I've used a flux cored wire for the root welding large bore schedule 40 304 SS. The internal root face had a sooty, blackened, crusty hard surface to it, not dissimilar to solar flux roots. It required a lot of grinding of the root on the outside to remove "wagon tracks" of the flux cored material before doing a hot pass. Any impurities missed in the grinding produced a material not unlike charred Swiss cheese. The contractor hadn't bothered to qualify the procedure before having us build many hundreds of feet of process piping using scores of expensive fittings, they attempted to do that post construction. I'd parted company with them by then. The flux cored wire saved a lot of argon but I think that's all it saved them.
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