Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: May 2015

Cast iron- It is an alloy of iron, carbon and silicon and it is hard and brittle. Carbon content may be within 1.7% to 3% and carbon may be present as free carbon or iron carbide Fe3C.

In general the types of cast iron are

(a) grey cast iron

(b) white cast iron

(c) malleable cast iron

(d) spheroidal or nodular cast iron

(e) austenitic cast iron

(f) abrasion resistant cast iron.

(a) Grey cast iron- Carbon here is mainly in the form of graphite. This type of cast iron is inexpensive and has high compressive strength. Graphite is an excellent solid lubricant and this makes it easily machinable but brittle. Some examples of this type of cast iron are FG20, FG35 or FG35Si15. The numbers indicate ultimate tensile strength in MPa and 15 indicates 0.15% silicon.

(b) White cast iron- In these cast irons carbon is present in the form of iron carbide (Fe3C) which is hard and brittle. The presence of iron carbide increases hardness and makes it difficult to machine. Consequently these cast irons are abrasion resistant.

(c) Malleable cast iron- These are white cast irons rendered malleable by annealing. These are tougher than grey cast iron and they can be twisted or bent without fracture. They have excellent machining properties and are inexpensive. Malleable cast iron are used for making parts where forging is expensive such as hubs for wagon wheels, brake supports. Depending on the method of processing they may be designated as black heart BM32, BM30 or white heart WM42, WM35 etc.

(d) Spheroidal or nodular graphite cast iron- In these cast irons graphite is present in the form of spheres or nodules. They have high tensile strength and good elongation properties. They are designated as, for example, SG50/7, SG80/2 etc where the first number gives the tensile strength in MPa and the second number indicates percentage elongation.

(e) Austenitic cast iron- Depending on the form of graphite present these cast iron can be classified broadly under two headings: Austenitic flake graphite iron designated, for example, AFGNi16Cu7Cr2 Austenitic spheroidal or nodular graphite iron designated, for example, ASGNi20Cr2. These are alloy cast irons and they contain small percentages of silicon, manganese, sulphur, phosphorus etc. They may be produced by adding alloying elements viz. nickel, chromium, molybdenum, copper and manganese in sufficient quantities. These elements give more strength and improved properties. They are used for making automobile parts such as cylinders, pistons, piston rings, brake drums etc.

(f) Abrasion resistant cast iron- These are alloy cast iron and the alloying elements render abrasion resistance. A typical designation is ABR33 Ni4 Cr2 which indicates a tensile strength in kg/mm2 with 4% nickel and 2% chromium.