3 - 5mm Steel... What Gas???

[projectsj]

Hi,

I was always under the impression that c02/Argon mix was best for welding steel.

However, Someone told me that if its not thin sheet metal (steel) that I should use pure c02 on metal thicker than 2mm. I will mostly be welding steel from 3 - 5mm thick.

Any suggestions...?

Also, What is the best wire to use for these thicknesses (3-5mm steel)? 0.6 or 0.8...?

Thanks very much in advance!

Ben.

 

[lexi]

Aint used Co2 myself but it is supposed to perform better on the thicker metals rather than the thinner. 0.8mm on those thicknesses I would say.
Alex

 

[malcolm]

0.8 definately. 0.6 is better for less than 2mm, and 1.0 above maybe 8mm. There's quite a lot of overlap in the use of the different thickness wires, but 0.8 is probably the best for what you want.

Pure CO2 has the advantage of giving slightly more penetration for the same setting, and the gas manufacturers do tend to add a large percentage of CO2 to their Argon/CO2 brews for thicker metal. They have a page that describes the different gasses: http://www.bocindustrial.co.uk/product_information/product_data/argoshield.asp

Pure CO2 can be a pain on thin stuff due to poor arc stability and lots of spatter.

 

[ralphy]

i guess i can offer my 2p worth of info in an attempt to try to provide some help.

1stly, gases such as Helium, Neon, Argon, Krypton, Xenon, Radon are known as noble gases (see periodic table, last column on the right from top to bottom type periodic table in google...). They have been called "noble" because they don't interact with any other elements / atoms we know of.

2ndly steel, no matter what form is based on iron ore + carbon. Co2 is basically a carbon gas. Steel and CO2 will interact no matter what you do to them (even if you do nothing they will intereact). You can actually see that because you will say that your weld is "dirty"..well that's because some of the CO2 molecule from your gas have been mixed with your Steel Carbon and are showing in your weld.

Some of you may be familiar with "oxide jacking" well that's CO2 and Carbon reacting in a steel environment creating carbon oxide jacking (CO)

Now the amount of interaction will depend on the steel you are using (assuming CO2 is CO2 regardless where you get it from) but in the field i work in we have high carbon steel with 100% CO2 to minimise interaction as the high carbon steel is pretty much "saturated" with carbon. (but we get oxide jacking)

CO2 is a very big molecule compared to noble gases and others in general, so i would be incline to believe that thin steel will be more prone to "Carbon poisoning" if welded with CO2 rather than thicker steel (using the same kind of steel of course)..

 

[ralphy]

malcolm, that's my kind of webpage the argoshield one!!!!

all gases being ionised during the process, the noble gas will provide a kind of funnel within which the CO2 will split giving out energy (heat) and penetration power. The extra oxygen will combine with the 2% of the mixture .. nice i like that!!!!

Ralphy

 

[weldequip]

Pure CO2 has the advantage of giving slightly more penetration for the same setting

,
I think you will find it's the other way round Malcolm. It's the Argon that has the penetration effect, being a 'hotter' gas. You will find when running a machine on straight CO2 that you have to up the amps to get the same kind of penetration as you would on Argoshield, sometimes by as much as two power settings up. The highest percentage of CO2 in a (standard) shielding gas is around 20-25%, the rest is Argon, with a touch of Oxygen. On very heavy section welding they introduce Helium (Helishield).
I'm no gases expert, BOC are the people, but from practical experience Argoshield is BY FAR the best all round gas.

 

[lexi]

Fascinating my man!
Alex

 

[Hitch]

Argoshield universal, Coogar 5, Ferromax 7 will do a nice job.
And wire 0.8mm

Helium based gasses are used for Stainless and alluminium.

