Discussion has been on going in the forum on Direct Current welding polarity and its effects on penetration. Without regard for welding process in this argument and specific process variables, lets concider strictly SMAW for this discussion.
Welding theory, suggusts that electron flow is from neg to positive. And yes that is arguable as most things in life.
DCEP is electrode positive work negative.
With this in mind, electrons are flowing from the plate (-) to the electrode (+).
The plate having greater surface area for the electrons to escape from generate greater heat at the plate surface resulting in deeper penetration, slower melting of the electrode,and cooler droplets transfering across the arc.
DCEN is electrode negative, work positive.
With this in mind, electrons are flowing from the electrode (-) to the plate (+).
The electrode having less surface area for the electrons to escape from generates an extreme heat,more then the plate surface resulting in faster melting of the electrode,wetter weld droplets and shallower penetration.
The effects could be seen if you choose to try an experiment with a E6010, a DCRP rod.
On DCRP the rod melts in a controlled manner,minimal splatter (concidering the rod) and deep penetration.
On DCSP the rods burns uncontollable,spits and hisses like two cats in a bag and splatter is high.
So...for SMAW,DCEP gives deeper penetration then DCEN.
Again...all things being equal, and broadly but simply speaking...does this make sense for SMAW or am I blowing smoke up your butts?
That is correct. Awhile back I was going to do an experiment to see how they compare but never found the spare time to do so. I wanted to see where AC fits into the picture. May someone knows for sure. In order from deepest penetration to lowest does the order go as follows: DCEP, AC, DCEN.???? Or does DCEN have greater penetration than AC. All things equal.
Food for thought
kam
Keep in mind too that SMAW rods have a preferred polarity to achieve better pen or digging per the mfg's recommendations. Check with the mfg's recommendations for best results.
John Wright
Plasma56
Your theory sounds logical but is completely opposite of what I've been taught and read in text books for 20 odd years. That doesn't mean that what I've been taught is absolutely correct but what you have done is to inspire me to experiment on exactly what you suggested. I'll keep you posted with my results.
Curt
My question would be, why does the electrode burn back and ball up when using electrode positive for GTAW(tig)? Does not happen when you use electrode negative. The electrode will ball up when using AC because of the positive (hot) side of the alternating current.
Right or wrong?
You can tell where the heat is when you are tigging it(check out your settings when your torch is too hot to handle)
John Wright
Probally a good test would be use a rod that is capable of being run on all 3 settings being: DCEN, DCEP & AC. Then compare the results. 6010 is generally run at DCEP only, which is a deep penetrating rod. If I were going to do the test I would us maybe a 6011, which can be run on all 3 currents.
Hope this helps
kam
Where did you get the theory that flow is from Neg to Pos? By definition thats bass ackwards.
If you have a vacuum things tend to flow toward it. Isn't Neg some what like a vacuum?
Here is a theory for you. If in Rev. polarity the work having a greater area thus more electrons and all those electrons try to jump on to the end of your poor puny little pointed Tungston it may just ball up so it can have more surface area to catch all those electrons.
Have you ever checked on the melting point of Tungston?
Quoted from Lincoln's Procedure Handbook of Arc Welding, (some parenthases and all misspelling are mine).
page 1.3-2
"Negative electrons are emitted from the cathode(-) and flow along with negative ions of the plasma to the positive anode. Positive ions flow in the reverse direction.................just as in a solid conductor, the principal flow of current in the arc is by electron travel. Heat is generated in the cathode (-) area mostly by the positive ions striking the surface of the cathode (-). Heat at the anode (+) is generated mostly by the electrons. These have been accelerated as they pass through the plasma by the arc voltage, and they give up their enery as heat when striking the anode (+)."
The Procedure Handbook discribes the difference in the heat generated between the anode (+) and cathode (-) for GTA:
page 1.3-3
"the electrode (tungsten) as a cathode (negative) can use about 10 times more current without melting than used as an anode (positive). This indicates that more heat is generated at the anode."
page 5.4-5
"With DC electrode negative, approximately 70% of the heat is developed at the work (+) and 30% at the electrode (-) so the electode tip will not melt and will maintain the conical shape when used within the recommended current range."
The melting point of pure tungsten is in the neighborhood of 6000 F.
6010 was designed to run on DCEP. I don't think it's valid to compare the characteristics of the arc and metal transfer switching the polarity.
From my own observations, penetration and metal transfer with SMAW (DCEP) is due more to the characteristics of the flux, the amperage setting, and the travel speed. I have used 6013, with the same aperage setting, to demonstrate the effects of polarity. On DCEN, the metal melts quicker and the puddle is more fluid, allowing (and requiring?) a faster travel speed without having the weld skip. The faster travel speed limits penetration. This technique was/is used for high speed sheet metal welding. With 6013 on DCEP, travel speed is slower to allow the puddle to flow out, building up more weld and penetrating deeper. 6013 on DCEP is often used as a general purpose rod because of it's ease of operation.
I'd say thats a pretty good trade off.
Well,polarity sure has the old conversation moving and shaking doesn't it.I enjoy the responses and the discussion that these topics bring out and wish we could all meet, have a few high test Canadian beers and listen as the discussion goes around the table.
Maybe we should?
Suffice to say,we can beat the bushes,howl loudly and as the arc sizzles, crackles, hisses,spits sputters and pulses take comfort that we all have been burn't afew time, we enjoy the wonders of this incredible skill we attempt to master,and will keep at the trade we like best till the last droplet solidifies.
Huh?
The high test is kicking in?
Back in "88" I was involved in proceedure qualification testing for an
X-60 pipeline. We did two complete tests. On the first test the entire weld was done using DCEP The second test was DCEN for the root. The hot pass, fill and cap was completed using DCEP.
The pipe was X-60, 20", .250 wall for the mainline and .375 for the creek and road crossings. The root was 5P+ and the rest was 7010-G, HYP (Hippy Rod) Because of the light wall pipe and the tendency for the X-60 to under cut on the root we did the one test DCEN to try and eliminate the under cut with the theroy that DCEN gave less penetration. Also we weren't sure how the 798 folks were going to go on this matter and could not get an accurate answer so we did it both ways.
As it turned out the entire project was done DCEP all the way out so I don't know if the DCEN is really benificial in this case or it's an old
wives-tale. I welded API-1104 down hill for over 20 years and always used DCEP. The few times I tried DCEN on the root I wasn't really able to tell a difference.
If there are any old pipeliners out there maybe they could shed some light on this
See ya- Ted
over herein the U.K,nearly all root runs with"stick"are done electrode neg.no matter what manufactor recommendations are.As has been said earlier,twothirds of the heat is in the positive pole,the other third in the negative,so running electrode negative helps prevent excessive burn thru on poor fit ups.
by the way,when T.I.G welding,greater penetration is always to be had with electrode positive,BUT try welding like that and you'll burn off the tungsten quick as a flash!one compromise is to use a much larger tungsten than normal,to handle the increased heat or to buy a nice "square-wave" machine which allows you to alter the time of each electrical cycle[50 hz in u.k.]spent on the positive or negative!
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