2_metallurgy_of_steel

2_metallurgy_of_steel - Design of Steel Structures Prof....

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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar Indian Institute of Technology Madras 1.2 Metallurgy of steel When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the influence of carbon on mechanical properties of iron is much larger than other alloying elements. The atomic diameter of carbon is less than the interstices between iron atoms and the carbon goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the original crystal lattice of iron. This mechanical distortion of crystal lattice interferes with the external applied strain to the crystal lattice, by mechanically blocking the dislocation of the crystal lattices. In other words, they provide mechanical strength. Obviously adding more and more carbon to iron (upto solubility of iron) results in more and more distortion of the crystal lattices and hence provides increased mechanical strength. However, solubility of more carbon influences negatively with another important property of iron called the ‘ductility’ (ability of iron to undergo large plastic deformation). The a-iron or ferrite is very soft and it flows plastically. Hence we see that when more carbon is added, enhanced mechanical strength is obtained, but ductility is reduced. Increase in carbon content is not the only way, and certainly not the desirable way to get increased strength of steels. More amount of carbon causes problems during the welding process. We will see later, how both mechanical strength and ductility of steel could be improved even with low carbon content. The iron-carbon equilibrium diagram is a plot of transformation of iron with respect to carbon content and temperature. This diagram is also called iron-iron carbon phase diagram (Fig. 1.2). The important metallurgical terms, used in the diagram, are presented below.
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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar Indian Institute of Technology Madras Fig 1.2. The iron – iron carbon phase diagram Ferrite ( α ): Virtually pure iron with body centered cubic crystal structure (bcc). It is stable at all temperatures upto 9100C. The carbon solubility in ferrite depends upon the temperature; the maximum being 0.02% at 723 o C. Cementite: Iron carbide (Fe3C), a compound iron and carbon containing 6.67% carbon by weight. Pearlite: A fine mixture of ferrite and cementite arranged in lamellar form. It is stable at all temperatures below 723 o C. Austenite ( γ ): Austenite is a face centred cubic structure (fcc). It is stable at temperatures above 723 o C depending upon carbon content. It can dissolve upto 2% carbon. The maximum solubility of carbon in the form of Fe3C in iron is 6.67%. Addition of carbon to iron beyond this percentage would result in formation of free carbon or graphite in iron. At 6.67% of carbon, iron transforms completely into cementite or Fe3C (Iron Carbide). Generally carbon content in structural steels is in the range of 0.12-
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Design of Steel Structures Prof. S.R.Satish Kumar and Prof. A.R.Santha Kumar
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2_metallurgy_of_steel - Design of Steel Structures Prof....

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