Ductile, or nodular, iron contains trace amountsof magnesium which, by reacting with the sulfurand O2 in the molten iron, precipitates outcarbon in the form of small spheres. Thesespheres improve the stiffness, strength, andshock resistance of ductile iron over gray iron.Different grades are produced by controllingthe matrix structure around the graphite, eitheras-cast or by subsequent heat treatment. Ductile iron is used in applications such ascrankshafts because of its good machinability,fatigue strength, and high modulus of elasticity;in heavy-duty gears because of its high yieldstrength and wear resistance; and in automobiledoor hinges because of its ductility. Because itcontains magnesium as an additional alloyingelement, ductile iron is stronger and more shockresistant than gray iron. But although ductile Fealso has a higher modulus of elasticity, itsdamping capacity and thermal conductivity arelower than those of gray iron.
