Uncle Grumpy
Devout Dirtbag
You could try acetone/atf mixture.
Beat me to it!
My father restores vintage farm equipment which are the pin-up girls of Rusted Crap Quarterly and he swears by the ATF/acetone mix. I've used it to soak seized suspension bolts on car restos with success too.
I have welded a piece of steel bar across a steel BB cup - that worked but in fairness the heat from the MIG welder probably helped crack the seal and contributed to the BB cups freedom.
Grumps