If you're using a small welder (especially a 120v one), preheating the weld area with a torch (even a propane one) will help. Another thing that can help is to grind all the old weld material off and put a 1/4" chamfer on the spindle, then do a multi pass weld.
Edit: upon looking a little closer at the picture, it does appear that the base metal failed rather than the weld. Rewilding that piece back in to the same hole is more likely to fail again without reinforcement.