Answer EKG Question 20

Q20. Cast iron is most widely used after steel in Marine Engineering. Most cast irons consist of graphite in steel like matrix. Discuss the variation of properties that may arise with reference to pearlitic greycast iron and spherical grey cast iron. Describe briefly the treatment necessary to produce these two types of Irons.

Answer:

Cast iron is produced by remelting pig iron in a cupola (a small type of blast furnace) wherein the composition of the iron is suitably adjusted. The fluidity of this material, makes it suitable for casting; other properties include; machinability, wear resistant, high compressive strength.

Simple matellargy of cast iron:-

Carbon can exist in two states, crystalline and non-crystalline. In the former state, diamond and graphite, the latter is pure carbon.
Pure iron (ferrite) is soft and ductile with considerable strength, when carbon is added to the iron it combines with it to form a hard brittle compound. This compound of iron and carbon called iron carbide or cementite (Fe,C) lies side by side with ferrite in laminations to form a structure called pearlite, so called because of its mother of pearl appearance. 

As more carbon is added to the iron, more iron carbide and hence more pearlite is formed, with a reduction in the amount of free ferrite. When the carbon content is approximately 0.9% the free ferrite no longer exists and the whole structure is composed of pearlite alone. Further increases in carbon to the iron produces free iron carbide with pearlite reduction.

The steel range terminates at approximately 2% carbon content and the cast iron range commences. Carbon content for cast iron may vary from 2% to 4%. This carbon may be present in either the form of cementite or graphite (combined or free carbon) depending upon certain factors one of which is the cooling rate. 

Grey or malleable cast iron is composed of pearlite and graphite and can be easily machined. Pearlite and cementite gives white cast iron which is brittle and difficult to machine and hence is not normally encountered in Marine work. The following diagram analyses the above in diagrammatic form.