If you are bit nervous using the grinder here's a reasonable way of doing it;
With suitable protecion,
Get one of the already mentioned slitting discs (welding suppliers do them)
Cut part way through the stuck race in line with the axis of the axle and bearing race.
Next get an assistant to either support race and axle with something imovable and hard like a dolly on blocks of wood
Get them to wedge a hammer underneath it (not the safest of ideas)
Then use a sharp cold chisel in the groove and hit it with another hammer it should shatter, however, if heat has been applied up to cherry red then you may have annealed it so it could take a bit more of a whacking to crack it.
New boat is here, very happy!