Metal Carbides
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Transcript of Metal Carbides
By Shamail Ahmad, Metallurgical Engineering Department, NED University of Engineering and Technology & Peoples
Steel Mills Ltd.
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Introduction
Classification of carbides
Interstitial Carbides
Covalent Carbides
Intermediate Carbides
Salt-Like Carbides
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M + C MC
These temperature ranges are some what 50-100 ۫ ۫ C lower when carbides are formed in Steels
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Interstitial Carbides
Intermediate Carbides
Covalent Carbides
Salt-Like Carbides
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Difference between Electronegativities of Metal and Carbon is quite large (except Cr3C2 which is a border line case)
(Atomic Radius of Carbon / Atomic Radius of Metal)<= 0.59
Bonding may be partly ionic and covalent, but primarily Metallic
High Thermal and Electrical Conductivities
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Their Compositions are often indefinite
Posses High Hardness, Melting Points and Chemical Inertness
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The carbides of these elements are also called Refractory Carbides ( for Chromium only Cr3C2 is refractory carbide)
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Group IV
Group V
Group VI
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Group VI-B carbides have variety of compositions and they mostly exist in Hexagonal and Orthorhombic Structures
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The difference between the electronegativities of Metal and Carbon is relatively small
The bonding is primarily covalent
Silicon and Boron carbides are considered to be refractory carbides
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Few metals of Group VII and VIII like Mn, Fe, Co, Ni and Cr (which is from VI) have too small radii to accommodate carbon atoms in interstitial spaces without considerable distortion of the lattice, so, they form intermediate carbides
Carbon atoms are close enough to form C-C and Carbon chains
Usually unstable chemically
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They are also called Salinic Carbides
Formed Between more Electropositive metals, normally with Group IA, IIA, IIIA elements
The difference of electronegativity >=2
The bond is at least 50% ionic
They are transparent to optical radiations and electrical insulators
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C4- ions carbides like Al4C3 and Be2C are called methanides because they are hydrolyzed to give methane
C2- ions carbide like Ca2C is called acetylides because it is hydrolyzed to give acetylene
Although covalent carbides posses high melting points but still they are not categorized as refractory carbides because they are decomposed at low temperature by dilute acids
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1. Hugh O. Pierson, Handbook of Refractory Carbides & Nitrides: Properties, Characteristics, Processing and Applications, 1st edn, William Andrew, 1997
2. George Krauss, George Roberts and Richard Kennedy, Tool Steels, 5th edn, ASM, 1998
3. Introduction and Background on Transition Metal Carbides, Chap. # 8, http://www.ropine.com/chapter8.html
4. R. Wilson, Metallurgy and Heat Treatment of Tool Steels, McGraw-Hill, London, 1975
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5. H. Brandis, E. Haberling, and H.H. Weigard, Metallurgical Aspects of Carbides in HighS peed Steels, Processing and Properties of High Speed Tool Steels, M.G.H. Wellsand L.W. Lherbier, Ed., TMS-AIME, 1980,p1-18
6. L.P. Tarasov, The Microhardness of Carbides in Tool Steels, Met. Prog., Vol 54 (No.6), 1948, p 846
7. L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, 1971
8. FD. Richardson, The Thermodynamics of Metallurgical Carbides and of Carbon in Iron, J. Iron Steel/ntf., Vol 175, 1953, p 3
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9. Wehr, M. R., Richards, J. A., Jr. and Adair, T . W., III, Physics of the Atom, Addison-Wesley Publishing Co., Reading, MA (1978)
10. March, J., Advanced Inorganic Chemistry, John Wiley & Sons, New York (1985)
11. Storms, E. K., The Refractory Metal Carbides, Academic Press, New York (1967)
12. Evans, R. C., An Introduction to Crystal Chemistry, Cambridge Univ. Press, Cambridge (1979)
13. Galasso, F. S., Structure and Properties of Inorganic Solids, Pergamon Press, New York (1970)
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14. Kosolapova, T. Ya., Carbides, Plenum Press, New York (1971)
15. Kisly, P. S., The Chemical Bond Strength and the Hardness of High Melting Point Compounds, in Science of Hard Materials, Institute of Physics Conf. Series No. 75, Adam Hilger Ltd., Bristol, UK (1984)
16. Toth, L. E., Transition Metal Carbides and Nitrides, Academic Press, New York (1971)
17. Aselage, T. L., and Tissot, R. G., Lattice Constants of Boron Carbide, J. Am. Ceramic Sot., 75(8):2207-2212 (1992)
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