Malleable cast iron is produced by controlled annealing of white iron to produce round graphite nodules. The material is ductile, tough, and has good machinability, but grindability is lessened by a tendency for wheel loading.
13.2.3 Nodular or Ductile Cast Iron
Nodular ductile cast iron has the carbon in the form of round graphite nodules but now also alloyed with numerous other elements such as silicon and magnesium. In this form, it is frequently used in automotive applications such as camshafts, crankshafts, and cylinder heads. The surface of the casting can be “chilled” by rapidly cooling it next to a metal chiller creating a hardened layer of white cast iron up to 55 HrC hardness under a softer, more ductile substrate. Again, a range of alloying elements is added to control the depth and hardness of this chilled layer. Chilled cast iron is particularly common in camshaft manufacture. Due to the highly abrasive nature of this layer combined with high productivity requirements and heat-sensitive nature of the material, chilled cast iron for this type of precision application is ground almost exclusively with CBN.
Steels are differentiated from cast iron in part by the level of carbon. Cast iron has typically 1.7 to 4.5% carbon while steels have only 0.05% to 1.5% carbon. The versatility of steel is evidenced by the enormous range of grades available selected for their processing, mechanical, electrical, magnetic, and corrosion-resistance properties. The major families of steel types are listed in Figure 13.2.