Arc physics is a field that has been rather complicated. (for me at any rate) I also believe that there has been much work in this field over the last twenty years or so. This means that what was considered a good theory a couple of years back is not necessarily the accepted theory today. In addition, there is bound to be different explanations for the same phenomenon by different authors.
In addition, there are different "governing" mechanism, to melting rates, depending on the materials being considdered and the electrode lengths or stick outs. AWS Handbook Volume 1 page 54 gives a bit of an explanation regarding the melting rates, but does not go into the base metal penetration issues.
From what I understand of this, the general metals we weld are so called "cold cathode" materials. They require a lot of energy to release electrons from the cathode. This is done at relatively low temperatures. When using these materials as the electrode, the cathode(emmiting electrons) will tend to have the most metal melted because it requires so much energy to release the electrons. This means that typically DCEN should result in higher melting rates of the electrode than DCEP.
One may then assume that the opposite should be the case for penetration, because then the greatest melting of the parent metal should occur when the parent metal is the cathode. (i.e. DCEP) I do however believe that there will not be a substantial difference because eventually all the energy ends up in the weld pool. (The metal melting off the electrode ends up in the weld puddle.) As such, there should be little difference in the penetration. (I may be missing something here.)
You may now be asking why it is that in the case of GTAW the electrode is colder when welding DCEN than DCEP? This is so because W is a thermionic material. This means that at high temperatures it readily releases electrons without the high energy consumption found in the "cold cathode" materials.
If there is someone with a better grasp of this, please help me if I have got the cat by the tail.
Regards
Niekie Jooste
well, Niekie gave a very technical explanation...I'll tell ya what I know from experience...
a lot of it depends on what kind of electrode you use. you could be using DCEP and E7018 and not get as much penetration as a self-shielded flux core wire feed welder running on DCEN. but w/ DCEP and E6010, the penetration might be about the same as w/ flux core on DCEN. now, here's another thing...a gas shielded flux core welder on DCEP penetrates more than all of the previous ones i mentioned. a GMAW machine running on DCEP has medium penetration. i'm not sure what submerged arc welding runs on, but i'd guess it's DCEP...anyway, this process penetrates the most because it uses such high amperages. and GTAW running on DCEN penetrates the least because the force of the arc on the base metal is so soft.
ofcourse, this is all assuming that you're welding with each of these processes at the correct amperages/voltages with reference to the position and thickness of base metal. theoretically, you can match the penetration of another process if you increase the amperage or voltage, but practically speaking, you wouldn't do that because the welding rod/wire would melt before it even got close to the puddle.
so, in order from least to greatest:
GTAW, GMAW, SMAW, FCAW-SS, FCAW-G, SAW