I missed the part about being magnetic. Based on that property, I would suspect a martensitic 400 series stainless.
From Crucible Industries, to anneal 440C, heat to 1650ºF, hold for 2 hrs. and cool slowly (<25ºF/hr) to 1200ºF, then air cool. At that rate, you can expect the initial cooling to take 18 hours.
To achieve this without a furnace, I would heat a pail of wood ash to the annealing temperature and place the bearing in the center to cool. Not perfect, but it may do the job.