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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