Cast irons 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. . 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. ) 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 ilicon, the cast iron solidifies as white and higher amount; it solidifies as gray at a moderate cooling rate. d) Amount of phosphorus: Phosphorus is also a strong graphitizer like silicon, and content varies from 0. 1 to 0. 3 %. Most of the phosphorus combines with iron and forms phosphide (Fe3P). This iron phosphide separates out as eutectic mixture with cementite and austenite. This ternary eutectic of iron phosphide, cementite and austenite is called Steadite. Steadite has a freezing temperature of about 980°C and is the last to solidify and therefore occupies interdendritic regions.
A relatively small percentage of phosphorus produces large volume of steadite and hence with higher amount of phosphorus, the steadite areas may merge to form a continuous network around the primary dendrites of austenite. Steadite is brittle and therefore, it reduces toughness and increases the brittleness of cast irons. Due to this, the amount of phosphorus must be carefully controlled to obtain optimum mechanical properties. However, phosphorus increases the fluidity of cast irons and makes them easy to cast into thin and complex sections.
Increasing silicon and phosphorus have almost similar effect as increasing the carbon on the microstructures of cast irons. Their effect in terms of carbon is considered and equivalent carbon is found out as below: Equivalent carbon = Total carbon + 1/3 (Silicon + Phosphorus) i. e. E. C = T. C + 1/3 (Si + P) e) Amount of sulphur: Sulphur combines with iron and forms iron sulfide (FeS) which is a hard and brittle compound. Due to its low melting point, it appears at interdendritic regions in a solidified casting and increases the brittleness of casting.
Addition of manganese reduces the detrimental effect of sulphur. Sulphur has greater affinity for manganese than for iron and hence, in the presence of manganese, the reaction product is manganese sulphide (MnS) instead of iron sulphide (FeS). MnS appears as small and widely distributed inclusions of rounded or polyhedral shape. Unless MnS is present in large amount, it has little effect on the properties of cast iron. The usual sulphur content cast iron is between 0. 06 to 0. 12%. Also sulphur in the form of FeS promotes the formation of iron carbide without participating in its formation.
It has a strong effect as carbide stabilizer and about sulphur is sufficient to neutralize the graphitizing influence of 0. 15% silicon. However sulphur is present as MnS, it has almost no influence on carbide or graphite formation. Sulphur increases the possibility of cracking at elevated temperatures called as red shortness. High Sulphur content reduces fluidity and responsible for presence of blow holes. f) Amount of Manganese: The most important effect of manganese is to reduce the brittleness likely to be introduced due to the formation of iron sulphide.
It takes sulphur by forming manganese sulphide. Any excess amount of manganese present after combining with all sulphur and forming MnS serves as an useful alloying element. The amount of manganese in any commercial cast iron varies between 0. 5 to 1. 0% (5 to 8 times the amount of sulphur). In addition to the above elements, cast irons may contain alloying elements nickel, chromium, molybdenum, magnesium, copper, aluminum, boron, etc. which are to obtain the desired properties and structure. d) The effect of heat treatment: The prolonged heating of a white iron will cause graphitization to occur.
This phenomenon is used as the basis for the production of malleable irons. Graphite is less dense than cementite and, if cementite decomposes into ferrite and graphite during service, this change will be accompanied by a reduction in the density of the iron, and a corresponding increase in dimensions. This phenomenon is termed the growth of cast irons, and irons for high-temperature service must be in a fully graphitized state before being put into use. Maurer Diagram: Presence of relative amounts of various elements in cast irons greatly influences their structure for the given conditions of casting.
The extent of graphitization or chill depth depends on the amount of graphitizing elements, particularly silicon, present in the cast iron with its equivalent carbon i. e. relative amounts of silicon and carbon determine whether a cast iron will contain cementite, graphite, or both as shown in Fig. In region I, cementite is stable and the structure is that of white cast iron. In region II, there is sufficient silicon to cause graphitization of all the cementite except the eutectoid cementite (i. e. the cementite in pearlite).
These results in gray cast iron with pearlitic matrix, In region III, the large amount of silicon promotes the decomposition of all the cementite and results in the formation of ferrite and graphite, giving a gray cast iron with ferritic matrix. In region IIa, the structures are typical of mottled cast iron and IIb, the matrix is pearlitic-ferritic. The properties of cast irons not only depend on the amount of graphite and type of matrix but also depend on the shape, size, and distribution of graphite present in the cast iron. 1. 4 White Cast Iron
