Hey Toddler,
I had never welded using GTAW process until this morning. But I read a lot, and I watched a fellow studen for a few minutes at weld school back in July. Anyway, I spent the weekend reading over the GTAW weld process, and what I read said to change cup size for each pass, as well as the length of tungsten. This morning, I tested on schedule 40 2" carbon pipe in and stainless in 6G position, and I asked the same question to a Journeyman Pipewelder who was testing next to me, and he said "NO," you don't have to change cup sizes for each pass. It's only a suggestion, and may or may not help, just depends on the welders ability to keep a steady hand, is what I got out of it.
Hello toddler, I would say that you are somewhat right when you say that many people will have many opinions on why they use a specific size cup, for any number of different reasons. I believe that some of that stems from the different users of the GTAW process. Different materials will require different levels of gaseous protection as they are welded, titanium is completely unforgiving when it isn't shielded properly, various stainlesses and other types of alloys which might be classified as "exotics" could be considered under a similar type of classification. In other senses, cup size can be tied to access issues relative to joint type and configuration. Many folks like to use gas lenses, certainly a personal preference of mine. Besides just the cup size and type issue, you should give consideration to the gas type(argon, helium, argon/helium mix), and flow rates(helium requires the highest flow rate and the argon/helium mixes will require slightly less and the argon will require the least of the three combinations). Cup sizes and flow rates will affect gas velocity at the opening of the cup and can affect turbulence issues or lack of shielding gas also.
You mentioned the changing of cup sizes when welding pipe and whether it is necessary. My opinion and this could be disputed by others, is that it is necessary as it is interpreted by certain folks who do complete GTAW on certain types and sizes of pipe. Changing the cup size as the weld progression extends from the root to the cap has to do with the travel of the cup as it is walked(rolled, turned, and rolled back) across the face of the weld while it is deep in the groove and as it extends out and the surface that the cup is walked on increases in width. By changing cup sizes as this process progresses it allows the operator to basically use the same hand and wrist action and not have to make significant changes in this motion. If you try to cap with the same cup that you put the root in with on a large heavy wall pipe, you would have to use significant roll of the wrist and might not be able to move far enough to cover the face of the cap. Also, if you were to try to make the root(in say a deep groove on thick wall pipe) with a large cup you would have a hard time getting consistency the way that would be possible with the much smaller cup. I am sure that there are many who have personal preferences on how they attack these different situations and that's why you will likely have so many different opinions. I believe you need to consider the specific types of welding that you will be covering and then try to narrow it down from there. If you are trying to provide for other welders in your facility you may need to consider their preferences as well as basing your choices and theirs on meeting the aesthetics and performance criteria of the affected welds. My $.02 and hopefully not adding to your confusion. Best regards, aevald
