Temperature in SMAW arc - anode/cathode - heat caused by? 1

[kingnero]

I'm having a discussion at work with a chemist.

What is the hottest side of an SMAW arc?

If you weld DCEN, this means the elektrode is negative = cathode.
Ions (with a relative large mass) jump from the workpiece into the elektrode, thereby causing heat due to the collision, and at the same time elektrons jump from the elektrode towards the workpiece.

The anode is the hottest point in an arc (correct?), but why exactly? Because the velocity of the electrons is higher when the collision takes place? Because of the larger mass of the ions, shouldn't they release more energy (heat) during the collision?

Another point: when MIG/MG welding, the highest deposition rate is when using DCEN, which is contradictory because the wire isn't the hottest part in the arc...


Anyone knows his chemistry enough to explain this in detail?


btw, excuse for the poor language, I don't know all the correct terms for this subject...

 

[unclesyd]

Conventional thinking says that the Anode is the hottest mainly due to Cathode Cooling.

Here are two papers that discuss the temperature of the welding arc. The first is general while the second gets into more detail.

http://eagar.mit.edu/EagarPapers/Eagar109.pdf

http://canteach.candu.org/library/20053426.pdf

From memory here is a book that goes into more detail than the papers.

Principles of Welding: Processes, Physics, Chemistry, and Metallurgy
RW (Messler)

There is another book "Physics of Welding" that covers the same information.

 

[Duwe6]

Bottom line is, observation has proven that Reverse - DCEP puts the 'heat on the rod' Straight - DCEN puts the 'heat on the stuff' meaning the base metal. That is why most SMAW - stick - welding is done Reverse, and all GTAW - tig - is done Straight.

For amusement, have a welder run his 'Tig Rig' on Reverse: both the tungsten rod and the side of the ceramic cup will evaporate in a few seconds.

Theory is wonderful, but sooner or later you have to go to the field and "ask the material" what the real answer is.

 

[kingnero]

Thx for your replies.

unclesyd, neither of the two links really explain what I was asking for, however they've certainly been interesting and I've printed them out for future reference. Thanks again.

Duwe6: about the same remark: I aprreciate your comments, however after being "in the field" (and a former welder) for about 10 years, I've seen the answers but I'd like to know - for arguments sake - what exactly causes the heat.

Which is (according to me, at least) either the mass of the ions, or the speed of the elektrons, that account for the greater energy emission. However I'm no chemist, yet merely interested in the arc mechanism.

 

[unclesyd]

Here are two more papers on the heat generated in the welding arc. To get a little deeper into arc temperatures on approach is to checkout Ohmic heating of the Plasma.

http://www.sut.ac.th/engineering/Metal/pdf/MetJoint/02_Heat f

low%20in%20welding.pdf

http://iopscience.iop.org/0022-3727/43/2/022001/pdf/0022-3727_

43_2_022001.pdf

 

[cloa]

Best to copy and paste the whole links.
"

http://iopscience.iop.org/0022-3727/43/2/022001/pdf/0022-3727_

43_2_022001.pdf"

"

http://www.sut.ac.th/engineering/Metal/pdf/MetJoint/02_Heat f

low%20in%20welding.pdf"

 

[racookpe1978]

If you weld DCEN, this means the elektrode is negative = cathode.
Ions (with a relative large mass) jump from the workpiece into the elektrode, thereby causing heat due to the collision, and at the same time elektrons jump from the elektrode towards the workpiece.

I think you're missing a minus sign: electrons are themselves negative, so they flow "opposite" current flow - one of the few things Franklin got wrong as he developed electrical theory back in the 1770's. Ions, on the other hand, are positive - they are the actual pieces of metal that were ionized and are traveling "towards" the molten mass as plasma. These ions ARE the deposited metal in the future weld.

So, in SMAW welding, if the "ions" went to the rod as in the quote above, your weld would be smaller and the rod larger as welding progressed. This doesn't happen, and the deposited metal from the rod ends up cooling from a plasma (at the rod tip) into a molten metal mass slowing cooling as the base metal surface - the base metal gets heated BY the inbound plasma.

On a TIG rig, the plasma is between the tungsten tip arcing over to the base metal, which melts the base metal. So the plasma is the hottest part of anything. The rod is fed (usually manually) into the molten pool and the rod metal melts. This further cools the melted pool and the base metal.

 

[kingnero]

lets take stick welding, DCEN:
electrons are negative and get repulsed in the arc by the (-) rod, and going to the (+) part of the current circle, which is the base metal.

That's what I always have understood, and this is apparently incorrect.

I do see your point about the ions, which are indeed the mass of the particles, and which go to build up as weld metal.
I fail to see however why they would "want" to go to the (+) part of the current circle (as pos. and pos. repulse each other)...

I'm totally confused now. The more I read about it, the more things I read that contradicts, depending on source.

 

[XL83NL]

From what Ive learned during my IWT course;

In the arc, one will find gas ions and electrons. The gas in the arc is both thermally and electrically in balance.
There's an electric voltage between the two electrodes, which creates an electrical current and, because of that, an electrical field.
The relative heavy ions will hit the cathode with a lot of impact; the cathode will be heated now so much, that new electrons will
be emitted/released from the cathodes surface. This is thermal emission.
These electrons move in the arc's gas towards the anode, and are accelerated by the electric field.
The quick moving electrons, when heading towards the anode, will then at their turn, bump into neutral gas atoms,
which are sensitized then too, and will start moving too. This reaction will cause a lot of movement in the arc of atoms, ions and electrons.
The electrons which collide onto the anode release their energy in the form of heat; this will make the temperature rise

If you take a look at the profile of standard TIG [GTAW] arc (see attach), you'll notice the highest temperatures are found close
to the electrode (or cathode in this case). However, the most heat is created at the anode. It's the same principle as a lighter, I guess (correct me If im wrong).


This is just my 2 pennies. I know the theory in the IWT-course is pretty simplified, and at some points inconsistent,
but it helped me during the course to understand a lot of things.
Hopefully this helps in stead of making things more difficult.


Now, another thing to remember is that if one uses basic electrodes (basic as in chemically basic, basoid, or alkaline, dunno the right word),
like e.g. for welding high strength carbon steel, the story is different, since basic electrodes contain CaF2, which creates more heat on the cathode.

 

[unclesyd]

The preview shows these links are complete, not truncated, but a posted bu cloa]/b] and again you may still have to cut and paste.

http://www.sut.ac.th/engineering/Metal/pdf/MetJoint/02_Heat flow in welding.pdf

http://iopscience.iop.org/0022-3727/43/2/022001/pdf/0022-3727_43_2_022001.pdf

 

[kingnero]

The links work, thanks for that.

@ XL83NL: I've had the same notes (done IWE in Belgium), however that still doesn't explains the full story...

The chemist I'm having this discussion with says the transfer of the molten metal takes part in the thermal cycle, but this far only the gas-ions and -electrons have been explained.

The thermal emision, which appears to be the greatest heat zone, is at the cathode, thus the (-) part of the arc. Whereas it is generally accepted that the (+) is the hottest part.
(Hence DCEP welding for surface reconditioning, and DCEN for penetration/root passes)

Also, the gas used makes for a great difference: Helium for instance will affect the penetration depth, CO2 will react further in the arc, stick welding will also differ, as you stated, depending on the used coating.

 

[XL83NL]

@ kingnero,

indeed, as I mentioned, the reasoning and explanations in IWT (and also probably IWE) courses is pretty minimal. perhaps unclesyd's links tell a better/full story, but I havent read those.