Remove worn tip by using a torch or air arc gouger to cut the weld taking care not to cut into the holder. Grind the remainder of the weld off the knife holder.
Tack weld the new tips onto the holders. Take care to align the tips at the correct angle.
Preheat the tips to 300° – 400° F. Confirm this temperature using a heat crayon or 1/16 inch 50/50 solder will also melt at this temperature. Set it up so you have a couple of knives preheating while you are welding one.
Weld the tips using 7018 rod or equivalent mig wire. As soon as you are finished welding each knife place it in a steel container and cover with fiberglass insulation so they cool gradually to shop temperature.