camaroextra

iTrader: (10)

Why do different metals need different shielding gas? If they are all Inert why does it matter if you use argon, C-25 or tri-mix.

And why do you have to switch the polarity if your using flux-core? Why does it matter which way the current is going?

And why exactly do you need shielding gas in the first place? I know that you get a crappy weld that doesnt stick without it, but why? What in the atmosphere messes up the weld?

Im not looking for a "cuz it werks bettar" answer. I wanna know the chemistry behind these distinctions. Thanks guys!

spy2520

iTrader: (3)

i believe using inert gas prevents corrosion/rust before the weld is created. from what i've heard, its because metal at extremely high temperatures oxides pretty quick...

camaroextra

iTrader: (10)

Yeah but C-25 and tri-mix have oxygen in them. And it solidifies so quick, less than a second.

I kinda thought it had something to do with the heat igniting something in the atmosphere that tainted the weld.

gun5l1ng3r

I have started to learn how to MIG weld w/ a Lincoln 135 something, and when I would get the tip close or too far away, the weld would literally start splattering, causing a crappy weld.

As for the polarity, I think that the metal actually has a tendency to 'move' or flow in the direction of the current.

I also believed that the reason for the inert gasses was to keep the content of the sourrounding atmosphere a constant mixture. I think that when you go from c-25 or tri-mix you also go to a different material being welded than if you used the argon/CO2 mix.

Just my $.02


EDIT-> Upon further thinking, reaction do tend to happen faster in the presence of more heat, so the second poster might be right, even if C25 and tri-mix have O2 in them.

ProdriveMS

You need shielding gas because the motlen metal reacts with the air in the atmosphere. Flux core provides the shielding gas by vaporizing the flux, thereby creating the shielding gas. It's cheap but not as effective as actually flowing gas around the weld. The direction the current flows matters a lot because that determines which side of the arc recieves the most heat. You can concentrate most of the arc heat in the workpiece or in the wire depending on the polarity. I hardly ever use a mig so I'm not sure about the different type of welding gasses. I always thought that argon was expensive so you tried to get away with the least expensive gas for the metal you're welding. You could use argon for everything but it's expensive. Someone with more mig experience will probably respond.

Al

RAMPANT

iTrader: (17)

Going from memory here:

The tri mix gases are typically used with regular carbon steel with trace amounts of oxygen or in the form of CO2. This provides a small amout of oxidation which creates extra heat in the weld puddle, giving better penetration. Argon is a very dense gas which causes quenching in the weld area, creating less heat and poor penetration. It is also more expensive. CO2 BTW is not inert, but very stable since it has 8 shared electrons in its outer shell (called a stable octet). If they use a helium which is light better penetration can be had due to its lower quenching, but you need more gas to shield the puddle.

It is kind of like heat treating with water or oil. Oil is lighter so it quenches slower and water more dense and quenches faster.

With Stainless Steels you will typically see pure inert gaes being used, if it is a mix it may be of Argon and Helium combined to generate more heat.

I have not been in this feild for 10 years or more but that is what was the standard back then.

BTW for heavy weldments 1/2"+ real Fluxcore rocks and will be stronger than GMAW (Mig) any day of the week.

MoNkEyZQ8

By Fluxcore do you mean SMAW?

MoNkEyZQ8

Moving the tip closer or farther away is causing the voltage to change, the longer your stick-out, the less voltage you have, and possibly less shielding gases. If you're too close, you may be dragging the cone in the puddle, causing a "crappy weld".

Try to keep a consistant 3/8th's stick-out.

Remember, watch your electrode angle, travel speed and stick-out.

RAMPANT

iTrader: (17)

Flux cored wire. Still uses a gas but also has a center filled with a flux material. A lot of structural steel companies use it for heavy weldments. IMO MIG should not be used for anything over 3/8" if that. It is a very cold welding process. I used to work for the CWB Canadian Welding Bereau, which is similar to the AWS American Welding Society and we typically had a 75% failure rate for MIG welder tests, properly known as GMAW. Stick is SMAW and Flux core is FCAW.