 

[malcolm]

malcolm, that's my kind of webpage the argoshield one!!!!
all gases being ionised during the process, the noble gas will provide a kind of funnel within which the CO2 will split giving out energy (heat) and penetration power. The extra oxygen will combine with the 2% of the mixture .. nice i like that!!!!
Ralphy

I've never really understood what's going on with these welding gases. I assume the nice stable arc is due to the argon. Both pure argon and argon/co2 feel much the same to use, but pure argon results in weird triangular looking welds with funny bubbly looking bits at the sides, but argon/co2 gives nice rounded welds. Pure CO2 on the other hand gives a nasty arc that keeps dropping out.

And how does pure Argon conduct electricity? Being a noble gas it's surely got all the electrons it needs. Why carry any more?

What's happening?there? And why throw in a bit of CO2 and oxygen? Could understand a bit of extra heat generated by them reacting making CO, but I don't understand why it makes the weld smoother than it would be if you just threw in a few more amps?

 

[ralphy]

oh dear...geek time again...

Right take a seat cause this may take long(ish)

Ok, Argon is a noble gas, sometimes refered to as an inert gas. One of its property is that it is "electrically stable" in its normal state. However, it is possible to ionise any atom, just add energy. Here the energy is the Amps from your "coil machine" (welder). This will remove some of the Argon electron and ionise it.

Since this process is exothermic heat will be generated. Heat will also be added to the mix via the Amps as a current trough a metal wire will make it melt. All that heat increases the chances fo ionisation. So in fact you have a plasma of Argon...(if weledquip reads this he will be able to talk to you about plasma and his telly!!)

Although ionised the Argon is still quite stable, much more stable than ionised CO2 for instance.

Now, Argon on its own, even ionised doesn't really enough energy. Even the heat from the Amps isn't strong enough at an atomic level, you need a bit more help.

Welding, i am not goign to teach you anything here, is basically about melting to metal together. For that process you need a bit of oxygen. Ok this is not a combustion, but there is a need for oxygen.

Argon on its own will do the trick, but oxygen is king. Oxygen on its own is quite tricky to deal with, and consequently the easiest way to deal with it / use it is via CO2.

CO2 can easily be turned into CO + O using heat. Because of the dissociation of the molecule you have an energy surplus which is released, ie heat.

Finally, oxygen on its own is added to the mix. Well you could see that as a cleanser agent. No matter how refined your process/ tools / material are you are likely to create some "nasties" as you are welding, either by trapping gas into the weld (bubbles or oxide) or disturbing the crystal structure of the material.

Throwing more power at the weld may only result in more crystal perturbation, too much ionisation of your gas mixing in wrong location / wrong way...as for the true reason behind the shape of the weld / plasma my gutt feelign is it has to do with the gas / atom molecules 3-D shape and their relative size....

alseep yet?

Ralphy

 

[ralphy]

ok.. i knew i had it somewhere....

here it is in plain english :

"The weld depth/width ratio increases substantially as a result from the additions of oxygen to the argon shielding gas in the range of 3 000 to 5000ppm both for the 10 and 20 L/min shielding gas flow rates. When the oxygen addition contents are below 2 000 ppm or over 6 000 ppm, the weld D/W ratio decreases to approximately 0.2. The oxygen in the weld pool plays an important role as an active element affecting the Marangoni convection mode. The inward Marangoni convection occurs on the liquid pool surface when the oxygen in the weld is over 100ppm, and hence the D/W ratio increases suddenly. The thicker oxide layer on the weld pool surface is not only a barrier for the oxygen to transfer and become a solute in the weld pool, but also prevents the weld pool from moving freely, and hence changes the weld pool shape."

Easy enough, no?!!!

Ralphy

 

[weldequip]

Oh, I see it clearly now!
You been drinking?!

 

[ralphy]

ok..ok..ok..oxygen in certain concentration within your gas mix will result in your weld to have a given width but also with a given depth...if you use too much oxygen you actually create an oxide layer on your weld as you are welding and this will result in weld problem...

better?.. see as clear as plasma tellies!!!!

Ralphy

 

[malcolm]

alseep yet?

