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Oakesmobile
01-17-2007, 07:09 PM
I've always used c25 mix for gmaw welding, but recently a friend of mine who has a lot of experience with gmaw was astonished that I wasn't using tri-mix. I was under the impression that C25 was the recommended gas for mild steel, and that tri mix is to be used for stainless. He seemed to think that you would have fine results using tri mix on mild steel, stainless and aluminum. What gives? Could I be using tri mix for all three types of metal? Am I asking a stupid question, I mean as long as the gas in question doesn't react with the metal welded, and the gas in question shields it from oxygen, it should be approved right?

tigman
01-17-2007, 08:34 PM
Not all tri-mixes are the same. If you look up Praxair they have a gas called helistar cs that was designed for Mild Steel. I have used it and really don't like it.

Depending on what welder you have C-25 will be good for short arc and if you machine has enough output for spray transfer C-15 is pretty much industry standard and will also work for short arc too.

Mike W
01-17-2007, 11:24 PM
This is a good read on gas. http://www.weldreality.com/MIG_welding_gases.htm

Oakesmobile
01-17-2007, 11:31 PM
Well tigman, I guess I don't know the answer to which tri mix, but I had a picture in my mind of a hel/arg/co2 blend. My machine is a HH140, but I have also used C25 on a HH175, a MM210, and a MM251. Perhaps my question could be better phrased in this way; Why do certain sheilding gas blends work best with certain processes and machines, and can I buy one type of gas that will work relatively well on a variety of metals and applications. Is there a rule that describes why some gases work better than others in certain situations, and if there is (which there certainly must be, there is some sort of physics/chemistry kind of thing happening) what is it?

Broccoli1
01-17-2007, 11:35 PM
must be, there is some sort of physics/chemistry kind of thing happening) what is it?

Nope- it's magic dat's all.


:)

bigdogjoe62
01-18-2007, 09:11 AM
we sell a tri mix for steel 90%argon/7.5%co2/2.5%oxygen works good use to call it stargon:cool: most people do use the 75/25:cool:

Pumpkinhead
01-18-2007, 12:32 PM
we sell a tri mix for steel 90%argon/7.5%co2/2.5%oxygen works good use to call it stargon:cool: most people do use the 75/25:cool:

dollars to donuts, after the "magic", the Ar/CO/O ratios are real similar, just more expensive.

TRG-42
01-18-2007, 06:14 PM
Here is an old thread which goes over this topic


http://hobartwelders.com/mboard/showthread.php?p=126776

I hope this answers some of your questions

Oakesmobile
01-18-2007, 06:34 PM
Thanks, TRG-42, that did answer some of my questions, as did MikeW's url. I am under the impression that with mild steel, I'm going to get the most consistent results with a Argon Co2 mix of some sort. I am a little confused however about O2 in the mix. Isn't the purpose of the shielding gas to block O2 from the easily oxidizable excited metal? Why put it in the gas? Also, If I wanted to weld stainless, could I do so with my C25 mix? What about Aluminim? I know it's not what's recommended, but seeing as how I have a giant freakin' tank (praxair t size) of C25, what's to lose?

Mike W
01-18-2007, 07:03 PM
Oakesmobile, your C-25 won't work on aluminum. Ed recommends argon with 3% CO2 for stainless over the more costly tri-mix.

Broccoli1
01-18-2007, 07:13 PM
Oakesmobile, your C-25 won't work on aluminum. Ed recommends argon with 3% CO2 for stainless over the more costly tri-mix.

Not the Broccoli Ed:D My trimix entails Tequila:p

Clay Walters
01-18-2007, 07:21 PM
Thanks, TRG-42, that did answer some of my questions, as did MikeW's url. I am under the impression that with mild steel, I'm going to get the most consistent results with a Argon Co2 mix of some sort. I am a little confused however about O2 in the mix. Isn't the purpose of the shielding gas to block O2 from the easily oxidizable excited metal? Why put it in the gas? Also, If I wanted to weld stainless, could I do so with my C25 mix? What about Aluminim? I know it's not what's recommended, but seeing as how I have a giant freakin' tank (praxair t size) of C25, what's to lose?

