Welding cast iron uses a cast iron rod and melts the base metal, fusing it into the weld. The result is a strong joint that’s part of the parent material. Brazing uses a brass or bronze filler that melts at a lower temperature than the cast iron itself, sticking to the surface without melting it.
Brazing is gentler on the part and lower-risk for cracks, but the joint is generally weaker. We choose between welding and brazing based on the part, the load, and how much heat the surrounding metal can take.