Microstructure of grey cast iron


For many applications it is important to know exactly what microstructure or phase fractions a steel is composed of at a certain carbon content. This ultimately necessitates a calculation. In order to carry out this, however, the entire iron-carbon phase diagram must be considered. Therefore, the complete phase diagram of the metastable system is briefly described below, before the determination of the microstructure and phase fractions is finally explained.

At higher carbon concentrations, further phase transformations occur, which lead to a different microstructure. Such ferrous materials are then no longer referred to as steels but as cast iron.

In the corresponding article on cast ironthe microstructure formation of such materials is described in more detail. In principle, however, the iron-carbon phase diagram of the metastable system ends at a carbon content of 6. Chemically, the cementite consists of three iron atoms each with an atomic mass of 56 u and one carbon atom with an atomic mass of 12 u.

Thus, the mass-related carbon content in the cementite is 6. In principle, the microstructure and phase fractions are determined by applying the lever rule. The lever arms must always be pulled to the corresponding microstructural or phase boundaries.

In the following, the microstructure and phase fractions at room temperature for an hyper- and hypoeutectoid steel will be determined as an example. In a hypoeutectoid hypoperlitic steel, the microstructure consists of ferrite and pearlite grains at room temperature.

For a steel with, for example, 0. Due to the lever rule, there is generally a linear relationship between the carbon content and the microstructure fractions. The explicit relationship is shown in a microstructure diagram below the phase diagram. After all, the microstructural component pearlite consists of a phase mixture consisting of ferrite as well as cementite.

The steel can thus also be characterized by the phase components ferrite and cementite instead of the microstructural components ferrite and pearlite. The procedure for determining the phase fractions is basically identical, but it must be noted that the lever arms must then be drawn up to the respective phase boundaries ferrite and cementite.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.

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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.

The other alloying elements added are chromium, molybdenum, copper, titanium, vanadium, etc. Based on the alloying elements added, the variation in the solidification of the cast iron and heat treatment used, the microstructure of the cast iron can vary.

Depending upon the application and the preferred mechanical properties, iron castings can be classified into the following. White cast iron When the white cast iron is fractured, white coloured cracks are seen throughout because of the presence of carbide impurities. White cast iron is hard but brittle. It has lower silicon content and low melting point. The carbon present in the white cast iron precipitates and forms large particles that increase the hardness of the cast iron.

It is abrasive resistant as well as cost-effective making them useful in various applications like lifter bars and shell liners in grinding mills, wear surfaces of pumps, balls and rings of coal pulverisers, etc. Grey is the most versatile and widely used cast iron. The presence of carbon leads to formation of graphite flakes that does not allow cracks to pass through, when the material breaks.

Instead, as the material breaks the graphite initiates numerous new cracks. The fractured cast iron is greyish in colour, which also gives it the name. The graphite flakes make the grey cast iron exhibit low shock resistance. They also lack elasticity and have low tensile strength. However, the graphite fakes gives the cast iron excellent machinability, damping features as well as good lubricating properties making them useful in many industrial applications. The graphite microstructure of the cast iron has a matrix that consists of ferrite, pearlite or a combination of two.

The molten grey iron has greater fluidity and they expand well during the solidification or freezing of cast iron. This has made them useful in industries like agriculture, automobile, textile mills, etc. Malleable cast iron is basically white iron that undergoes heat treatment to convert the carbide into graphite.

The resultant cast iron has properties that vary from both grey and white cast iron. In case of malleable cast iron, the graphite structure is formed into irregularly shaped spheroidal particles rather than flakes that are usually present in gray cast iron.

This make the malleable cast iron behave like low-carbon steel. There is considerable shrinkage that results in reduced production of cast iron as well increased costs. Malleable cast iron can be identified easily by the blunt boundaries. Ductile cast iron is yet another type of ferrous alloy that is used as an engineering material in many applications.

To produce ductile iron, small amount of magnesium is added to the molten iron, which alters the graphite structure that is formed. The magnesium reacts with oxygen and sulphur in the molten iron leading to nodule shaped graphite that has earned them the name-nodular cast iron. Like malleable iron, ductile iron is flexible and exhibits a linear stress strain relation. It can be casted in varied sizes and into varying thickness. Different Types Of Cast Iron.

How is cast iron classified? Types of cast iron White cast iron When the white cast iron is fractured, white coloured cracks are seen throughout because of the presence of carbide impurities.Gray ironor grey cast ironis a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite.

It is used for housings where the stiffness of the component is more important than its tensile strengthsuch as internal combustion engine cylinder blockspump housings, valve bodies, electrical boxes, and decorative castings. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors.

A typical chemical composition to obtain a graphitic microstructure is 2. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. A moderate cooling rate forms a more pearlitic matrix, while a fast cooling rate forms a more ferritic matrix.

