Hello Pipe 1105,
There is really no need to use the GTAW. It is advantageous to use GTAW on small bore piping ( 2" and under) but anything bigger is fine to weld entirely with E7018.
Regards,
Shane Feder
We usually recess the flange, 1/2" to 3/8" and whether you use GTAW or SMAW to weld it...depends on the WPS you are welding to. If a welder is certified (to do either process)...than like Shane says (and IF the weld spec allows) you should be able to stick weld the whole thing with 7018 or it can be welder's preference depending on the size of the pipe.
They do not ususally give us a choice......it the pipes are to be SMAW-ed (per wps) than that is what we do. On a slip-on flange, we'll run a pass of 6010 and then go with 7018 over that. We recess back 1/2" to stay away from the raised face or mating surface of the flange and still put in a nice fillet on the ID of the pipe, you know, sometimes those slip-on's have plenty of slop in em'.