Hello BarraCuda, as nosetackle said, it is basically due to cost and time. You cannot argue that GTAW welding is a much cleaner and when properly done, a much more precise type of welding, that is why a great majority of high pressure steam piping, process piping, boiler tubes, and other extreme duty applications utilize the GTAW process. At the same time they may only use it for the root pass and possibly one or two more fill passes, after that they may complete the weld out with SMAW, GMAW, or FCAW processes. In just about any piping application where internal bead configuration is a consideration due to the type of material that is contained in the system, GTAW will provide less chance of restrictions or spots where things can hang up or as with certain steam and chemical applications less possibility of internal eddy erosion. On some jobs there may be individuals that only weld in the root pass and some fill passes with GTAW and then the rest of the weld out is completed by someone else with one of the other processes. When you look at the time consideration you might compare it like this: a 6" sch.80 pipe that is welded completely with the GTAW process, as might be the case in some nuclear power plant applications, can take between 8 to 10 hours of weld time to complete one joint. The same 6" sch.80 pipe that is root passed and single fill passed with GTAW and then completed with SMAW might only require 4 to 5 hours to complete(both of these techniques when properly done would provide a sound welded joint on the pipe, yet nuclear applications might require the completely GTAW welded configuration), it doesn't take long to figure out that for the sake of economic efficiency you would want to utilize the SMAW or any of the other processes as much as possible whenever possible. In most cases engineers, codes, and normal protocol and application determines which processes will be used and for what reasons. Hope this helps with your question. Regards, aevald
BarraCuda:
One reason comes to mind that I would like to share with you. Take a WPS that has a GTAW root pass and a SMAW E7018 fill for example. The E7018 is not an open root no backing filler metal. The GTAW filler metal in the root is to provide the backing for the SMAW E7018.
The same can be said for a pipe groove weld that has a SMAW E6010 root and SMAW E7018 fill.
Regards, Donnie Mann
Actually, 7018 can be used to produce a sound root pass. As a matter of fact, atleast one state here in New England required an open root carbon pipe weld with E-7018 ALL the way out. Now, they give you the option to use either 7018 or 6010 for he root pass.
Let's just say, it can be done(with 7018) but it is not easy. Generally speaking, it is much easier to produce a sound root & Hot pass using GTAW and an E-70-S wire with the balance to be done with SMAW E7018. THe mechanical properties for either type rod are basically the same no matter which process is used and as far as time is concerned.....SMAW is faster.
I was recently on a job welding a gas line (4"-Sch-40) where we were required to pass 100% X-ray of every joint (GTAW All the way Out) we did per ASME-IX, B 31.1 while another contractor running the main had to pass 100% X-ray.........per API 1104 and he got to use SMAW with 5P (E6010) all the way out.
Go figure.
I agree 100% that you can use E7018 to make a sound root pass and I have done it. Geralds reply at the end echos my sentiments that it takes more skill.
Another reason we use GTAW for the root pass on boilers is to eliminate slag falling down inside the tubes which could raise alot of havoc with the turbine if a piece of it got into the blades or a paper machine to name just a couple.
Mike
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