To achieve a fully ferritic matrix the alloy must be annealed. The graphite takes on the shape of a three-dimensional flake.

In two dimensions, as a polished surface, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches at which stresses concentrate and it therefore behaves in a brittle manner. Grey iron also has very good damping capacity and hence it is often used as the base for machine tool mountings.

Class 20 has a high carbon equivalent and a ferrite matrix. Higher strength gray irons, up to class 40, have lower carbon equivalents and a pearlite matrix. Gray iron above class 40 requires alloying to provide solid solution strengtheningand heat treating is used to modify the matrix. Class 80 is the highest class available, but it is extremely brittle.

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These grades are a measure of the tensile strength-to- Brinell hardness ratio. Gray iron is a common engineering alloy because of its relatively low cost and good machinabilitywhich results from the graphite lubricating the cut and breaking up the chips. It also has good galling and wear resistance because the graphite flakes self lubricate. The graphite also gives gray iron an excellent damping capacity because it absorbs the energy and converts it into heat.

Gray iron also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increased fluidity when casting.

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From Wikipedia, the free encyclopedia. Retrieved Categories : Cast iron Ferrous alloys Iron. Namespaces Article Talk.In this article we will discuss about:- 1. Introduction to Cast Irons 2. Composition and Cooling Rate of Cast Irons 3. Comparison of Properties 4. Science of Development of Microstructures. As the higher carbon contents make them more brittle, industrial cast irons have carbon normally in the range of 2. Cast irons, being brittle, cannot be forged, rolled, drawn, etc.

As casting is the only and exclusive suitable process to shape these alloys, these are called cast irons. Although cast irons are inferior to steel in mechanical properties, these are superior in damping capacity, sliding quality, and resistance to wear, and of course cost.

microstructure of grey cast iron

Carbon, in cast iron, may be in the combined form as cementite, or in the free form as graphite, or both. This depends on the chemical composition including the presence of nuclei of graphite and the rate of cooling of the casting from the molten state:.

But, more the graphite formed, lower are the mechanical properties. Silicon mainly controls the form of carbon present in the cast iron. Silicon is a strong graphitiser. Depending on its content and the rate of coolingsilicon not only helps to precipitate graphite during solidification, but may also graphitise the secondary as well as eutectoid cementite. Once the graphite flake has formed, its shape cannot be changed later by any method. Silicon lowers the eutectic composition approximately by 0.

Silicon also lowers eutectoid carbon content. Depending on the silicon content and the rate of cooling, the carbon content of pearlite decreases to be as low as 0.

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For high strengths, carbon is kept on lower side to have low volume of graphite and silicon on higher side keeping a balance to get good machinability. Si Equ. Sulphur 0. Manganese 0. Manganese, thus, has indirect effect to promote graphitisation as it removes sulphur, which promotes cementite formation. When phosphorus is less than 0.

This increases the range of eutectic solidification, and thus, helps to form graphite, and improves castability even of thin and intricate sections.

Microstructure and tensile properties of grey cast iron

In Fe-C alloys, although graphite is more stable phase but cementite formation is kinetically favoured as it is easier and quicker only 6. A high rate of cooling prevents the formation of graphite at all stages from liquid to the eutectoid reaction. Table Grey iron is the cheapest and the easiest to cast to get sound castings. Compacted graphite irons have superior mechanical properties, even at elevated temperatures than grey iron, but are expensive and are not typically heat-treated.

Meehanite irons is better than grey iron but are a bit expensive. Malleable irons are difficult to cast as white ironand there is restriction about the section-size than S. These generally cost more in the final form than S. For distinguishing the irons, S. However, breathing a freshly polished surface of S. The graphitic cast irons have graphite embedded in steel matrix, i.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.

Cast Iron Types – Microstructure

Carbon C ranging from 1. Iron alloys with lower carbon content are known as steel. Cast iron tends to be brittleexcept for malleable cast irons.

With its relatively low melting point, good fluidity, castabilityexcellent machinabilityresistance to deformation and wear resistancecast irons have become an engineering material with a wide range of applications and are used in pipesmachines and automotive industry parts, such as cylinder headscylinder 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.

Cast iron was used in ancient China for warfare, agriculture, and architecture. The amounts of cast iron used for cannon required large scale production. Cast iron was also used in the construction of buildings.

Cast iron is made from pig ironwhich is the product of melting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron[4] often along with substantial quantities of iron, steel, limestone, carbon coke and taking various steps to remove undesirable contaminants. Phosphorus and sulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced.

Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2—3. If desired, other elements are then added to the melt before the final form is produced by casting. Cast iron is sometimes melted in a special type of blast furnace known as a cupolabut in modern applications, it is more often melted in electric induction furnaces or electric arc furnaces.

