Yesterday I tried it again, flattened 3 bearing balls, cleaned the surfaces, welded them to a handle with mild steel.
First I thought that my fire was too oxidizing so I made sure the whole welding area was well coated with borax. I sprinkled the borax while the steel was still black so not much scale formed.
Forge welded them 3 times and I used a piece of wood as an anvil so the heat wouldn't be sucked out of the pieces so fast. But all of them came apart after hammering hard on the sides.
Maybe I should have tried to forge weld mild steel to mild steel first. I'm thinking about that the chromium from the bearings maybe messed up the welding. I should have used more steel because after one hammer blow the temperature was already too low.
I hope next time all works out.
Thanks for the advises!