I intend on welding the bottom flange of a 15" x 60 plf section erected in 1907, to the top flange of a new section to create a built-up section for an existing facility.
I would do this before cutting out a piece to test.
A quick test to see if it is readily weldable is to make a one side fillet weld,7018 electrode, a 1/4" 2"x2" tab in an inconspicuous place and try to forcibly remove it.
Try first in the hard direction, hitting one time on the weld side. Then hit it from the easy side. It shouldn't break out the hard way. The weld should fail hitting from the easy side.