which one has deeper penetration?

By Niekie3 (***)

Date 05-28-2002 19:57

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

By welder_guy2001 (***)

Date 05-28-2002 23:19

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

By kam

Date 05-29-2002 15:42

Considering all things equal (using same amps and volts), and thinking of just a SMAW application. Could you rate these three settings from least amount of penetration to most penetration. My guess would be: (from least penetration to most) AC, DCEN & DCEP. My experience is mainly with GMAW but have done alittle with all types of welding. Would like to expand my knowledge a bit. Also how do the penetration patterns differ. Alot of the rods you buy can be run with all three settings, but what is the right one?

Thanks

kam

By welder_guy2001 (***)

Date 05-29-2002 23:56

well, i don't know what rods run with all 3 polarities, but if there were a rod that ran on all 3, i'd say the ranking would be DCEN, AC, and DCEP. i only say that because when running on AC, the current is constantly switching back and forth from DCEP to DCEN 60 times/sec (or 50 times/sec in Europe and Canada). so with AC, you're getting the best of both worlds.

most rods either run on AC and DCEP or they're strictly DCEN. E7018 can run on AC and DCEP. E6010 runs on DCEP only. E6011 can run on AC and DCEP. E6013 is DCEP i think. some hard surfacing rods run on DCEN. basically, if the electrode number ends in "8" it can probably run on AC or DCEP...if it ends in "0" it's DCEP, and so on. there are charts in most general welding books that tell what rods run on what polarity

By kam

Date 05-30-2002 12:22

I might have to do a few tests see what I can find out. I will run a few beads on a flat 1/2" plate and do a cross section & etch.

Looking at a catalog from Hobart I see there are 3 types of rods which can be run at all 3 current types. These are rods with the last digit ending in 3, 4, and 7.

Rethinking my rating for penetration, I would probally go with the order you put them rather than mine. I know DCEP has greater penetration than DCEN but not really sure if AC has greater penetration than DCEN.

I'll run a few test in the next day or 2 and let you know what I find out.

By weber

Date 05-30-2002 18:06

Thanks a lot for all inputs.
As I mentioned above my question is just about SMAW.
For example, does an E6013 electrode have deeper penetration with DCRP or DCEN?
As KAM said, I think testing is the best way to know the answer.

Remember KAM that before welding of second pass, you should emerge the work piece into the water to cool it down again. Preheat increases penetration.