Gas, wire, and materiel. The oxygen will release in-process and corrupt the weld. You'll want pure Argon.

TRG-42
01-19-2007, 10:25 AM
I am a little confused however about O2 in the mix. Isn't the purpose of the shielding gas to block O2 from the easily oxidizable excited metal? Why put it in the gas? Also, If I wanted to weld stainless, could I do so with my C25 mix? What about Aluminim? I know it's not what's recommended, but seeing as how I have a giant freakin' tank (praxair t size) of C25, what's to lose?


The purpose of shielding a welding arc ( stick / gas shielded flux cored / flux cored / MIg etc ) is to keep both Oxygen and Nitrogen ( you will actually pick up Ni porosity far easier because there is a higher concentration in the atmosphere )

The following info is truely useless trivia : O2 combines with the Carbon in the steel and formes Carbon Monoxide. As the weld metal cools the gas tries to float to the top but the bead solidifies before this can happen resulting in porosity

with nitrogen, Nitrogen is soluble in weld metal at high temperatures but evolves out as it cools. Bead solidifies before this can happen resulting in porosity

Oxygen in Sheilding Gas ( mainly cabon steels and stainless )

Although you can generally say Oxygen is bad for weld metal , SMALL additions of oxygen can be very beneficial

Small Additons of Oxygen ( 1% to 5% ) -Most popular by gas suppliers are 2% and 5%

Increase arc stability ( significant improvement )

Improve wetting and Bead shape - ie increses travel speed

Lowers transition current ( to spray weld )

Better mechanical properties ( mainly in low alloy MIg wires )
- high strengths MIG wires such as ER80-D2s ( moly ) , ER80-Ni1 ( nickel ) , ER100s ( Cr , ni , Mo ) are generally recommended to be run with 98Ar2O2 shielding

Examples of this gas are PRaxair Stargold O2 and O5, Airgas Ox2 and Ox5, Agamix 3

Oxygen Bearing Tri Mixes

Another family of carbon steel gas that containes oxygen are the Ar / CO2 / O2 blends. Typically the O2 content is about 2 % . Benefits with the O2 are similar to ArO2 blends, but in addition, there is CO2 . These gases provide the advatanges of a O2 blend but with the added ability to short arc well

This is my main carbon steel welding gas in my garage. It can spray / pulse spray / short arc very well . this blend is great on red coat primer ( to lazy to clean it off )

Examples of this gas are Praxair Stargon, BOC Argoshield universal, Airgas AB Trimix or AL Blueshield 13

Stainless

The standard spray gas for austenitic stainless is 98Ar02 . STandard short arc gas is a He based tri mix 90He7.5Ar2.5Co2

To answer your question regarding short arcing stainless with a 75Ar25CO2
- stainless weld pool will pick up Carbon from the CO2 .
- this causes sensitization where chromium carbides percipitate out of the heat effected zone . This essentially steals chrome from the solution, therefore reducinig its corrosion resistance
- most stainless fillers are "L grade" denoting low carbon. Example, ER308L . Its counterproductive to use a L grade filler and then dump carbon back into it with your shielding gas

Is this acceptable - only you can determine if this is ok for your application. For most home type applications I wouldn't worry about it . Contrary to popular belief, sensitization does NOT make the weld any more suceptible to cracking

Lastly other "bad" gases are also added to shielding gases

Hydrogen - only found in gases to weld austentic stainless. Huge increase in travel speed. Typically only 1 to 2%

Nitrogen - typically found in gases to weld all stainlesses . Only way to get a universal stainless gas that can short arc, spray etc

Oakesmobile
01-20-2007, 08:58 AM
TRG-42, thanks that finally starts to make some sense. I'm sure that there is a lot more to it, but that's a great start. ;)