IRON CARBON PHASE DIAGRAM

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A PRESENTATION WHICH GIVES A BASIC IDEA ABOUT THE IMPORTANCE OF PHASE DIAGRAM IN UNDERSTANDING THE MICRO& MACRO STRUCTURE OF STEEL, CAST-IRON & SO ON..........

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IRON-CARBON PHASE DIAGRAM:

BY VENU MANOHAR R IRON-CARBON PHASE DIAGRAM

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INTRODUCTION. IRON-CARBON PHASE DIAGRAM. PLAIN CARBON STEEL & CAST IRON. HEAT TREATMENT OF STEELS. CONCLUSION. REFERENCES.

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IRON –CARBON PHASE DIAGRAM Crystal structure- The atoms of liquid metal during solidification arrange themselves into a systematic pattern called a crystal structure. Unit cell- Unit cell is the basic building block of crystal structure and defines the crystal structure by virtue of its geometry and the atom positions within. Classification of unit cells: Simple cubic structure. Body centered cubic structure. Face centered cubic structure.

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Simple cubic structure: Body centered cubic structure:

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ex: Cr, W, Fe (  ), Tantalum, Molybdenum Face centered cubic structure: ex: Al, Cu, Au, Pb, Ni, Pt, Ag.

IRON CARBON DIAGRAM:

IRON CARBON DIAGRAM

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α-Ferrite - solid solution of C in BCC Fe Stable form of iron at room temperature. The maximum solubility of C is 0.022 wt% at 723 °C Transforms to FCC γ-austenite at 912 °C γ-Austenite - solid solution of C in FCC Fe The maximum solubility of C is 2.11 wt % at 1148 °C . Transforms to BCC δ-ferrite at 1394 °C Is not stable below the eutectic temperature (727 ° C) unless cooled rapidly.

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δ-Ferrite solid solution of C in BCC Fe The same structure as α-ferrite Stable only at high T, above 1394 °C Melts at 1538 °C. Cementite : These are extremely hard in nature. Cementite increases gradually with increase in carbon%. Pearlite: Its combination of about 87% of ferrite & 13% of cementite.

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Plain carbon steels Low carbon steels: They contain less than 0.3% of carbon. Its popularly known as “mild steel”. They are very soft & ductile, can be easily machined & easily welded. Due to low carbon content they are “unresponsive” to Heat treatment. Medium carbon steels: carbon content range is of 0.3 to 0.55%. Its popularly called “Machinery steel”. They are used for making shafts, rail axels, gears etc.

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High carbon steel: carbon content range is 0.55 to 1.6%. They are called Hard steels or Tool steels. They have high tensile strength & harder than other steels. They are used in making of cutting tools, leaf & helical springs etc.

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Cast Iron Alloy of iron & carbon containing more than 2.11% of carbon. Cast iron are classified on the basis of distribution of carbon content in their micro-structure. They include: Grey cast iron: Formed when the carbon content in the alloy exceeds the amount that can be dissolved. Carbon precipitates & remains as “Graphite Flakes”.

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White cast iron: formed when most of the carbon content in the alloy forms iron carbide & they are no graphite flakes. Ductile cast iron: Also called nodular or spheroidal graphite cast iron. In ductile irons, the carbon is in the form of spherical nodules rather than flakes , thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name.

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Heat treatment of steel Controlled heating & cooling of made either of plain or alloy steel, for the purpose of changing their structure in order to obtain certain desirable properties like hardness, strength or ductility. Major heat treatment processes include : Normalising. Annealing . Quenching. Tempering.

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Normalising Normalising involves heating the steel to about 40 o C above its upper critical limit and is then cooled in air . The structure produced by this process is pearlite or eutectoid steels . Because the steel is cooled in air the process results in a fine pearlite formation with improved mechanical properties. Annealing : Heating the component to a temperature slightly above critical temperature holding it for sufficient time & then slowly cooling it in the furnace.

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Conclusion & References Iron carbon phase diagram can be said as the Map of steel & cast-iron. By changing the carbon content we can get different varieties of steel & cast-iron Also by controlling the heating & cooling rate of the component we can get metals whose physical properties are different. References : Fundamentals of material science by William D callister. http://www.roymech.co.uk. http://www4.hcmut.edu.vn/~dantn/Matter/Hardening.html. Design of machine elements by V B Bhandari.