Types Of Cast Iron And Their Properties Pdf

types of cast iron and their properties pdf

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Cast Iron | Types, Advantages, Disadvantages, Uses, Properties

As cast irons are relatively inexpensive, very easily cast into complex shapes and readily machined, they are an important engineering and structural group of materials. Unfortunately not all grades are weldable and special precautions are normally required even with the so-called weldable grades. Cast irons can be conveniently grouped according to their structure which influences their mechanical properties and weldability; the main groups of general engineering cast irons are shown in Fig.

Grey cast irons contain 2. Their structure consists of branched and interconnected graphite flakes in a matrix which is pearlite, ferrite or a mixture of the two Fig. The graphite flakes form planes of weakness and so strength and toughness are inferior to those of structural steels. The mechanical properties of grey irons can be greatly improved if the graphite shape is modified to eliminate planes of weakness.

Such modification is possible if molten iron, having a composition in the range 3. This produces castings with graphite in spheroidal form instead of flakes, known as nodular, spheroidal graphite SG or ductile irons Fig. Nodular irons are available with pearlite, ferrite or pearlite-ferrite matrices which offer a combination of greater ductility and higher tensile strength than grey cast irons. By reducing the carbon and silicon content and cooling rapidly, much of the carbon is retained in the form of iron carbide without graphite flakes.

However, iron carbide, or cementite, is extremely hard and brittle and these castings are used where high hardness and wear resistance is needed. These are produced by heat treatment of closely controlled compositions of white irons which are decomposed to give carbon aggregates dispersed in a ferrite or pearlitic matrix. As the compact shape of the carbon does not reduce the matrix ductility to the same extent as graphite flakes, a useful level of ductility is obtained. Malleable iron may be divided into classes.

Whiteheart, Blackheart and Pearlitic irons. Whiteheart malleable castings are produced from high carbon white cast irons annealed in a decarburising medium. Carbon is removed at the casting surface, the loss being only compensated by the diffusion of carbon from the interior. Whiteheart castings are inhomogenous with a decarburised surface skin and a higher carbon core.

Blackheart malleable irons are produced by annealing low carbon 2. The resulting structure, of carbon in a ferrite matrix, is homogenous with better mechanical properties than those of whiteheart irons. These have a pearlitic rather than ferritic matrix which gives them higher strength but lower ductility than ferritic, blackheart irons. This depends on microstructure and mechanical properties.

For example, grey cast iron is inherently brittle and often cannot withstand stresses set up by a cooling weld. As the lack of ductility is caused by the coarse graphite flakes, the graphite clusters in malleable irons, and the nodular graphite in SG irons, give significantly higher ductility which improves the weldability. The weldability may be lessened by the formation of hard and brittle microstructures in the heat affected zone HAZ , consisting of iron carbides and martensite.

As nodular and malleable irons are less likely to form martensite, they are more readily weldable, particularly if the ferrite content is high. White cast iron which is very hard and contains iron carbides, is normally considered to be unweldable. Braze welding is frequently employed to avoid cracking.

Braze welding is often called 'Bronze welding' in the UK. Bronze welding is a varient of braze welding employing copper-base fillers, it is regulated by BS This standard has been withdrawn, but no direct replacement has been identified. As oxides and other impurities are not removed by melting, and mechanical cleaning will tend to smear the graphite across the surface, surfaces must be thoroughly cleaned, for example, by means of a salt bath.

In general, low heat inputs conditions, extensive preheating and slow cooling are normally a pre-requisite to avoid HAZ cracking. Oxy-acetylene because of the relatively low temperature heat source, oxy-acetylene welding will require a higher preheat than MMA. Penetration and dilution is low but the wide HAZ and slow cooling will produce a soft microstructure. Powder welding in which filler powder is fed from a small hopper mounted on the oxy-acetylene torch, is a very low heat input process and often used for buttering the surfaces before welding.

MMA widely used in the fabrication and repair of cast iron because the intense, high temperature arc enables higher welding speeds and lower preheat levels. The disadvantage of MMA is the greater weld pool penetration and parent metal dilution but using electrode negative polarity will help to reduce the HAZ. In oxy-acetylene welding, the consumable normally has slightly higher carbon and silicon content to give a weld with matching mechanical properties.

The most common MMA filler rods are nickel, nickel - iron and nickel - copper alloys which can accommodate the high carbon dilution from the parent metal and produces a ductile machinable weld deposit. In MIG welding, the electrode wires are usually nickel or Monel but copper alloys may be used. Flux cored wires, nickel-iron and nickel-iron-manganese wires, are also available for welding cast irons.

Powders are based on nickel with additions of iron, chromium and cobalt to give a range of hardnesses.

The potential problem of high carbon weld metal deposits is avoided by using a nickel or nickel alloy consumable which produces finely divided graphite, lower porosity and a readily machinable deposit.

However, nickel deposits which are high in sulphur and phosphorus from parent metal dilution, may result in solidification cracking. The formation of hard and brittle HAZ structures make cast irons particularly prone to HAZ cracking during post-weld cooling. HAZ cracking risk is reduced by preheating and slow post-weld cooling. As preheating will slow the cooling rate both in weld deposit and HAZ, martensitic formation is suppressed and the HAZ hardness is somewhat reduced.

