Types of Cast Irons

Cast Iron
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  TYPES OF CAST IRONS Cast irons generally contain more than 2% C and a variety of alloying elements. These aregenerally classified by a rather simple and archaic system. Classificationis done on the basis ofthe appearance of their fracture surface, their microstructure and properties. There has been two class of cast irons historically, one having a gray fracture appearance and other having a white fracture appearance, named as  gray cast iron and white cast iron respectively. Those irons having both gray and white appearance are called mottled iron. It is interesting to note that these names still apply today. Over the years, other cast irons have been evolved which have their name derived from their mechanical property, such as malleable iron and ductile iron . More recently compacted graphite iron and austempered ductile iron have been introduced. There are four factors which lead to the different types of cast irons namely, the carbon content, the alloy, the impurity content, the cooling rate and the heat treatment after casting. These parameters control the composition as well as the form of parent matrix phase present. [2][3][14] Cast irons can be broadly classified into these 5 categories. 1. Gray cast irons: It is the most common type of cast iron found. It has a gray fracture surface due to high volume of graphite  flakes. Carbon in graphite form is more stable than carbide form. During cooling if it is subjected to a controlled cooling rate and adequate alloying addition then carbon gets precipitated out as graphite flakes. It has high Si content  because it promotes the formation of graphite during solidification. Gray cast irons have negligible ductility but are useful because they can be easily casted to complex shapes and are inexpensive. These also have very low impact resistance. 2. White cast iron: Rapid solidification of gray iron results in white cast iron. It has white fracture surface. Graphite flakes are not present in this type of cast irons rather; an iron carbide network is  present that gives a white fracture surface. The Si content is lower to minimize the graphitizing effect. They are hard and have excellent abrasion resistance. But they also have excessive brittleness and poor machinability. To enhance wear resistance generally Mo, Cr, Ni are added to it. 3. Mottled iron: This type of cast iron is not intentionally produced and results from a transition between gray and white cast irons. It is not necessarily a desirable material. 4. Malleable cast iron: It is produced by heat treatment of white cast iron in which the iron carbide network decomposes or breaks down into temper carbon . This process is called malleablization which involves two stages of annealing as the first stage of annealing and the second stage of annealing. Because of the absence of hard and brittle carbide phase, iron  becomes malleable. Disadvantage of this type of cast iron is its limited section thickness and prolonged annealing cycles. 5. Spheroidal graphite cast iron or ductile iron: It is produced by adopting special alloy addition and proper cooling rates so that the carbon can be converted to spherical forms which can be used in those fields where carbon in flake form or temper form can’t be used. The nodules are formed during  solidification and not during heat treatment. It can be of three types namely, ferritic,  pearlitic/ferritic, martensitic. It has excellent mechanical properties which can be  compared to steels. There is a subclass of ductile iron named as Austempered ductile iron . It has the same nodular or spherical graphite as in ductile iron but the matrix is a combination of bainite and stabilized austenite. Austempering is necessary to get this type of cast iron structure. Here graphite is present in compact form and shape of graphite is controlled by minor alloying addition. Austemperd ductile irons have excellent mechanical properties such has tensile strength, ductility and wear resistance. [2][3][14] Cast Irons and Alloy Cast Iron Cast iron is a cheap alloy. Ordinary cast iron is an alloy containing a total of up to 10% of the elements carbon, silicon, manganese, sulphur and phosphorus; the balance being iron. Alloy cast irons, contain also varying amounts of nickel, chromium, molybdenum, vanadium and copper. Graphitization Cementite (Fe 3 C) is a metastable compound, and under some circumstances it can be made to dissociate or decompose. Thus, the true equilibrium diagram for iron and carbon is not that presented in Fe-Fe 3 c phase diagram but rather as shown in Figure 11.1. Figure 11.1 extends to 100 wt% carbon such that graphite is the carbon-rich phase, instead of cementite at 6.7 wt% C. This tendency to form graphite is regulated by: 1-Composition: Graphite formation is promoted by the presence of silicon and to less degree  phosphorus, nickel and copper. If silicon content is lower than 1 wt% graphitization may not takes  place. 2-Cooling rate: Slower cooling rates during solidification favor graphitization. While higher rate of cooling during solidification tends to favor the formation of cementite. This effect is illustrated by casting a 'stepped bar' of cast iron of a suitable composition (Fig. 11.2). Here, the thin sections have cooled so quickly that solidification of cementite has occurred, as indicated by the white fracture and high Brinell values. The thicker sections, having cooled more slowly, are graphitic and consequently softer. 3-Heattreatment 2  FIGURE 11.1 The true equilibrium iron  –   carbon phase diagram with graphite instead of cementite as a stable phase. FIGURE 11.2 The effect of thickness of cross-section on the rate of cooling, and hence upon the microstructure of a grey cast iron. Types of Cast Iron 1-White Cast Iron If silicon content is lower and the cooling rates during solidification are higher, the resulting structure will contain cementite, and the fracture surface will appears bright .Since white cast iron is extremely hard and brittle is not used as such but they are made as the first step for conversion into the malleable iron. It is possible to form white cast iron structure on the surface layers of grey cat iron by chilling the surface. This is called chilled iron and is used for making wear resistance surfaces for iron rolls and  ploughs 2-Grey Cast Iron If silicon content is higher and the cooling rates during solidification are lower, complete graphitization takes place and the 3  resulting structure will contain graphite flakes only. Then it is called grey cast iron, the fracture surface may be dull and grey. The important engineering properties of grey cat iron are; 1. High compressive strength. 2. Moderate tensile strength 3. Good wear resistance 4. High damping capacity The shortcoming of grey iron is the brittleness du to the flake form of graphite which introduces sharp notches at the edges. The most important applications of cast iron are machine beds, ingot moulds, lamp spots and others. 3-Spheroidal-graphite (SG) Cast Iron Also known as nodular cast iron or ductile cast iron. In SG cast iron the graphite flakes are replaced  by spherical particles of graphite as shown in Fig. 11.6, so that the sharp stress raisers are eliminated. The formation of this spheroidal graphite is effected by adding small amounts of cerium or magnesium to the molten iron just before casting. 4-Compacted-graphite (CG) Iron The mechanical properties of this type is intermediate between those of ordinary grey flake-graphite irons and those of SG iron. The graphite flakes produced are short and stubby and have rounded edges as shown in figure 11.7. CG iron is produced when molten iron of near-eutectic composition is treated with a single alloy containing appropriate amounts of magnesium, titanium and cerium. CG has good resistance to scaling and 'growth' at high temperatures, so CG iron attractive as a heat-resisting material and it was developed srcinally for the manufacture of ingot moulds and vehicle  brake components. 5-Malleable Cast Irons The names of the two srcinal malleabilising processes, the Blackheart and the White heart, refer to the color of a fractured section after heat treatment has been completed. Another process used for 4

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Jul 23, 2017
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