Graphitization
A specialized form of selective corrosion in cast iron where the metallic iron matrix is dissolved, leaving behind a porous, interconnected graphite skeleton while the component loses its mechanical strength.
Graphitization is a problem that mainly occurs when processing cast iron, if the part gets too hot or is heated for too long. The carbon in the structure transforms, the material loses strength and becomes brittle. When welding, you have to make sure not to overheat the workpiece and let it cool down slowly and evenly afterwards. If you don't control the heat input, you'll get hard, cracked spots or the structure will break away under the weld. Keep the heat-affected zones small and let the part cool in air or in an insulating environment after welding, then it won't happen to you.