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Author: Subject: Create a carbon free electric arc ?
metalresearcher
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[*] posted on 10-9-2020 at 10:19
Create a carbon free electric arc ?


I do sometimes (stainless) steel melting with my DC welder and carbon rods. That works well, but, even when using only a carbon cathode and the metal bath with a thick steel wire embedded in the refractory bottom using as anode, still picks up carbon from the cathode.
But when using a steel cathode the arc does not sustain.

Is there a way to make a sustainable elctric arc from a material which does not picked up by liquid steel ?
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1KEE
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[*] posted on 10-9-2020 at 10:50


Why not use a TIG (Tungsten Inert Gas) welding torch powered by your stick welder and using Argon as a shield gas? Pretty common process, look up TIG welding.

The tungsten electrode (cathode) does not need to touch the molten metal (except to initiate the arc if you don't have high frequency/high voltage starting like a "real" TIG welding machine)...

Or am I misunderstanding the question, and your problem is making an electrical connection to the molten metal without contaminating it?
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metalresearcher
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[*] posted on 10-9-2020 at 12:06


No, it is purely the vaporizing carbon which is contaminating the steel. But can I just use a tungsten stick or won't that sustain the arc ?
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[*] posted on 10-9-2020 at 12:52


Quote: Originally posted by metalresearcher  
But can I just use a tungsten stick or won't that sustain the arc ?


Hot tungsten oxidizes in air... There are two benefits to Argon, one is protecting the hot W electrode and the metal being melted from the atmosphere. The other is Ar ionizes easier than air, and has a longer arc length, allowing you to keep the electrode further away from the material being melted. The TIG torch has a ceramic cup which keeps the Ar gas and heat in place...

A TIG torch kit for a stick welder is about $100, then you will need an Ar cylinder so maybe another $150.

I maybe biased towards this process because I already have a TIG welder and Ar cylinders, so it’s hard for me to think of doing it any other way :-)

An alternative maybe RF induction heating, induction heating supplies are pretty cheap on EBay.

[Edited on 9-10-2020 by 1KEE]
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[*] posted on 10-9-2020 at 14:08


You need a minimum current and minimum voltage to sustain an arc and those minimums are a dependent the the material of the electrode and the atmosphere. Carbon electrodes have a lower minimum voltage and fluxed welding electrodes have lower minimum voltages compared to plain metal electrodes.

I have tried to use my cheap ac transformer type stick welder with plain metal electrodes to melt sand but most of the time the arc could not be maintained. That welder was difficult to get going even with fluxed electrodes. It had a low open circuit voltage and being ac also makes it harder to maintain the arc.

Eventually a purchased a solid state welder that has a much high open circuit voltage. But best of all is my plasma cutter which has a RF HT start. Using steel or copper electrodes you can start an arc with about 10mm separation and drew it out to 50mm to 100mm.

I suspect your problem is is too low an open circuit voltage particularly if it is only rectified AC. You could add a flux (salt or borax) reduce to arc voltage.

You could also preheat the electrodes. It much harder to get the arc going with cold electrodes and work piece.




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[*] posted on 10-9-2020 at 18:27


>> You need a minimum current and minimum voltage to sustain an arc and those minimums are a dependent the the material of the electrode and the atmosphere.

Yep. The Argon gas helps, it's easier to initiate and arc than in air. And maintains a longer arc under the same voltage, current and electrode conditions than air.

>> Carbon electrodes have a lower minimum voltage and fluxed welding electrodes have lower minimum voltages compared to plain metal electrodes.

Yes. The tungsten electrodes are doped with Thorium (or some rare earth, Cerium and Lanthanum seem popular for those fearing the Th) to improve thermionic emission... And ground to a point for the same reason. As the melting point of W is pretty high, the point stays reasonably sharp (unless you're running AC, in which case the bombardment of the tungsten electrode by electrons heats it up more, and it tends to melt and ball up). Since the OP mentioned the sample to be melted as the anode, and the electrode the cathode, I'm assuming he has a rectifier (DC) welder.

>> Eventually a purchased a solid state welder that has a much high open circuit voltage

Just for random trivia, the welder I have is a solid state Miller Dynasty 280DX. It runs in "stick welding" and "TIG welding mode". Here's the specs on output:

TIG mode:
Open circuit voltage 60 V
200Amps at 18.0V
280Amps at 21.2V

Stick mode:
Open circuit voltage 60 V
200Amps at 28.0V
280Amps at 31.2V

So at least on this particular machine, it seems that in Stick (flux covered electrode) mode it runs higher voltage than in TIG mode. So maybe the TIG (W electrode / Ar gas) requiresa less voltage to maintain an arc?

>> But best of all is my plasma cutter which has a RF HT start. Using steel or copper electrodes you can start an arc with about 10mm separation and drew it out to 50mm to 100mm.

Yes, for sure, the plasma cutter is way higher voltage! Like 90 to 130V DC if I remember correctly. But less current. The bad thing is less of the energy will be dissipated directly into the metal the electrons are hitting, and more into the (much longer) arc. You'll be putting more watts into heating gas and less into heating metal. But yeah, the arc length will be much longer.

>> I suspect your problem is is too low an open circuit voltage particularly if it is only rectified AC. You could add a flux (salt or borax) reduce to arc voltage.

Good idea... flux should be helpful... I have never tried melting with a TIG torch using flux, now I'm tempted to try.

>> You could also preheat the electrodes. It much harder to get the arc going with cold electrodes and work piece.

Yep. That's the advantage of a "proper" TIG welder, it has a circuit that superimposes high frequency, high voltage AC on top of the high current, low voltage, DC. Makes starting the arc super easy without touching the electrode.

In the absence of that, "scratch starting" and "lift arc" are used. Scratching is quickly scathing the electrode against the work piece and pulling away. Lift arc is when the power supply is off, the electrode is placed on the work piece, power is turned on (quickly heating up the electrode tip) then pulling it away.
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metalresearcher
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[*] posted on 10-9-2020 at 21:50


Thanks for the tip.
I have a (simple disposable) Ar cylinder for other experiments (not used yet for arc melting) and a DC inverter welder up to 200 Amp. During welding, no hum is heard and during melting, it only hums when pulling the anode further away from the metal, while the arc is still running.
When melting steel, I always use a flux (silica sand + CaO / CaCO3) which helps a lot. It works fine and, thanks to the flux, the arc is sustainable, only startup is somewhat difficult, because of cold metal / electrodes. The problem for me is absorbing C from the electrodes by the metal.

But I am considering a TIG welder for purposes where it is designed for, it makes better welds than stick welding. As a side effect I can also use it for better arc melting.

[Edited on 2020-9-11 by metalresearcher]
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[*] posted on 11-9-2020 at 14:29


>> I have a (simple disposable) Ar cylinder

If it's a disposable Ar cylinder, it's probably fairly low pressure. TIG welding usually runs 5 to 10 Liters per minute, so you will run out fairly quick... I normally use a 60 or a 150 Cubic Foot (~ 1000 or ~4000 Liters) cylinder.

>> But I am considering a TIG welder for purposes where it is designed for, it makes better welds than stick welding. As a side effect I can also use it for better arc melting.

You will like the TIG setup for melting much better than the carbon electrode.... But even a TIG torch on the stick welder should work reasonably well.
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