Do you need to use a flux for forge welding?
Yes, you should use flux when forge welding. Borax removes any oxidation. Oxidation causes a layer over the steel, giving you an imperfect weld between the two pieces of steel. It bonds with the oxidation, becomes viscose, and squeezes out of the joint. You might see this oozing out of your pieces, this is known as slag. It helps seal the weld.