Welding

Is the load on it during the welding operation or is this a part being prepared for such a load?

You generally need to take the load off during the welding process. If you are reinforcing it, it is surely for a reason i.e. more load.

Half inch plate will take alot of heat to weld and will probably make the tube bend due to thermal difference between the opposite sides of the tube (unless great care is taken during the welding process in terms of tack welds, welding sequence, pre-heating and cooling). This is both a problem while the post is under load but is also a problem for the post welding behaviour of the tube.

Imperfections in the alignment of the post (due to differential shrinkage to opposite sides of the post) and inbuilt stresses due to the welding process itself will not be an easy buckling problem to solve (this assumes that the post is slender and in the case of a 12" post this would be if it is longer than about 150").

Is the post stocky i.e. less than 100" long?

Bridge builders come across this problem when needing to reinforce large components. However, it is somewhat specialised and really requires an expert and an expert company to undertake this kind of work.