What produces grey cast iron?

Compound of iron and carbon

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Chemical compound

Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure.[4] It is a hard, brittle material,[4] normally classified as a ceramic in its pure form, and is a frequently found and important constituent in ferrous metallurgy. While cementite is present in most steels[5] and cast irons, it is produced as a raw material in the iron carbide process, which belongs to the family of alternative ironmaking technologies. The name cementite originated from the theory of Floris Osmond and J. Werth, in which the structure of solidified steel consists of a kind of cellular tissue, with ferrite as the nucleus and Fe3C the envelope of the cells. The carbide therefore cemented the iron.

Metallurgy

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In the iron&#;carbon system (i.e. plain-carbon steels and cast irons) it is a common constituent because ferrite can contain at most 0.02wt% of uncombined carbon.[6] Therefore, in carbon steels and cast irons that are slowly cooled, a portion of the carbon is in the form of cementite.[7] Cementite forms directly from the melt in the case of white cast iron. In carbon steel, cementite precipitates from austenite as austenite transforms to ferrite on slow cooling, or from martensite during tempering. An intimate mixture with ferrite, the other product of austenite, forms a lamellar structure called pearlite.

The iron-carbon phase diagram

While cementite is thermodynamically unstable, eventually being converted to austenite (low carbon level) and graphite (high carbon level) at higher temperatures, it does not decompose on heating at temperatures below the eutectoid temperature (723 °C) on the metastable iron-carbon phase diagram.

Mechanical properties are as follows: room temperature microhardness 760&#; HV; bending strength 4.6&#;8 GPa, Young's modulus 160&#;180 GPa, indentation fracture toughness 1.5&#;2.7 MPa&#;m.[8]

Pure form

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Cementite changes from ferromagnetic to paramagnetic upon heating to its Curie temperature of approximately 480 K (207 °C).[9]

A natural iron carbide (containing minor amounts of nickel and cobalt) occurs in iron meteorites and is called cohenite after the German mineralogist Emil Cohen, who first described it.[10]

Other iron carbides

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There are other forms of metastable iron carbides that have been identified in tempered steel and in the industrial Fischer&#;Tropsch process. These include epsilon (ε) carbide, hexagonal close-packed Fe2&#;3C, precipitates in plain-carbon steels of carbon content > 0.2%, tempered at 100&#;200 °C. Non-stoichiometric ε-carbide dissolves above ~200 °C, where Hägg carbides and cementite begin to form. Hägg carbide, monoclinic Fe5C2, precipitates in hardened tool steels tempered at 200&#;300 °C.[11][12] It has also been found naturally as the mineral Edscottite in the Wedderburn meteorite.[13]

References

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Bibliography

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• Crystal structure of cementite at NRL
• Hallstedt, Bengt; Djurovic, Dejan; von Appen, Jörg; Dronskowski, Richard; Dick, Alexey; Körmann, Fritz; Hickel, Tilmann; Neugebauer, Jörg (March ). "Thermodynamic properties of cementite (Fe3C)". Calphad. 34 (1): 129&#;133. doi:10./j.calphad..01.004.

• Le Caer, G.; Dubois, J. M.; Pijolat, M.; Perrichon, V.; Bussiere, P. (November ). "Characterization by Moessbauer spectroscopy of iron carbides formed by Fischer&#;Tropsch synthesis". The Journal of Physical Chemistry. 86 (24): &#;. doi:10./ja030.

• Bauer-Grosse, E.; Frantz, C.; Le Caer, G.; Heiman, N. (June ). "Formation of Fe7C3 and Fe5C2 type metastable carbides during the crystallization of an amorphous Fe75C25 alloy". Journal of Non-Crystalline Solids. 44 (2&#;3): 277&#;286. Bibcode:JNCS...44..277B. doi:10./-(81)-2.

Alloy of iron and carbon

Micrograph of grey cast iron

Gray iron, or grey cast iron, is 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.[1] It is the most common cast iron and the most widely used cast material based on weight.[2]

It is used for housings where the stiffness of the component is more important than its tensile strength, such as internal combustion engine cylinder blocks, pump 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.[3]

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Its former widespread use[clarify] on brakes in freight trains has been greatly reduced in the European Union over concerns regarding noise pollution.[4][5][6][7] Deutsche Bahn for example had replaced grey iron brakes on 53,000 of its freight cars (85% of their fleet) with newer, quieter models by &#;in part to comply with a law that came into force in December .[8][9][10]

Structure

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A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon by weight. Graphite may occupy 6 to 10% of the volume of grey iron. Silicon is important for making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides; at 3% silicon almost no carbon is held in chemical form as iron carbide. 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.[1][11] Rapid cooling partly or completely suppresses graphitization and leads to the formation of cementite, which is called white iron.[12]

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.[12][13] The presence of graphite flakes makes the grey iron easily machinable as they tend to crack easily across the graphite flakes. Grey iron also has very good damping capacity and hence it is often used as the base for machine tool mountings.

Classifications

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In the United States, the most commonly used classification for gray iron is ASTM International standard A48.[2] This orders gray iron into classes which correspond with its minimum tensile strength in thousands of pounds per square inch (ksi); e.g. class 20 gray iron has a minimum tensile strength of 20,000 psi (140 MPa). 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 strengthening, and heat treating is used to modify the matrix. Class 80 is the highest class available, but it is extremely brittle.[12] ASTM A247 is also commonly used to describe the graphite structure. Other ASTM standards that deal with gray iron include ASTM A126, ASTM A278, and ASTM A319.[2]

In the automotive industry, the SAE International (SAE) standard SAE J431 is used to designate grades instead of classes. These grades are a measure of the tensile strength-to-Brinell hardness ratio.[2] The variation of the tensile modulus of elasticity of the various grades is a reflection of the percentage of graphite in the material as such material has neither strength nor stiffness and the space occupied by graphite acts like a void, thereby creating a spongy material.

Properties of ASTM A48 classes of gray iron[14] Class Tensile
strength (ksi) Compressive
strength (ksi) Tensile modulus,
E (Mpsi) 20 22 83 10 30 31 109 14 40 57 140 18 60 62.5 187.5 21 Properties of SAE J431 grades of gray iron[14] Grade Brinell hardness t/h&#; Description G 120&#;187 135 Ferritic-pearlitic G 170&#;229 135 Pearlitic-ferritic G 187&#;241 150 Pearlitic G 207&#;255 165 Pearlitic G 217&#;269 175 Pearlitic &#;t/h = tensile strength/hardness

Advantages and disadvantages

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Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which 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.[3] Grey iron cannot be worked (forged, extruded, rolled etc.) even at temperature.

Relative damping capacity of various metals[15] Materials Damping capacity&#; Gray iron (high carbon equivalent) 100&#;500 Gray iron (low carbon equivalent) 20&#;100 Ductile iron 5&#;20 Malleable iron 8&#;15 White iron 2&#;4 Steel 4 Aluminum 0.47 &#;Natural log of the ratio of successive amplitudes

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.[12] Gray iron is generally considered easy to weld.[16] Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact and shock resistance is almost non-existent.[16]

See also

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Notes

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References

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Further reading

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