Why are so many different gases used for welding?

In an episode of Mythbusters, they mention ordering methane from the welding supply. I thought welding was done with just oxygen and acetylene for gas welding, and one of
the noble gases for arc/plasma welding. A quick search reveals over a dozen different gases are used for welding. And while most of them are of the neutral, non-reactive type, there are at least 3 different fuels (acetylene, propane, proylene).

Knowing nothing about welding (or any sort of fabrication, for that matter), I’m curious why welders need* so many choices, especially for non-fuel gases.

  • by need, I mean there are enough welders buying a particular gas that it’s economically viable for a welding store to keep them in stock.

Oxygen Methane welding is used for Aluminum (although not popular and oxygen/hydrogen is preferred). Aluminum needs lower temperature - especially when welding sheets.

For the non fuel gases (Nitrogen, Argon,) they are used for an inert atmosphere. If you are welding Stainless steel for a pressure part, you will need Argon and TIG welding.

The combustible gases are used for welding as well as cutting. Here are somethings that goes into deciding what fuels / inerts to pick :

1> Metal type - Stainless, carbon steel, Aluminum, etc.
2> Thickness of the metal
3> Weld strength (penetration, fusing, heat affected zone, etc.)
4> Weld Aesthetics

Propane is often used with cutting torches because it’s less expensive than acetylene. The cutting results from the oxidization of hot metal with oxygen, the fuel gas is needed to initially heat the metal prior to cutting and to keep it hot enough to oxidize. The fuel gas can be cut off entirely and the cutting will continue with just oxygen. Propylene is also used for cutting and provides a little more heat than propane. Carbon dioxide is also used as a shielding gas.

Propane burns in air more cleanly than acetylene does. Although you can fix that by using a more complex system, (or by using oxygen), propane is generally a better choice for just heating up metal.

Is it possible that “welding supply” shops sell some gases that aren’t used for welding, just because they have become the de facto supplier of specialty gases?

What I mostly see are gas suppliers carrying other welding supplies because they do steady business with welders.

Others are giving good insight, but to sum up-

Welding gases are used for two things -

  1. As fuel (methane, acetylene, propane, propylene, etc?)
  2. As a shielding gas where the heat is generated via electricity. (Argon, CO2, Helium, etc?)

I’m not very familiar with the first point, but I can speak with some knowledge about the second-

Think of welding as trying to maintain a fire at a set level indefinitely. Every little change in atmospheric conditions (humidity/wind gusts/local oxygen content) will cause the your fire to vary. Obviously you would be in better control of the fire if you could have it indoors in a room where you carefully controlled those parameters. That is in effect what the use of shielding gas accomplishes.

That said, you accomplish various things by using various shielding gases/combinations. I was going to wax poetic about the various effects; but I’ll just say using the same settings on the same machine with a different gas mixture DEFINITELY changes the way the puddle behaves/penetrates/ and finishes, and point you to this page to for a pretty good overview of which gases are used where-

Shielding Gases

Also, to blow your mind, in Stick/ARC/SMAW welding, the controlled atmosphere is provided by the flux burning off as you weld. Gas in powdered form, via chemistry.

specifically why do welders need to have access to argon, helium and nitrogen (and xenon?). If you need an inert gas for some purpose why not use the cheapest one? (nitrogen).

Nitrogen is only so inert. Molecular nitrogen is pretty nonreactive, but inside a welding arc you may reach temperatures where there is enough monatomic nitrogen around to cause problems. In this form nitrogen is extremely reactive.

The noble gasses don’t have this problem. Xenon is expensive enough that you would never consider it. About 250 times that of argon.

Acetylene is a great fuel gas, but has a couple of down sides. It is unstable, and requires some care in handling. Cylinders of acetylene don’t contain the compressed gas (this would be closer to a bomb than a useful storage mechanism). Rather the cylinder contains cardboard soaked in acetone, into which the acetylene dissolves. This all makes it a more expensive proposition. On the up side, that triple bond across the middle holds a a lot of energy, and you can get a very hot flame. Hotter than is easily obtained with methane or propane.

Different gases have different properties that influence the welding arc as well as the behaviour of the melt pool. Gases differ in their ionisation energy, heat capacity, mobility, etc. and can change e.g. the temperature of the arc, the size/spread of the arc leading to differing power density distributions available for the process. They differ in their ability to provide shielding from the surrounding atmosphere. They influence the surface tension of the melt pool surface and will lead to different flow patterns within the melt pool and the appearance of the weld surface by changing the wetting behaviour and the contact angle of the molten material at the interface with the solid material.

Certain steels also must be preheated before welding, i.e. A514. So welders will bring out propane torches to bring the parts to temperature.

Wow. I didn’t realize the chemistry of it was so complex. I’d kind of assumed it was more about technique and accumulated skill than abstract knowledge (the kind you can get from a book). Thanks for enlightening me.

Is any inert gas other than nitrogen or argon ever used? While in principle helium is even more inert than argon, you’d be extremely hard-pressed to find any situation where argon would be too reactive, and argon is one of the cheapest of all gases (it’s the third-most-abundant gas in the atmosphere, after only nitrogen and oxygen).

And one other advantage to propane is that you can get a blowtorch that uses propane much more cheaply and easily than equipment for acetylene or any other fuel gas. You probably won’t be able to weld with it, but there’s still a lot you can do with a blowtorch.

I don’t know why, but helium improves the weld quality by better penetration in non-ferrous metals. Most shielding gases are mix of argon/helium or argon/helium/CO2.

Yes - Helium (hence Heliarc), and CO2, for MIG welding. CO2 may not be a true inert gas, but it’s close enough for most welding.

Besides welding there is brazing and soldering which are done with open flame on metals that melt too low of temperature to be electric welded. Such as copper pipe for heating systems, tin working, brass and bronze fittinhgs. I have also seen CO2 used for the inert welding gas.