Dremel grinds......
Seriously, be dead careful and grind the race away, the heat and progressive weakening mean it will relax and start top spin before you cut through, otherwise try shattering it before you cut right through.
I was once told that electric arc welding would crystalise bearing race metal making it brittle so put a blob of wleding on the race then chisel it, but I suspect again the heat had more to do with that trick working.