DCEN, DCEP ..

It's plain English which you seem to have a problem comprehending. A 7018 is an AC or DC+ rod. not recommended for DC-, so your wise ass remark doesn't apply.

The charistics of the rod and the flux can have a great deal to do with the penetration of the weld. If you were using a bare solid electrode then DCEN would clearly give the deepest penetration and DCEP would clearly give the widest bead. (Exactly like it does with TIG).

But by adding, subtracting, varying the mix, and quantity of things like, cellouse, potassium silicate, sodium silicate, iron powder, iron-oxide, and lime-fluoride (the last for low-hydrogen) among other things, you can change the characteristics of what would be at one extreme, say a wide shallow bead to a deep penetrating bead, that would be completely different than what the polarity would suggest.

This is why there is a lot of confusion about what is deep or not deep penetration. People are basing their conclusions on what they experience, and I would bet that there is no one out there (unless he's and engineer with some company that makes welding rods) has tried every different rod out there. The reason the rod which people use may or may not do something, could very well be (and most likely is) the results of the composition of flux coating and to a lesser extent the metal in the rod itself. (Heck we haven't even got around to discussing what happens when the % of alloys is changed in a rod).

From the chart I have handy ("Welding:Principles and Practice")

E7018 DCRP, AC mild penetration very high rate deposition smooth and slightly convex bead

I guess it "sits there so even and pretty" because the flux coating is designed to make it do so

Dan

iron-oxide, and

shallow bead

experience,

engineer

something,

composition of

Summed it up nicely, Jim....

Thank you ever so much for sharing.

STeve

You're welcome!

That's old terminology which I hope will dissapear. DCEN and DCEP have obvious meanings.

As to the effects of polarity in TIG, consider:

Force on a charged partical is electric field times charge and F=ma. Integrate over the arc length and we get Potential times charge = mass times velocity = momentum. So all singly charged particles crossing the same potential get the same momentum. (Multiply charged ions change things a little but not much.) So light particles (electrons) are moving much faster than heavy ones. However energy is proportional to velocity squared so the light, fast particles carry much more energy than the heavy, slow ones. If you run your TIG DCEN, electrons flow to the work carrying far more energy than the ions flowing to the tungsten

- most of the energy goes into the work. Reverse that, DCEP, and those energetic little electrons are heating the tungsten. This is usually undesireable. :-)

Ted

Eleetrons have a negative charge. They flow from the negative to the positive. Straight polarity refers to the current flowing in the negative to positive direction

the direction of the current is where the heat will go.

the experts can correct me if i'm wrong, but this is what i know of it.

so, as example, rod + and piece - (dcep), electrons jump/flow from piece to rod... the tip of the rod will get more heat than the piece. provides a shallow broader weld.

reverse it (dcen) to get more penetration. current flow from rod to piece, more heat flow into the piece. provides a deeper narrower weld.

i believe you need to choose the correct type of rod for dcen or dcep to achieve the best weld of each mode.

think of the flow as a high speed molten metal flow... the electron flow carries velocity (direction + speed), and when the metal becomes a molten pool the electron flow can/will place force vectors into or out of the metal....

it gets slightly more complex when moving to MIG and TIG using pulsed arc of HF overlay, in AC or DC modes....

I agree with half of what you say. DCEP does get the tip of the rod hotter, so the molten filler metal is hotter. But I think the hotter filler metal provides a higher temperature puddle and more penetration.

With DCEN more heat is generated in the work piece, but the filler metal is not as hot, the puddle is not as hot, and therefore the penetration is less.

Dan

Kryptoknight wrote:

are you sure about that? in both cases the puddle should be about the same temp in the puddle, but because of the electron velocity (force vectors) either into the piece or into the rod, this determines if its a shallow broad weld or a narrower deeper weld.

this is just how i understand it, but i may be wrong.... :)

I may try some MIG welding on thin metal with both DCEN and DCEP. I think that might be the best experiment. No flux involved and the thin metal ought to show the difference is penetration best. One experiment is worth a thousand theories.

Dan

As the OP, I think I'll chime in here. The original question was only about stick welding. I'm sure that with one polarity, you get good penetration, and with the other, you get really light penetration. I now know that DCEP, or reverse polarity, gives good penetration. I've seen this in many definitive places in the last couple of days, including both the Miller and Lincoln Web sites.

Also, if I were going to run an experiment, the way I'd do it is to lay 2 pieces of 1/4x1" flat bar, each about 4" long, next to each other so their long edges butt, and run two beads, one at each end, one with each polarity. Then I'd saw out two transverse "slices" on the bandsaw, each maybe 3/8" wide, straight across the bead, keeping track of which was which, then polish the edge and do an etch with any handy acid, then rinse. The penetration will be very obvious, and this would reveal the answer. The key will be in keeping track of which is which, of course, might want to stamp the parts before slicing.

GWE

I thought some more and decided that running a test using MIG in short circuit transfer mode may not relate to stick welding. Running in spray mode would. But it might be hard to get into spray mode using DCEN.

Dan

yes, you are correct, DCEP provides a deeper weld.