IMHO, heated air does rise because the heated air has the room to rise(air gets lighter as heated). The heat in metal, on the other hand, has nowhere to go, and not enough time to do it. So I don't think it makes to much difference, either weld up or down, the heat will just radiate out from the heat source until disipated in surrounding metal.
Ron
Ron ,I agree with some of your thinking, however you discount the effect of the hot gases that would naturally flow upward ahead of your weld preheating the material. This effect cannot happen in downward welding as the heat would rise back up over something already welded.
Jody's claim of reduced voltage required for the same penetration in upward travel would sustantiate this preheat effect.
To further simplify the issue, when you apply a torch to solder a pipe in the middle of a pipe. The solder will melt quite easily on the top of the pipe, but no so easily on the bottom of the pipe.
When you want to heat something up you apply the heat to the lowest point, Why ,cause heat rises better than it travels downward.
Feb 06, 2010 Rating
Heat rises? by: Ron
IMHO, heated air does rise because the heated air has the room to rise(air gets lighter as heated). The heat in metal, on the other hand, has nowhere to go, and not enough time to do it. So I don't think it makes to much difference, either weld up or down, the heat will just radiate out from the heat source until disipated in surrounding metal.
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