AIT Welder 2nd Period Practice Test

Graphite form in cast iron determines how it behaves under stress, balancing strength and ductility. In ductile or nodular cast iron, elements such as magnesium cause carbon to appear as spheroidal nodules rather than flakes. These round graphite nodules deflect cracks and blunt their growth, reducing stress concentrations and allowing more plastic deformation before failure. That combination gives high tensile strength along with good ductility, making ductile iron ideal for parts that must withstand impact and bending. By contrast, grey cast iron has flake graphite that concentrates stress and makes the material brittle; white cast iron is hard but very brittle due to cementite; malleable cast iron is heat-treated to form graphite clusters and offers better ductility than gray iron, but not to the same extent as nodular iron. Therefore, the nodular graphite in ductile cast iron is what grants its noted strength and ductility together.