1. Cast iron is defined as an alloy of iron with greater than 2% of carbon and usually more than 0.1% of Si. 2. Since high carbon content tends to make the cast iron very brittle, so they cannot be forged, rolled, drawn or pressed into desired shape, hence desired shape is obtained by casting hence known as “cast irons”. 3. Most commercially manufactured types are in the range of 2.5 to 4% with other elements such as Si, Mn, P, S in substantial amount. 1.1 Characteristics and advantages as compared steels or other materials 1. Cast iron has lower melting temperature (1140-1250°C) than steels (1380-1500°C), so it can be easily melted. 2. It is least expensive casting material. As all raw materials are relatively cheap – pig iron, cast iron scrap, steel scrap, lime stone, coke, and iron ore. 3. It possesses high casting properties such as high fluidity, low shrinkage, casting soundness, ease of production, and higher yield. 4. Cast iron can provide a very wide range of metallic properties ranging from a high yield to high ductility and toughness. 5. They possess a very high compressive strength, about 3 to 4 times that of its tensile strength. 6. Cast irons can be machined easily.
7. They provide high wear and abrasion resistance.
8. An important characteristic of cast iron is its high damping capacity. It is that property which permits a material to absorb vibrational stresses. The relative damping capacity of steel, nodular iron and gray iron is shown in Fig.
9. Alloyed cast irons possess high corrosion and heat resistance. 10. Ductility of cast iron is very low and it cannot be rolled or cold worked at room temp. 1.2 Types of cast irons
The best method of classifying cast iron is according 3 metallographic structure (Microstructure). There are four variables to be considered which lead to the different types of cast iron, namely (1) the carbon content, (2) the alloy and impurity content, (3) the cooling rate during and after freezing, (4) the heat treatment after casting. These variables control the condition of the carbon and also its physical form of cast iron. Gray cast iron
White cast iron
Malleable cast iron
Nodular cast iron
Mottled cast iron
Chilled cast iron
Alloyed cast iron
(a) Ni- Hard
(b) Ni- Resist
1.3 Process of Graphitization
The carbon may be combined as iron carbide in cementite, or it may exist as free carbon in graphite. The shape and distribution of the free carbon particles will greatly influence the mechanical & physical properties of cast iron. Cementite decomposes to ferrite + graphite
Fe3C 3 Fe () + C (graphite)
Factors promoting Graphitization
1. High carbon content
2. High silicon content
3. Slow cooling rate
4. Addition of certain alloying elements such as Cu, Ni, Al also promoting Graphitization, while Mn, Mo, Cr, S which limits the Graphitization. 1.4 The structure of cast iron is affected by the following factors:- a) The rate of solidification: Slow rates of solidification allow for graphite formation and castings made in sand moulds tend to solidify gray. More rapid solidification will tend to give white irons structures. Metal chills are sometimes inserted into parts of sand moulds in those areas where a high surface hardness is required. b) Amount of total carbon: Carbon is a graphitizer. With increasing carbon tendency of graphitization i.e. formation of graphite by the decomposition of cementite (Fe3C -» 3Fe + C) becomes more and hence leads; to the formation of gray cast iron. c) Amount of silicon: Silicon is a strong graphitizer and promotes graphitization i.e. decomposition of cementite to iron and graphite and hence its amount is controlled to control amount of graphitization. The amount of silicon varies from 0.5 to 3.0 % in various commercial cast irons. With lower amount of silicon, the cast iron solidifies as white and higher amount; it solidifies as gray at a moderate...
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