Preheating can also dissipate shrinkage stresses and reduce distortion, lessening the likelihood of weld cracking and HAZ. As cracking may also result from unequal expansion, especially likely during preheating of complex castings or when preheating is localised on large components, preheat should always be applied gradually. Also, the casting should always be allowed to cool slowly to avoid thermal shock. An alternative technique is 'quench' welding for large castings which would be difficult to preheat.

The weld is made by depositing a series of small stringer weld beads at a low heat input to minimise the HAZ. These weld beads are hammer peened whilst hot to relieve shrinkage stresses and the weld area is quenched with an air blast or damp cloth to limit stress build up.

Because of the possibility of casting defects and their inherent brittle nature, repairs to cast iron components are frequently required. For small repairs, MMA, oxy-acetylene, braze and powder welding processes can all be used. This is shown schematically in figure 3. Finally, to avoid cracking through residual stresses, the weld area should be covered to ensure the casting will cool slowly to room temperature. If you would like more information on any aspect of cast irons, contact materials twi.

This Job Knowledge article was originally published in Connect, March It has been updated so the web page no longer reflects exactly the printed version. Support for SMEs. Software Products. Go to Technical knowledge Search. Login Login. Members' Portal. Weldability of materials - cast irons. Click here to see our latest technical engineering podcasts on YouTube. Material types Cast irons can be conveniently grouped according to their structure which influences their mechanical properties and weldability; the main groups of general engineering cast irons are shown in Fig.

Main groups of engineering cast irons. Grey cast irons Grey cast irons contain 2. Nodular cast irons The mechanical properties of grey irons can be greatly improved if the graphite shape is modified to eliminate planes of weakness. Microstructures of a grey cast iron and. White cast irons By reducing the carbon and silicon content and cooling rapidly, much of the carbon is retained in the form of iron carbide without graphite flakes.

Malleable irons These are produced by heat treatment of closely controlled compositions of white irons which are decomposed to give carbon aggregates dispersed in a ferrite or pearlitic matrix. Whiteheart malleable irons Whiteheart malleable castings are produced from high carbon white cast irons annealed in a decarburising medium. Blackheart malleable irons Blackheart malleable irons are produced by annealing low carbon 2.

Pearlitic malleable irons These have a pearlitic rather than ferritic matrix which gives them higher strength but lower ductility than ferritic, blackheart irons. Weldability This depends on microstructure and mechanical properties. Welding process Braze welding is frequently employed to avoid cracking. Filler alloys In oxy-acetylene welding, the consumable normally has slightly higher carbon and silicon content to give a weld with matching mechanical properties.

Weld imperfections The potential problem of high carbon weld metal deposits is avoided by using a nickel or nickel alloy consumable which produces finely divided graphite, lower porosity and a readily machinable deposit.

Repair of castings Because of the possibility of casting defects and their inherent brittle nature, repairs to cast iron components are frequently required. Repair of crack in cast iron from one side. If air arc or MMA gouging is used, the component must be preheated locally to typically degrees C.

After gouging, the prepared area should be lightly ground to remove any hardened material. Preheat the casting to the temperature given in Table 1. Butter the surface of the groove with MMA using a small diameter 2. Remove slag and peen each weld bead whilst still hot.

Fill the groove using nickel 3 or 4mm diameter or nickel-iron electrodes for greater strength. FAQ: What are the different types of cast iron which can be welded? FAQ: How do you weld cast iron? For more information please email: contactus twi.

Weldability of materials - cast irons

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As cast irons are relatively inexpensive, very easily cast into complex shapes and readily machined, they are an important engineering and structural group of materials. Unfortunately not all grades are weldable and special precautions are normally required even with the so-called weldable grades. Cast irons can be conveniently grouped according to their structure which influences their mechanical properties and weldability; the main groups of general engineering cast irons are shown in Fig. Grey cast irons contain 2. Their structure consists of branched and interconnected graphite flakes in a matrix which is pearlite, ferrite or a mixture of the two Fig. The graphite flakes form planes of weakness and so strength and toughness are inferior to those of structural steels. The mechanical properties of grey irons can be greatly improved if the graphite shape is modified to eliminate planes of weakness.

The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing. Carbon C ranging from 1. Iron alloys with lower carbon content are known as steel. Cast iron tends to be brittle , except for malleable cast irons. With its relatively low melting point, good fluidity, castability , excellent machinability , resistance to deformation and wear resistance , cast irons have become an engineering material with a wide range of applications and are used in pipes , machines and automotive industry parts, such as cylinder heads , cylinder blocks and gearbox cases. It is resistant to damage by oxidation. The earliest cast-iron artefacts date to the 5th century BC, and were discovered by archaeologists in what is now Jiangsu in China.


White Cast Iron (combined carbon Fe. Malleable Cast Iron (free carbon as irregular. Chilled Cast Iron (white cast iron at the surface. Grey Cast Iron (Flake Graphite) Spheroidal Graphite (SG) / Ductile Cast Iron / Alloy Cast Iron.


TYPES OF CAST IRONS

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Cast iron is a ferrous alloy that is made by re-melting pig iron in a capola furnace until it liquefies. The molten iron is poured into molds or casts to produce casting iron products of the required dimensions. Based on the application of cast iron, the alloying elements added to the furnace differ. The commonly added alloy elements are carbon followed by silicon.

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