There is one pro who does that. I use O2 propane to weld frames. Gas is a lot of fun for welding since the heat can be deployed differently. For instance, you can heat the hole in the donut since one doesn't have to ground the arc to get heat. Very cool for light tube welding since that kind of welding puts more of the heat in the rod from the torch, and gas makes that easier. It is easy with gas to fill holes, as another example. Of course what is "easy" is mostly of interest to the beginer since the old hands don't make mistakes and can do it all anyway.
There is also a whole religious issue about heat within welding on whether the arc from TIG is too hot and creates porocity one doesn't get with gas welding. There does seem to be some evidence to support that, though the end position is the same: It all works in this case. I does seem to be easier to seal pressure vessels with gas welding (no personal experience).
Gas is also a different paradigm as to how it handles the whole oxydization thing. Rather than excluding oxygen you play with the flame chemistry. This is a very simple approach compared to monkeying around with purge gasses etc...
Cost wise the set-up costs of gas welding are very attractive since we all need a torch for brazing or lugs anyway. Not sure what the running costs are like, I haven't put in the hours to know how quickly I am going to rip through my bottles.
Safety wise one is playing the risks of explosions off against the risk of electrocution. As someone who works alone, I do worry a little about the straight DC of my TIG. I have never had so much as a shock, but it worries me a little, because there won't be anyone around to turn me off if I plug myself into the grid. I believe my propane is safer than acet. is with all it's creepy pressure and handling issues. In both systems one has the safety issues around compressed gas, and fire. I have started a fire with TIG, and didn't notice until stuff started to get hot or stinky.
Downside wise the look of gas welds is often not all that great. Of course, there are ways around that, but naked TIG has a very mechanical perfection in the best hands, and that is hard to get with gas, or maybe I just haven't seen the same level of gas welding. Certainly the stuff from training videos in WWII looks good enough...
If weight is not a problem, gas welding is fine. However, I fail to see any advantages that make up for the problems. I have done some gas welding on lugs. I don't do enough of it to feel comfortable though. TIG and brazing are the way to go.
Both gas and Tig are a art form at least in my opinion. And I worry more when I tig with AC than with DC about getting shocks. Hot, humid, and or wet conditions when using alternating current are sometimes not a good combination. Don't ask me how I know this. In the last few years the technology in inverter based power supplies for tig welders has enabled some amazing improvements over the older tried and true transformer based units. Iv'e had a couple Sycrowave 250's and 350's over the years and currently use a Dynasty 300 that so far as held up great. Each has their place.
gas welded designs are generally made of thicker, lower strength tubing. I have never heard of gas welding a high strength steel if it's even possible. Furthermore, if I were to spend the time to learn a skill like that, I would learn tig.
Ok, I agree I have no idea what you would do to weld some super alloy, mostly from the perspective that the same rods that are used with TIG are not used with gas welding (though based on long observation of TIG threads, the "right" rods aren't always used there either).
I'm not so sure I really agree with the thickness thing, you can straight forwardly weld .6mm tubes with gas, that is business as usual with ultralight aircraft. Is there a lot of thinner welding than that gets done on bikes? Cause that isn't pushing it with gas, that is without moving down into jewelery range torch heads. For big guys like me the light tubing is out of reach anyway.
As far as skill is concerned it is the same skill for the most part, except no pedal. Some courses teach TIG by introducing gas welding first. I am not sure why they do that, except that if they are teaching all the skills, gas, MIG, stick, TIG, I suppose it makes sense to start with gas.
I do think that if one wants to start turning out welded frames for the punters it makes more sense to splash out for a Dynasty 200, than to fritz around with gas. For the Hobbyist, not so sure. Marginal cost of 75-100 bucks is pretty attractive for welding with gas. I spent more than that on the diamond wheel that sharpens my tungsten, and new it was hundreds.
I knew you'd disagree, but the fatigue issue is very real with bike frames. Whereas with aircraft there is structural redundancy, and many members just have to be there and not carry much of a load. Furthermore you can't sell a welded frame even if you could make a light enough frame with a reasonable fatigue life. Case closed as far as I'm concerned.
As a "sanctimonious Luddite", you ought to be all over this stuff! You kinda lost me though, moving on to fatigue. How did we get there? Beyond that you make a lot of sense. My main interest in framebuilding over the last few years has been how they don't do it. I am getting near to having exhausted that strand and will probably be moving on to more conventional pastures by next summer. People who want to know how it is conventionally done should probably block my posts.