Cast iron's properties are changed by adding various alloying elements, or alloyants. Next to carbonsilicon is the most important alloyant because it forces carbon out of solution. A low percentage of silicon allows carbon to remain in solution forming iron carbide and the production of white cast iron. A high percentage of silicon forces carbon out of solution forming graphite and the production of grey cast iron.

Other alloying agents, manganesechromiummolybdenumtitanium and vanadium counteracts silicon, promotes the retention of carbon, and the formation of those carbides. Nickel and copper increase strength, and machinability, but do not change the amount of graphite formed. The carbon in the form of graphite results in a softer iron, reduces shrinkage, lowers strength, and decreases density. Sulfurlargely a contaminant when present, forms iron sulfidewhich prevents the formation of graphite and increases hardness.

The problem with sulfur is that it makes molten cast iron viscous, which causes defects. To counter the effects of sulfur, manganese is added because the two form into manganese sulfide instead of iron sulfide. The manganese sulfide is lighter than the melt, so it tends to float out of the melt and into the slag.

The amount of manganese required to neutralize sulfur is 1. Nickel is one of the most common alloying elements because it refines the pearlite and graphite structure, improves toughness, and evens out hardness differences between section thicknesses. Chromium is added in small amounts to reduce free graphite, produce chill, and because it is a powerful carbide stabilizer; nickel is often added in conjunction.

A small amount of tin can be added as a substitute for 0. Copper is added in the ladle or in the furnace, on the order of 0. Molybdenum is added on the order of 0. Titanium is added as a degasser and deoxidizer, but it also increases fluidity. In malleable iron melts, bismuth is added, on the scale of 0.Cast Iron is considered the most widely used metal-matrix composite from the s. Follow microstructural development in Cast Iron types to learn about graphite nucleation mechanism, lediburite formation, and eutectic and eutectoid reactions within cast iron.

Cast Iron is being used in a wide area of industry i. This wide application of cast iron types is mainly due to following three reasons.

With understanding of microstructure development, we can estimate cast iron types based on alloying elements, cooling mechanism and post-casting heat treatments. Cast iron types are. Within topic, we have mentioned all proceeding in equilibrium cooling due to equilibrium reactions followed in the phase diagram. As we have explained in the topic, as we cross the eutectic solidification line, we have Cementite and Austenite in the microstructure. From here, equilibrium generates graphite flakes in a matrix with carbon getting decreased in austenite.

This results in a microstructure comprising of Pearlite and Cementite called White cast iron. One unique characteristic of Cast Iron type is that it is only a unique unit of cast iron with carbon present in the form of carbide.

In the rest of the cast irons, carbon as a separate identity is present as well. White cast iron consists of carbides and pearlite. Carbides are extremely hard and brittle. When a crack appears, due to brittle nature, it flows straight through the material.

microstructure of grey cast iron

No secondary or minor cracks appear due to the brittle nature of carbides which results in light reflection from the fracture surface. This results in white cast iron. These carbides are a reason for high compressive strength, high hardness, and high-temperature properties. A typical microstructure of White cast iron with composition in Hypo-eutectic region is shown below.

This microstructure is of white cast iron with In this microstructure, we clearly see two phases appearing in the microstructure. While you perform etching of material, the reaction between etchant and one of a phase of cast iron happens. Etching basically indicates chemical removal of material for exposing certain microstructural features. So, here due to etching, microstructure divides into two phases. The white phase is Cementite and the black phase depicts pearlite.

The matrix of pearlite appears to be black here. Pearlite consists of ferrite and cementite. Etchant reacts with ferrite and causes the removal of ferrite. This pearlite in high-resolution microscope appears as black and white lines but, here, the black phase indicates pearlite.

An optical microscope, when light strikes the cast iron etched surface, the light comes up straight after striking cementite and deflects when it comes from pearlite. This causes cementite to appear as white and pearlite as black.

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Grey cast iron or commonly cast iron are used for large number of cast irons having graphite flakes in ferritic or Pearlitic matrix. These cast iron types are developed by equilibrium or slow cooling. Cast iron types of these types normally contain 2. In grey cast iron, graphite flakes are embedded in Pearlitic and ferritic matrix.

When crack flows through graphite and crack is initiated, they cause nucleation of countless cracks which distorts the light being reflected.Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. Fourlakidis and I. FourlakidisI. Svensson Published Materials Science.

Gray cast iron is a widely used construction material with a unique combination of properties such as very good thermal conductivity, vibration damping ability, and good machinability. The producti Save to Library. Create Alert. Launch Research Feed.

microstructure of grey cast iron

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Cast Irons: Composition and Properties | Alloys | Iron | Metallurgy

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Fracture and fracture toughness of cast irons. Comparison of mechanical properties in flake graphite and compacted graphite cast irons for piston rings. Evaluation of eutectic growth in grey cast iron by means of inverse modelling.

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