EFM had shear pins installed for this reason...Using cotter pins can result in your auger snapping instead of your shear pin. Call up EFM or one of the dealers and get 5-10 shear pins delivered ASAP.
Onto the broken auger, my guess is that you sucked up something harder/larger than the coal you are supposed to be burning (maybe a rock or some ashpalt?). This caused the initial auger failure. When you welded the auger back together, if there is any offset or misalignment then the auger will be stressed at every rotation. This will take a toll on your auger, bushings, and gearbox in short order. I think you best option at this point is to get a new/good used pot auger, and a new pot bushing. Install the new pot bushing being careful to drive it to the correct depth and make sure you don't deform it, then install the new pot auger.