Some information, there is lots out there.

http://www.thefabricator.com/ArcWeld...icle.cfm?ID=10

gun5l1ng3r

Yeah, I figured out how important the electrode angle is to welding. It is LOT harder than it looks!

Manic Mechanic

iTrader: (3)

I like a book on how to build racing cars by Carroll Smith titled “Engineer To Win”. In it he writes this passage.

"One of the critical aspects during the refinement of steel is to prevent oxidation. This is why deoxidizers are added to melts and why high-alloy steels are refined under vacuum. At ordinary temperatures, any oxygen combined with the iron in any steel will be in the form of iron oxide, which is weak, brittle and totally undesirable in steel. At elevated temperatures iron combines readily with oxygen to form iron oxides which then remain as inclusions in the cooled steel. It is therefore necessary to protect steel being welded from the oxygen in the atmosphere. In TIG and MIG welding, this protection is provided by the inert gas (argon or helium) that shields the weld area. In arc welding and in the various brazing processes, the protection is provided by the flux. In oxyacetylene fusion welding the protection is provided by the shielding action of the outer envelope of the gas flame.

HOWEVER, it is not quite that simple. In order to provide the required protection the envelope must be of the right composition. Pretty obviously, for instance, an atmosphere rich in oxidizers is going to be of no help at all in shielding the molten metal from oxidation—and that is exactly what we have with an oxidizing flame. When you see someone attempting to weld steel with even a slight excess of oxygen you are watching ignorance at work—and the creation of a burned and brittle weld."

Vernon

camaroextra

iTrader: (10)

Sweet.

Why cant I weld steel with pure Argon?

MoNkEyZQ8

TIG or MIG?

RAMPANT

iTrader: (17)

Books by Carroll Smith = GREAT RACER RESCOURCE

Rothman

iTrader: (4)

You can weld steel with pure Argon if you are doing tig. When using mig you use tri mix because helium is lighter than air, argon is heavier than air and the CO2 helps everything flow better. The mix helps the mig arc stableize and ensures that it is enveloped at all times in all positions by an inert gas. You can use just argon to mig, it can work if you keep your nozzle a lot closer to the weld. Most of the time though, you will end up with the weld bubbling out and being hollow. If you plan to use just argon for mig, don't do any vertical or horizontal position type welding. To ensure the best results stick with the flat position. I have had to mig with just argon before and just to let you know, it can be done and can work well if your very carefull.

camaroextra

iTrader: (10)

Thank you Rothman

BadBlackZ

iTrader: (1)

I have just recently been made a MCE certified welder, we use 100% carbon dioxide for FCAW (flux) and 90/10 for GMAW (hard wire). And yes you have to watch the stick out because if you are too far away there will be no shielding gas and you will end up with a weld full of porosity ( small holes all throughout the weld). And for the polarity, the weld will more of less get pulled down to the metal, at least this is what i have learned.

Kano

iTrader: (3)

fcaw- electrodes ending in 3 ,4 ,6, 7, 8, 10, 11, 13 and 14 are formulated to be used without shielding gas
1, 2 , 5, 9 and 12 require some external shielding to aid in protecting the molten metal

fcaw
dcep 1 2 3 4 6 9 12
dcen 7 8 10 11 13 14
both dcep and dcen 5

with dcen your + ions flow from the workpiece to the electrode
and your - electrons flow from the electrode to the work piece
This will penetrate deep and narrow
the heat balance would be approx 70% on the workpiece and 30% at the electrode

DCEP is the exact opposite

The shielding gas is to protect the molten metal from impurities in the air
Im not sure of the chemical composition of the impurities but like said above
can lead to discontinuities like porosity...your weld will look like a sponge
A flux will help float impurities off the metal and protect from impurities in the air

Shielding gas is also sensitive to drafts and wind which may blow the gas away leaving the weld unprotected

1fastWS6

iTrader: (3)

Interesting thread! I've been MIG, TIG, gas and stick welding for almost 20 years and didn't know much of the science behind why certain gasses are used and stuff. I just learned recently that you can get better penetration using FCAW.

Craig

2002_Z28_Six_Speed

iTrader: (19)

CANAM doesn't exist anymore, unfortuntely. I like his books.