Cast Iron

Cast Iron is a group of iron-carbon alloys with carbon content greater than 2%, about 2 to 4% C and 1 to 3% Si .Other alloying metallic and non-metallic elements are added in order to control and vary specific properties. Besides chemical composition, other important factors which affect their properties are the solidification process, solidification rate, and subsequent heat treatments. Cast iron make excellent casting alloys, have a wide range of strength and hardness, and in most cases are easy to machine. They are alloyed to produce superior wear, abrasion, and corrosion resistance.

Classifications of Cast Iron

There are four basic types of cast iron, which can be differentiated from each other by distribution of the carbon in their microstructure. Since their chemical composition overlap (table 1), they cannot be distinguished by chemical analyses. These four basic metallurgical types are:

Table 1

Element Gray iron % White iron % Malleable iron % Ductile iron%
C 2.5 - 4 1.8 - 3.6 2.00 - 2.60 3.0 -4.0
Si 1.0 -3.0 0.5 - 1.9 1.10 - 1.60 1.8 - 2.8
Mn 0.25 - 1.0 0.25 - 0.8 0.2 - 1.00 0.10 - 1.00
S 0.02 - 0.25 0.06 - 0.20 0.04 - 0.18 0.03 max
P 0.05 - 1.0 0.06 - 0.18 0.18 max 0.10 max

White cast Iron

In white cast iron, the carbon in the molten iron remains combined with iron in the form of iron carbide or cementite, which is a hard brittle compound (fig 1) .White cast iron, is therefore hard and brittle, and shows a white crystal fracture surface. It has considerable strength and excellent wear resistance but insignificant ductility.

Gray cast Iron

In this case the carbon in the iron separates or graphitizes during solidification to form separate graphite flakes (fig 2). Graphite flakes surrounded by a matrix of either Pearlite or a-Ferrite. Exhibits gray fracture surface due to fracture occurring along Graphite plates. These cast irons have excellent machinability at hardness and good wear resistance but insignificant ductility.

Ductile cast iron

Ductile iron has its free carbon in the form of spheres instead of flakes for this reason it is sometimes referred to as nodular cast iron. Figure 3 show graphite nodules surrounded by a matrix of α-Ferrite, Bainite, or Austenite. Ductile cast iron has a good range of yield strengths along with reasonable ductility.

Malleable cast Iron

This type of cast iron has most of its carbon in the form of irregularly shaped nodules of graphite( fig 4).Malleable cast iron is first as white iron of a suitable composition. Then during an annealing treatment often called malleablizing, the graphite nucleates and grows from the cementite of the white iron to form nodule. A wide range of mechanical properties can be obtained in malleable iron by varying the annealing heat treatment. Consists of tempered Graphite in a α-Ferrite or Pearlite matrix.

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