No, but I'm still quite lost. Argon getting ionised would explain why it can sustain an arc (thanks for that) but I'm not at all sure that nice stable argon changing to argon missing a bit wwould be exothermic. If that were the case surely they'd charge us even more money for the stuff. And I was fairly sure that CO plus O2 to CO2 is exothermic rather than the other way around. Otherwise we'd be killed even ehwn we do have a decent supply of oxygen to the fire. But welders have an electricity knob to play with so no matter.

Can certainly imagine a bit of oxygen coming off when you surprise CO2 with electricity. But you've lost me from there. Nasties in the weld I can quite imagine, and oxidising them as they come out might well be a good thing.

I should imagine the guys that came up with the nice shielding gases we have today did so by trial and error. Given the state of most companies I guess that's still what happens. I'm very keen to understand the science behind the gases if you can cope with my silly questions.

 

[weldequip]

 

[lexi]

ok.. i knew i had it somewhere....

here it is in plain english :

"The weld depth/width ratio increases substantially as a result from the additions of oxygen to the argon shielding gas in the range of 3 000 to 5000ppm both for the 10 and 20 L/min shielding gas flow rates. When the oxygen addition contents are below 2 000 ppm or over 6 000 ppm, the weld D/W ratio decreases to approximately 0.2. The oxygen in the weld pool plays an important role as an active element affecting the Marangoni convection mode. The inward Marangoni convection occurs on the liquid pool surface when the oxygen in the weld is over 100ppm, and hence the D/W ratio increases suddenly. The thicker oxide layer on the weld pool surface is not only a barrier for the oxygen to transfer and become a solute in the weld pool, but also prevents the weld pool from moving freely, and hence changes the weld pool shape."

Easy enough, no?!!!

Ralphy

Yeah ...The Marongoni Convection Mode.......Sounds like a religous brainwashing experience!! Good to know what goes on inside it all though.
Alex

 

[ralphy]

ok...

Excited gas = heat generated, no matter how, no matter which gas (noble or not). You may need heat to start the process up (such as electricity) but once the process has started it generates heats. So plasma, or ionised gas is well and truly excited..hot stuff baby..

Fission generates heat : that's what happens in nuclear reactor and trust me it generates bags of heat. so that's CO2 into CO + O

CO + O giving CO2 needs either pressure and temperature (or both) to be truly effective. CO + O is not fusion (which is very exothermic) and that's a totally different kettle of fish!!!

not sure i get this one though "Otherwise we'd be killed even ehwn we do have a decent supply of oxygen to the fire."

Yes, you are right, original welding gases development were trial and error. But fortunately there were some good ground rules before, for instance noble gases ok, gases such as CO2 will react with C in the material..etc..as for ratio, well 100% trial and error..science is all about trying and seeing what can be done!!!

Don't worry about the crystal bit. All materials are in fact crystal (ie 3-D assembly of atoms) each of those will have different properties (melting point, solidification point, mixing behaviour) which are influenced by the crystal shape itself.

So your gas needs to be suited to the atom of the material and its crystal form otherwise you are likely to introduce defects into the non welded bit and the weld too (crystal malformation...)

Just realised that we have hijacked "projectsj" thread...ooppss.. sorry dude!!

Ralphy

more questions?!!!

 

[weldequip]

Want a PORTAMIG?!

 

[BillJ]

Fission generates heat : that's what happens in nuclear reactor and trust me it generates bags of heat. so that's CO2 into CO + O

Now, my formal nuclear physics training stopped at A-level (although I've read plenty about it in the 25 years since then) but I do know that's not fission. Fission is the splitting of one atom into atoms of lighter elements and does release a lot of energy (ask the Japanese). CO2 -> CO + O doesn't involve any new elements. It's just a simple chemical decomposition of the CO2 molecule.

It's more likely, I'd have thought, to be 2CO2 -> 2CO + O2, or even 3CO2 -> 3CO + O3 (ozone).

But I'm neither a chemist nor a nuclear physicist.

 

[weldequip]

Who got us on to this??!