There's a nut welded on my countershaft and I don't like that.
How do I remove it while minimizing the chance of damage to the countershaft's threads, as damage would necessitate:
a. welding a nut on
b. splitting the cases
My suggestion is to take a Dremel tool or a angle grinder and clean out the weld, then you'll be able to back the nut off. The threads are already buggered from the welding which they were accosted with. Clean up the weld the best you can off of the shaft (and I mean really clean them up), then back the nut off, which should pull whatever debris off of them which is left. If you are planning on replacing the nut anyway, then splitting it is not a bad option, but you'll still need to cut down through the welds first, or you'll completely hose the threads in the process. Either way, once you get the nut off of there, you are going to have to chase those threads with a die, otherwise you'll most likely kill a new nut when you go to put it on.
I don't see how you are going to get around not utilizing a new nut on there.
With all of this said, there was a reason the nut was welded onto the shaft in the first place. I'd suggest to keep it from backing off (obviously??). When you go to put a new nut back on there, if you don't want it welded, I'd suggest you use some red Locktite. This will keep it on there without issue and you won't have to worry about the weld.
I agree ,grind the weld. But I will add those steels are not intended to be welded and have a significant risk of cracks in the HAZ ( heat affected zone) . So try to remove some of the HAZ, I would guess if you remove 1/8 " of metal past the fusion line for those small welds. Assuming you want to reuse any parts.