece to get more heat and therefore more fusion over a larger area??

the rod on DC- should be a 'tamer' arc and be easier to control where blow thru is a problem.

MIG

So how about for MIG? With gas it's generally run DC+ and that results in less penetration. But we've got control over voltage and wire speed (amps) to a certain extent on MIG, and the extra control might well influence yer average elenbaas-heller equation like Shenion says.

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

TIG

So how about TIG? We've more control over TIG than with any other process. We have complete control over amps, and that's completely independently of any filler that we might want to put in. So we look after the tungsten and if things are getting too hot we can turn them down.

when TIG welding DC+ will overheat the tungsten so effectively you will reduce penetration.

From that my assumption is have TIG on DC- and turn the amps down if it's too hot.

That's a summary for me to refer to as I'm constantly being confused by all this. Have I got it all right?

 

[TechnicAl]

Malcolm
If you dont bother with the whys and wherefores

MMA
DC+ most common, best pen
DC- only when there is a chance of burn thro

MIG
DC+ always

TIG
DC- always
AC for Aluminium

FCW
DC- for gasless
DC+ for gas shielded

OK there are obscure applications that can prove me wrong but this fits 99% of the time.

 

[clem dog]

Ok so now im really confused!! I really only know how to nail bits of metal together! Think ill sell my welder and stick to drills and bolts!! Do 6013 rods not state on packaging that they prefer to run on positive? Forgive me my ignorance but this is really all new!!

 

[madkayaker]

is your welder AC or DC?

 

[clem dog]

Its an oxford oil cooled welder prob ac. I just remember reading that on the packets of rods.

 

[madkayaker]

Oxford oil cooled are AC so no need to worry about polarity!